N 11. The systems of the human body. Prepositions.

Organ Systems

Organ systems are composed of two or more different organs that work together to provide a common function.  There are 10 major organ systems in the human body

 Skeletal System:

Major Role:

The main role of the skeletal system is to provide support for the body, to protect delicate internal organs and to provide attachment sites for the organs.

Major Organs:

Bones, cartilage, tendons and ligaments

Muscular System:

Major Role:

The main role of the muscular system is to provide movement.  Muscles work in pairs to move limbs and provide the organism with mobility.  Muscles also control the movement of materials through some organs, such as the stomach and intestine, and the heart and circulatory system.

Major Organs:

Skeletal muscles and smooth muscles throughout the body.

Circulatory System:

Major Role:

The main role of the circulatory system is to transport nutrients, gases (such as oxygen and CO2), hormones and wastes through the body.

Major Organs:

Heart, blood vessels and blood.

Nervous System:

Major Role:

The main role of the nervous system is to relay electrical signals through the body.  The nervous system directs behaviour and movement and, along with the endocrine system, controls physiological processes such as digestion, circulation, etc.

Major Organs:

Brain, spinal cord and peripheral nerves.

Respiratory System:

Major Role:

The main role of the respiratory system is to provide gas exchange between the blood and the environment.  Primarily, oxygen is absorbed from the atmosphere into the body and carbon dioxide is expelled from the body.

Major Organs:

Nose, trachea and lungs.

Digestive System:

Major Role:

The main role of the digestive system is to breakdown and absorb nutrients that are necessary for growth and maintenance.

Major Organs:

Mouth, esophagus, stomach, small and large intestines.

Excretory System:

Major Role:

The main role of the excretory system is to filter out cellular wastes, toxins and excess water or nutrients from the circulatory system.

Major Organs:

Kidneys, ureters, bladder and urethra.

Endocrine System:

Major Role:

The main role of the endocrine system is to relay chemical messages through the body.  In conjunction with the nervous system, these chemical messages help control physiological processes such as nutrient absorption, growth, etc.

Major Organs:

Many glands exist in the body that secrete endocrine hormones.  Among these are the hypothalamus, pituitary, thyroid, pancreas and adrenal glands.

Reproductive System:

Major Role:

The main role of the reproductive system is to manufacture cells that allow reproduction.  In the male, sperm are created to inseminate egg cells produced in the female.

Major Organs:

Female (top): ovaries, oviducts, uterus, vagina and mammary glands. 

Male (bottom): testes, seminal vesicles and penis.

Female:

Male:

Lymphatic/Immune System: 

The main role of the immune system is to destroy and remove invading microbes and viruses from the body.  The lymphatic system also removes fat and excess fluids from the blood.

Major Organs:

Lymph, lymph nodes and vessels, white blood cells, T- and B- cells.

Major organ systems

·                    Circulatory system: pumping and channeling blood to and from the body and lungs with heart, blood, and blood vessels.

Diagram of the human circulatory system.  Arteries are shown red, veins are shown blue.

The circulatory system (or cardiovascular system) is an organ system that moves substances to and from cells; it can also help stabilize body temperature and pH (part of homeostasis). While the most primitive animal phyla lack circulatory systems, some invertebrate groups have open circulatory system. Vertebrates and annelids have a closed circulatory system.

Open circulatory system

An open circulatory system is an arrangement of internal transport present in animals such as molluscs and arthropods, in which fluid (called plasma) in a cavity called the hemocoel (or haemocoel) bathes the organs directly and there is no distinction between blood and interstitial fluid; this combined fluid is called hemolymph / haemolymph. Muscular movements by the animal during locomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores.

Hemolymph fills all of the interior hemocoel of the body and surrounds all cells. Hemolymph is composed of water, inorganic salts (mostly Na+, Cl-, K+, Mg2+, and Ca2+), and organic compounds (mostly carbohydrates, proteins, and lipids). The primary oxygen transporter molecule is hemocyanin.

There are free-floating cells, the hemocytes, within the hemolymph. They play a role in the arthropod immune system.

Closed circulatory system

The main components of the circulatory system are the heart, the blood, and the blood vessels.

The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods (squid and octopus) are closed, meaning that the blood never leaves the system of blood vessels consisting of arteries, capillaries and veins.

Arteries bring oxygenated blood to the tissues (except pulmonary arteries), and veins bring deoxygenated blood back to the heart (except pulmonary and portal veins). Blood passes from arteries to veins through capillaries, which are the thinnest and most numerous of the blood vessels and these capillaries help to join tissue with arterioles for transportation of nutrition to the cells.

Circulatory system

The human circulatory system (simplified). Red indicates oxygenated blood, blue indicates deoxygenated.

(Not depicted are the intricate network of capillaries, as well as the entire lymphatic system.)

Latin systema cardiovasculare

The circulatory system is an organ system that passes nutrients (such as amino acids, electrolytes and lymph), gases, hormones, blood cells, etc. to and from cells in the body to help fight diseases, stabilize body temperature and pH, and to maintain homeostasis.

 

This system may be seen strictly as a blood distribution network, but some consider the circulatory system as composed of the cardiovascular system, which distributes blood,and the lymphatic system, which returns excess filtered blood plasma from the interstitial fluid (between cells) as lymph. While humans, as well as other vertebrates, have a closed cardiovascular system (meaning that the blood never leaves the network of arteries, veins and capillaries), some invertebrate groups have an open cardiovascular system. The most primitive animal phyla[clarify] lack circulatory systems. The lymphatic system, on the other hand, is an open system providing an accessory route for excess interstitial fluid to get returned to the blood.

 

Two types of fluids move through the circulatory system: blood and lymph. Lymph is essentially recycled blood plasma after it has been filtered from the blood cells and returned to the lymphatic system. The blood, heart, and blood vessels form the cardiovascular (from Latin words meaning 'heart'-'vessel') system. The lymph, lymph nodes, and lymph vessels form the lymphatic system. The cardiovascular system and the lymphatic system collectively make up the circulatory system.

Human cardiovascular system

Cross section of a human artery

 

The essential components of the human cardiovascular system are the heart, blood, and blood vessels. It includes: the pulmonary circulation, a "loop" through the lungs where blood is oxygenated; and the systemic circulation, a "loop" through the rest of the body to provide oxygenated blood. An average adult contains five to six quarts (roughly 4.7 to 5.7 liters) of blood, accounting for approximately 7% of their total body weight. Blood consists of plasma, red blood cells, white blood cells, and platelets. Also, the digestive system works with the circulatory system to provide the nutrients the system needs to keep the heart pumping.

Pulmonary circulation

 

The pulmonary circulatory system is the portion of the cardiovascular system in which oxygen-depleted blood is pumped away from the heart, via the pulmonary artery, to the lungs and returned, oxygenated, to the heart via the pulmonary vein.

 

Oxygen deprived blood from the vena cava, enters the right atrium of the heart and flows through the tricuspid valve (right atrioventricular valve) into the right ventricle, from which it is then pumped through the pulmonary semilunar valve into the pulmonary artery to the lungs. Gas exchange occurs in the lungs, whereby CO2 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the heart.

 

Systemic circulation

 

Systemic circulation is the circulation of the blood to all parts of the body except the lungs. Systemic circulation is the portion of the cardiovascular system which transports oxygenated blood away from the heart through the Aorta from the left atrium where the blood has been previously deposited from pulmonary circulation, to the rest of the body, and returns oxygen-depleted blood back to the heart. Systemic circulation is, distance-wise, much longer than pulmonary circulation, transporting blood to every part of the body.

View from the front, which means the right side of the heart is on the left of the diagram (and vice-versa)

 

Coronary circulation

 

The coronary circulatory system provides a blood supply to the muscles of the heart. As it carries oxygenated blood to muscles, it is by definition a part of the systemic circulatory system. It arises from the aorta and drains through the coronary sinus into the right atrium. Back flow of blood through its opening during atrial systole is prevented by the Thebesian valve. The smallest cardiac veins drain directly into chambers of the heart.

Heart

 

The heart pumps oxygenated blood to the body and deoxygenated blood to the lungs. In the human heart there is one atrium and one ventricle for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total: left atrium, left ventricle, right atrium and right ventricle. The right atrium is the upper chamber of the right side of the heart. The blood that is returned to the right atrium is deoxygenated (poor in oxygen) and passed into the right ventricle to be pumped through the pulmonary artery to the lungs for re-oxygenation and removal of carbon dioxide. The left atrium receives newly oxygenated blood from the lungs as well as the pulmonary vein which is passed into the strong left ventricle to be pumped through the aorta to the different organs of the body.

 

Closed cardiovascular system

 

The cardiovascular systems of humans are closed, meaning that the blood never leaves the network of blood vessels. In contrast, oxygen and nutrients diffuse across the blood vessel layers and enters interstitial fluid, which carries oxygen and nutrients to the target cells, and carbon dioxide and wastes in the opposite direction. The other component of the circulatory system, the lymphatic system, is not closed.

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Oxygen transportation

Main article: Blood#Oxygen transport

 

About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with hemoglobin molecules. About 1.5% is physically dissolved in the other blood liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species.

An animation of a typical human red blood cell cycle in the circulatory system

 

This animation occurs at real time (20 seconds of cycle) and shows the red blood cell deform as it enters capillaries, as well as changing color as it alternates in states of oxygenation along the circulatory system.

Magnetic resonance angiography of aberrant subclavian artery

Development

 

The development of the circulatory system initially occurs by the process of vasculogenesis. The human arterial and venous systems develop from different embryonic areas. While the arterial system develops mainly from the aortic arches, the venous system arises from three bilateral veins during weeks 4 - 8 of human development.

 

Arterial development

 

The human arterial system originate from the aortic arches and from the dorsal aortae starting from week 4 of human development. Aortic arch 1 almost completely regresses except to form the maxillary arteries. Aortic arch 2 also completely regresses except to form the stapedial arteries. The definitive formation of the arterial system arise from aortic arches 3, 4 and 6. While aortic arch 5 completely regresses.

 

The dorsal aortae are initially bilateral and then fuse to form the definitive dorsal aorta. Approximately 30 posterolateral branches arise off the aorta and will form the intercostal arteries, upper and lower extremity arteries, lumbar arteries and the lateral sacral arteries. The lateral branches of the aorta form the definitive renal, suprarrenal and gonadal arteries. Finally, the ventral branches of the aorta consist of the vitelline arteries and umbilical arteries. The vitelline arteries form the celiac, superior and inferior mesenteric arteries of the gastrointestinal tract. After birth, the umbilical arteries will form the internal iliac arteries.

Venous development

 

The human venous system develops mainly from the vitelline veins, the umbilical veins and the cardinal veins, all of which empty into the sinus venosus.

Measurement techniques

Electrocardiogram—for cardiac electrophysiology

Sphygmomanometer and stethoscope—for blood pressure

Pulse meter—for cardiac function (heart rate, rhythm, dropped beats)

Pulse—commonly used to determine the heart rate in absence of certain cardiac pathologies

Heart rate variability—used to measure variations of time intervals between heart beats

Nail bed blanching test—test for perfusion

Vessel cannula or catheter pressure measurement—pulmonary wedge pressure or in older animal experiments.

Health and disease

Other vertebrates

Two-chambered heart of a fish

 

The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods (squids, octopuses and relatives) are closed, just as in humans. Still, the systems of fish, amphibians, reptiles, and birds show various stages of the evolution of the circulatory system.

 

In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation. The heart of fish is therefore only a single pump (consisting of two chambers).

 

In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart.

 

In reptiles, the ventricular septum of the heart is incomplete and the pulmonary artery is equipped with a sphincter muscle. This allows a second possible route of blood flow. Instead of blood flowing through the pulmonary artery to the lungs, the sphincter may be contracted to divert this blood flow through the incomplete ventricular septum into the left ventricle and out through the aorta. This means the blood flows from the capillaries to the heart and back to the capillaries instead of to the lungs. This process is useful to ectothermic (cold-blooded) animals in the regulation of their body temperature.

 

Birds and mammals show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought[citation needed] that the four-chambered heart of birds evolved independently from that of mammals.

 

Open circulatory system

 

The open circulatory system is a system in which a fluid in a cavity called the hemocoel bathes the organs directly with oxygen and nutrients and there is no distinction between blood and interstitial fluid; this combined fluid is called hemolymph or haemolymph.[6] Muscular movements by the animal during locomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores (ostia).

 

Hemolymph fills all of the interior hemocoel of the body and surrounds all cells. Hemolymph is composed of water, inorganic salts (mostly Na+, Cl-, K+, Mg2+, and Ca2+), and organic compounds (mostly carbohydrates, proteins, and lipids). The primary oxygen transporter molecule is hemocyanin.

 

There are free-floating cells, the hemocytes, within the hemolymph. They play a role in the arthropod immune system.

 

Absence of circulatory system

Flatworms, such as this Helicometra sp., lack specialized circulatory organs

 

Circulatory systems are absent in some animals, including flatworms (phylum Platyhelminthes). Their body cavity has no lining or enclosed fluid. Instead a muscular pharynx leads to an extensively branched digestive system that facilitates direct diffusion of nutrients to all cells. The flatworm's dorso-ventrally flattened body shape also restricts the distance of any cell from the digestive system or the exterior of the organism. Oxygen can diffuse from the surrounding water into the cells, and carbon dioxide can diffuse out. Consequently every cell is able to obtain nutrients, water and oxygen without the need of a transport system.

 

Some animals, such as jellyfish, have more extensive branching from their gastrovascular cavity (which functions as both a place of digestion and a form of circulation), this branching allows for bodily fluids to reach the outer layers, since the digestion begins in the inner layers.

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History of discovery

 

The earliest known writings on the circulatory system are found in the Ebers Papyrus (16th century BCE), an ancient Egyptian medical papyrus containing over 700 prescriptions and remedies, both physical and spiritual. In the papyrus, it acknowledges the connection of the heart to the arteries. The Egyptians thought air came in through the mouth and into the lungs and heart. From the heart, the air travelled to every member through the arteries. Although this concept of the circulatory system is only partially correct, it represents one of the earliest accounts of scientific thought.

 

In the 6th century BCE, the knowledge of circulation of vital fluids through the body was known to the Ayurvedic physician Sushruta in ancient India.[7] He also seems to have possessed knowledge of the arteries, described as 'channels' by Dwivedi & Dwivedi (2007).[7] The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BCE. However their function was not properly understood then. Because blood pools in the veins after death, arteries look empty. Ancient anatomists assumed they were filled with air and that they were for transport of air.

 

The Greek physician, Herophilus, distinguished veins from arteries but thought that the pulse was a property of arteries themselves. Greek anatomist Erasistratus observed that arteries that were cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood that entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries but with reversed flow of blood.[8]

 

In 2nd century AD Rome, the Greek physician Galen knew that blood vessels carried blood and identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the liver from chyle, while arterial blood gave vitality by containing pneuma (air) and originated in the heart. Blood flowed from both creating organs to all parts of the body where it was consumed and there was no return of blood to the heart or liver. The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves.

 

Galen believed that the arterial blood was created by venous blood passing from the left ventricle to the right by passing through 'pores' in the interventricular septum, air passed from the lungs via the pulmonary artery to the left side of the heart. As the arterial blood was created 'sooty' vapors were created and passed to the lungs also via the pulmonary artery to be exhaled.

 

In 1025, The Canon of Medicine by the Persian physician, Avicenna, "erroneously accepted the Greek notion regarding the existence of a hole in the ventricular septum by which the blood traveled between the ventricles." Despite this, Avicenna "correctly wrote on the cardiac cycles and valvular function", and "had a vision of blood circulation" in his Treatise on Pulse.[9][verification needed] While also refining Galen's erroneous theory of the pulse, Avicenna provided the first correct explanation of pulsation: "Every beat of the pulse comprises two movements and two pauses. Thus, expansion : pause : contraction : pause. [...] The pulse is a movement in the heart and arteries ... which takes the form of alternate expansion and contraction."

 

In 1242, the Arabian physician, Ibn al-Nafis, became the first person to accurately describe the process of pulmonary circulation, for which he is sometimes considered the father of circulatory physiology. Ibn al-Nafis stated in his Commentary on Anatomy in Avicenna's Canon:

 

"...the blood from the right chamber of the heart must arrive at the left chamber but there is no direct pathway between them. The thick septum of the heart is not perforated and does not have visible pores as some people thought or invisible pores as Galen thought. The blood from the right chamber must flow through the vena arteriosa (pulmonary artery) to the lungs, spread through its substances, be mingled there with air, pass through the arteria venosa (pulmonary vein) to reach the left chamber of the heart and there form the vital spirit..."

 

In addition, Ibn al-Nafis had an insight into what would become a larger theory of the capillary circulation. He stated that "there must be small communications or pores (manafidh in Arabic) between the pulmonary artery and vein," a prediction that preceded the discovery of the capillary system by more than 400 years. Ibn al-Nafis' theory, however, was confined to blood transit in the lungs and did not extend to the entire body.

Image of veins from William Harvey's Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus

 

Michael Servetus was the first European to describe the function of pulmonary circulation, although his achievement was not widely recognized at the time, for a few reasons. He firstly described it in the "Manuscript of Paris" (near 1546), but this work was never published. And later he published this description, but in a theological treatise, Christianismi Restitutio, not in a book on medicine. Only three copies of the book survived, the rest were burned shortly after its publication in 1553 because of persecution of Servetus by religious authorities. Finally William Harvey, a pupil of Hieronymus Fabricius (who had earlier described the valves of the veins without recognizing their function), performed a sequence of experiments, and published Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus in 1628, which "demonstrated that there had to be a direct connection between the venous and arterial systems throughout the body, and not just the lungs. Most importantly, he argued that the beat of the heart produced a continuous circulation of blood through minute connections at the extremities of the body. This is a conceptual leap that was quite different from Ibn al-Nafis' refinement of the anatomy and bloodflow in the heart and lungs." This work, with its essentially correct exposition, slowly convinced the medical world. However, Harvey was not able to identify the capillary system connecting arteries and veins; these were later discovered by Marcello Malpighi in 1661.

 

In 1956, André Frédéric Cournand, Werner Forssmann and Dickinson W. Richards were awarded the Nobel Prize in Medicine "for their discoveries concerning heart catheterization and pathological changes in the circulatory system."

 

·                    Digestive system: digestion and processing food with salivary glands, esophagus, stomach, liver, gallbladder, pancreas, intestines, rectum, and anus.

Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones.

 

When food enters the mouth, its digestion starts by the action of mastication, a form of mechanical digestion, and the contact of saliva. Saliva, which is secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food. After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus. It will then travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains hydrochloric acid and pepsin. As these two chemicals may damage the stomach wall, mucus is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, which are waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. After some time (typically an hour or two in humans, 4–6 hours in dogs, somewhat shorter duration in house cats), the resulting thick liquid is called chyme. When the pyloric sphincter valve opens, chyme enters the duodenum where it mixes with digestive enzymes from the pancreas, and then passes through the small intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly acidic about 5.6 ~ 6.9. Some vitamins, such as biotin and vitamin K (K2MK7) produced by bacteria in the colon are also absorbed into the blood in the colon. Waste material is eliminated from the rectum during defecation.

Digestive systems

 

Digestive systems take many forms. There is a fundamental distinction between internal and external digestion. External digestion is more primitive, and most fungi still rely on it. In this process, enzymes are secreted into the environment surrounding the organism, where they break down an organic material, and some of the products diffuse back to the organism. Later, animals form a tube in which internal digestion occurs, which is more efficient because more of the broken down products can be captured, and the internal chemical environment can be more efficiently controlled.

 

Some organisms, including nearly all spiders, simply secrete biotoxins and digestive chemicals (e.g., enzymes) into the extracellular environment prior to ingestion of the consequent "soup". In others, once potential nutrients or food is inside the organism, digestion can be conducted to a vesicle or a sac-like structure, through a tube, or through several specialized organs aimed at making the absorption of nutrients more efficient.

Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, bringing the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is transferred to the recipient cell. 4- Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors.

Secretion systems

 

Bacteria use several systems to obtain nutrients from other organisms in the environments.

Channel transport system

 

In a channel transupport system, several proteins form a contiguous channel traversing the inner and outer membranes of the bacteria. It is a simple system, which consists of only three protein subunits: the ABC protein, membrane fusion protein (MFP), and outer membrane protein (OMP)[specify]. This secretion system transports various molecules, from ions, drugs, to proteins of various sizes (20 - 900 kDa). The molecules secreted vary in size from the small Escherichia coli peptide colicin V, (10 kDa) to the Pseudomonas fluorescens cell adhesion protein LapA of 900 kDa.[5]

Molecular syringe

 

One molecular syringe is used through which a bacterium (e.g. certain types of Salmonella, Shigella, Yersinia) can inject nutrients into protist cells. One such mechanism was first discovered in Y. pestis and showed that toxins could be injected directly from the bacterial cytoplasm into the cytoplasm of its host's cells rather than simply be secreted into the extracellular medium.[6]

Conjugation machinery

 

The conjugation machinery of some bacteria (and archaeal flagella) is capable of transporting both DNA and proteins. It was discovered in Agrobacterium tumefaciens, which uses this system to introduce the Ti plasmid and proteins into the host, which develops the crown gall (tumor).[7] The VirB complex of Agrobacterium tumefaciens is the prototypic system.[8]

 

The nitrogen fixing Rhizobia are an interesting case, wherein conjugative elements naturally engage in inter-kingdom conjugation. Such elements as the Agrobacterium Ti or Ri plasmids contain elements that can transfer to plant cells. Transferred genes enter the plant cell nucleus and effectively transform the plant cells into factories for the production of opines, which the bacteria use as carbon and energy sources. Infected plant cells form crown gall or root tumors. The Ti and Ri plasmids are thus endosymbionts of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.

 

The Ti and Ri plasmids are themselves conjugative. Ti and Ri transfer between bacteria uses an independent system (the tra, or transfer, operon) from that for inter-kingdom transfer (the vir, or virulence, operon). Such transfer creates virulent strains from previously avirulent Agrobacteria.

Release of outer membrane vesicles

 

In addition to the use of the multiprotein complexes listed above, Gram-negative bacteria possess another method for release of material: the formation of outer membrane vesicles. Portions of the outer membrane pinch off, forming spherical structures made of a lipid bilayer enclosing periplasmic materials. Vesicles from a number of bacterial species have been found to contain virulence factors, some have immunomodulatory effects, and some can directly adhere to and intoxicate host cells. While release of vesicles has been demonstrated as a general response to stress conditions, the process of loading cargo proteins seems to be selective.

Venus Flytrap (Dionaea muscipula) leaf

Gastrovascular cavity

 

The gastrovascular cavity functions as a stomach in both digestion and the distribution of nutrients to all parts of the body. Extracellular digestion takes place within this central cavity, which is lined with the gastrodermis, the internal layer of epithelium. This cavity has only one opening to the outside that functions as both a mouth and an anus: waste and undigested matter is excreted through the mouth/anus, which can be described as an incomplete gut.

 

In a plant such as the Venus Flytrap that can make its own food through photosynthesis, it does not eat and digest its prey for the traditional objectives of harvesting energy and carbon, but mines prey primarily for essential nutrients (nitrogen and phosphorus in particular) that are in short supply in its boggy, acidic habitat.

Trophozoites of Entamoeba histolytica with ingested erythrocytes

 

Phagosome

 

A phagosome is a vacuole formed around a particle absorbed by phagocytosis. The vacuole is formed by the fusion of the cell membrane around the particle. A phagosome is a cellular compartment in which pathogenic microorganisms can be killed and digested. Phagosomes fuse with lysosomes in their maturation process, forming phagolysosomes. In humans, Entamoeba histolytica can phagocytose red blood cells.

Specialised organs and behaviours

 

To aid in the digestion of their food animals evolved organs such as beaks, tongues, teeth, a crop, gizzard, and others.

 

A Catalina Macaw's seed-shearing beak

Squid beak with ruler for size comparison

Beaks

 

Birds have beaks that are specialised according to the bird's ecological niche. For example, macaws primarily eat seeds, nuts, and fruit, using their impressive beaks to open even the toughest seed. First they scratch a thin line with the sharp point of the beak, then they shear the seed open with the sides of the beak.

 

The mouth of the squid is equipped with a sharp horny beak mainly made of cross-linked proteins. It is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain any minerals, unlike the teeth and jaws of many other organisms, including marine species. The beak is the only indigestible part of the squid.

 

Tongue

 

The tongue is skeletal muscle on the floor of the mouth that manipulates food for chewing (mastication) and swallowing (deglutition). It is sensitive and kept moist by saliva. The underside of the tongue is covered with a smooth mucous membrane. The tongue also has a touch sense for locating and positioning food particles that require further chewing. The tongue is utilized to roll food particles into a bolus before being transported down the esophagus through peristalsis.

 

The sublingual region underneath the front of the tongue is a location where the oral mucosa is very thin, and underlain by a plexus of veins. This is an ideal location for introducing certain medications to the body. The sublingual route takes advantage of the highly vascular quality of the oral cavity, and allows for the speedy application of medication into the cardiovascular system, bypassing the gastrointestinal tract.

 

Teeth

 

Teeth (singular tooth) are small whitish structures found in the jaws (or mouths) of many vertebrates that are used to tear, scrape, milk and chew food. Teeth are not made of bone, but rather of tissues of varying density and hardness. The shape of an animal's teeth is related to its diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing.

 

The teeth of carnivores are shaped to kill and tear meat, using specially shaped canine teeth. Herbivores' teeth are made for grinding food materials, in this case, plant parts.

Crop

 

A crop, or croup, is a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion. In some birds it is an expanded, muscular pouch near the gullet or throat. In adult doves and pigeons, the crop can produce crop milk to feed newly hatched birds.

 

Certain insects may have a crop or enlarged esophagus.

Rough illustration of a ruminant digestive system

Abomasum

 

Herbivores have evolved cecums (or an abomasum in the case of ruminants). Ruminants have a fore-stomach with four chambers. These are the rumen, reticulum, omasum, and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material. Solids clump together to form the cud (or bolus). The cud is then regurgitated, chewed slowly to completely mix it with saliva and to break down the particle size.

 

Fibre, especially cellulose and hemi-cellulose, is primarily broken down into the volatile fatty acids, acetic acid, propionic acid and butyric acid in these chambers (the reticulo-rumen) by microbes: (bacteria, protozoa, and fungi). In the omasum water and many of the inorganic mineral elements are absorbed into the blood stream.

 

The abomasum is the fourth and final stomach compartment in ruminants. It is a close equivalent of a monogastric stomach (e.g., those in humans or pigs), and digesta is processed here in much the same way. It serves primarily as a site for acid hydrolysis of microbial and dietary protein, preparing these protein sources for further digestion and absorption in the small intestine. Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs. Microbes produced in the reticulo-rumen are also digested in the small intestine.

A flesh fly "blowing a bubble", possibly to concentrate its food by evaporating water

Specialised behaviours

 

Regurgitation has been mentioned above under abomasum and crop, referring to crop milk, a secretion from the lining of the crop of pigeons and doves with which the parents feed their young by regurgitation.

 

Many sharks have the ability to turn their stomachs inside out and evert it out of their mouths in order to get rid of unwanted contents (perhaps developed as a way to reduce exposure to toxins).

 

Other animals, such as rabbits and rodents, practise coprophagia behaviours - eating specialised faeces in order to re-digest food, especially in the case of roughage. Capybara, rabbits, hamsters and other related species do not have a complex digestive system as do, for example, ruminants. Instead they extract more nutrition from grass by giving their food a second pass through the gut. Soft faecal pellets of partially digested food are excreted and generally consumed immediately. They also produce normal droppings, which are not eaten.

 

Young elephants, pandas, koalas, and hippos eat the faeces of their mother, probably to obtain the bacteria required to properly digest vegetation. When they are born, their intestines do not contain these bacteria (they are completely sterile). Without them, they would be unable to get any nutritional value from many plant components.

 

In earthworms

 

An earthworm's digestive system consists of a mouth, pharynx, esophagus, crop, gizzard, and intestine. The mouth is surrounded by strong lips, which act like a hand to grab pieces of dead grass, leaves, and weeds, with bits of soil to help chew. The lips break the food down into smaller pieces. In the pharynx, the food is lubricated by mucus secretions for easier passage. The esophagus adds calcium carbonate to neutralize the acids formed by food matter decay. Temporary storage occurs in the crop where food and calcium carbonate are mixed. The powerful muscles of the gizzard churn and mix the mass of food and dirt. When the churning is complete, the glands in the walls of the gizzard add enzymes to the thick paste, which helps chemically breakdown the organic matter. By peristalsis, the mixture is sent to the intestine where friendly bacteria continue chemical breakdown. This releases carbohydrates, protein, fat, and various vitamins and minerals for absorption into the body.

 

Overview of vertebrate digestion

 

In most vertebrates, digestion is a multi-stage process in the digestive system, starting from ingestion of raw materials, most often other organisms. Ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four steps:

Ingestion: placing food into the mouth (entry of food in the digestive system),

Mechanical and chemical breakdown: mastication and the mixing of the resulting bolus with water, acids, bile and enzymes in the stomach and intestine to break down complex molecules into simple structures,

Absorption: of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, and

Egestion (Excretion): Removal of undigested materials from the digestive tract through defecation.

 

Underlying the process is muscle movement throughout the system through swallowing and peristalsis. Each step in digestion requires energy, and thus imposes an "overhead charge" on the energy made available from absorbed substances. Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures. Examples may be seen in humans, who differ considerably from other hominids (lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc.).

 

The major part of digestion takes place in the small intestine. The large intestine primarily serves as a site for fermentation of indigestible matter by gut bacteria and for resorption of water from digesta before excretion.

 

In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches. The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with defecation.[2]

 

Human digestion process

 

Upper and Lower human gastrointestinal tract

 

The whole digestive system is around 9 meters long. In a healthy human adult this process can take between 24 and 72 hours. Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal.

 

Phases of gastric secretion

 

Cephalic phase - This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.

Gastric phase - This phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myenteric reflexes. This activates the release of acetylcholine, which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete gastric acid. Gastric acid is about 0.5% hydrochloric acid (HCl), which lowers the pH to the desired pH of 1-3. Acid release is also triggered by acetylcholine and histamine.

Intestinal phase - This phase has 2 parts, the excitatory and the inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.

 

Oral cavity

 

In humans, digestion begins in the Mouth, otherwise known as the "Buccal Cavity", where food is chewed. Saliva is secreted in large amounts (1-1.5 litres/day) by three pairs of exocrine salivary glands (parotid, submandibular, and sublingual) in the oral cavity, and is mixed with the chewed food by the tongue. Saliva cleans the oral cavity, moistens the food, and contains digestive enzymes such as salivary amylase, which aids in the chemical breakdown of polysaccharides such as starch into disaccharides such as maltose. It also contains mucus, a glycoprotein that helps soften the food and form it into a bolus. An additional enzyme, lingual lipase, hydrolyzes long-chain triglycerides into partial glycerides and free fatty acids.

 

Swallowing transports the chewed food into the esophagus, passing through the oropharynx and hypopharynx. The mechanism for swallowing is coordinated by the swallowing center in the medulla oblongata and pons. The reflex is initiated by touch receptors in the pharynx as the bolus of food is pushed to the back of the mouth.

 

Pharynx

 

The pharynx is the part of the neck and throat situated immediately behind the mouth and nasal cavity, and cranial, or superior, to the esophagus. It is part of the digestive system and respiratory system. Because both food and air pass through the pharynx, a flap of connective tissue, the epiglottis closes over the trachea when food is swallowed to prevent choking or asphyxiation.

 

The oropharynx is that part of the pharynx behind the oral cavity. It is lined with stratified squamous epithelium. The nasopharynx lies behind the nasal cavity and like the nasal passages is lined with ciliated columnar pseudostratified epithelium.

 

Like the oropharynx above it the hypopharynx (laryngopharynx) serves as a passageway for food and air and is lined with a stratified squamous epithelium. It lies inferior to the upright epiglottis and extends to the larynx, where the respiratory and digestive pathways diverge. At that point, the laryngopharynx is continuous with the esophagus. During swallowing, food has the "right of way", and air passage temporarily stops.

 

Esophagus

 

The esophagus is a narrow muscular tube about 20-30 centimeters long, which starts at the pharynx at the back of the mouth, passes through the thoracic diaphragm, and ends at the cardiac orifice of the stomach. The wall of the esophagus is made up of two layers of smooth muscles, which form a continuous layer from the esophagus to the colon and contract slowly, over long periods of time. The inner layer of muscles is arranged circularly in a series of descending rings, while the outer layer is arranged longitudinally. At the top of the esophagus, is a flap of tissue called the epiglottis that closes during swallowing to prevent food from entering the trachea (windpipe). The chewed food is pushed down the esophagus to the stomach through peristaltic contraction of these muscles. It takes only about seven seconds for food to pass through the esophagus and now digestion takes place.

 

Stomach

 

The stomach is a small, 'J'-shaped pouch with walls made of thick, distensible muscles, which stores and helps break down food. Food reduced to very small particles is more likely to be fully digested in the small intestine, and stomach churning has the effect of assisting the physical disassembly begun in the mouth. Ruminants, who are able to digest fibrous material (primarily cellulose), use fore-stomachs and repeated chewing to further the disassembly. Rabbits and some other animals pass some material through their entire digestive systems twice. Most birds ingest small stones to assist in mechanical processing in gizzards.

 

Food enters the stomach through the cardiac orifice where it is further broken apart and thoroughly mixed with gastric acid, pepsin and other digestive enzymes to break down proteins. The enzymes in the stomach also have an optimum conditions, meaning that they work at a specific pH and temperature better than any others. The acid itself does not break down food molecules, rather it provides an optimum pH for the reaction of the enzyme pepsin and kills many microorganisms that are ingested with the food. It can also denature proteins. This is the process of reducing polypeptide bonds and disrupting salt bridges, which in turn causes a loss of secondary, tertiary, or quaternary protein structure. The parietal cells of the stomach also secrete a glycoprotein called intrinsic factor, which enables the absorption of vitamin B-12. Mucus neck cells are present in the gastric glands of the stomach. They secrete mucus, which along with gastric juice plays an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid. Other small molecules such as alcohol are absorbed in the stomach, passing through the membrane of the stomach and entering the circulatory system directly. Food in the stomach is in semi-liquid form, which upon completion is known as chyme.

 

After consumption of food, digestive "tonic" and peristaltic contractions begin, which helps break down the food and move it onward.[16] When the chyme reaches the opening to the duodenum known as the pylorus, contractions "squirt" the food back into the stomach through a process called retropulsion, which exerts additional force and further grinds down food into smaller particles.[16] Gastric emptying is the release of food from the stomach into the duodenum; the process is tightly controlled with liquids being emptied much more quickly than solids.[16] Gastric emptying has attracted medical interest as rapid gastric emptying is related to obesity and delayed gastric emptying syndrome is associated with diabetes mellitus, aging, and gastroesophageal reflux.[16]

 

The transverse section of the alimentary canal reveals four (or five, see description under mucosa) distinct and well developed layers within the stomach:

Serous membrane, a thin layer of mesothelial cells that is the outermost wall of the stomach.

Muscular coat, a well-developed layer of muscles used to mix ingested food, composed of three sets running in three different alignments. The outermost layer runs parallel to the vertical axis of the stomach (from top to bottom), the middle is concentric to the axis (horizontally circling the stomach cavity) and the innermost oblique layer, which is responsible for mixing and breaking down ingested food, runs diagonal to the longitudinal axis. The inner layer is unique to the stomach, all other parts of the digestive tract have only the first two layers.

Submucosa, composed of connective tissue that links the inner muscular layer to the mucosa and contains the nerves, blood and lymph vessels.

Mucosa is the extensively folded innermost layer. It can be divided into the epithelium, lamina propria, and the muscularis mucosae, though some consider the outermost muscularis mucosae to be a distinct layer, as it develops from the mesoderm rather than the endoderm (thus making a total of five layers). The epithelium and lamina are filled with connective tissue and covered in gastric glands that may be simple or branched tubular, and secrete mucus, hydrochloric acid, pepsinogen and rennin. The mucus lubricates the food and also prevents hydrochloric acid from acting on the walls of the stomach.

 

Small intestine

 

It has three parts: the Duodenum, Jejunum, and Ileum.

 

After being processed in the stomach, food is passed to the small intestine via the pyloric sphincter. The majority of digestion and absorption occurs here after the milky chyme enters the duodenum. Here it is further mixed with three different liquids:

Bile, which emulsifies fats to allow absorption, neutralizes the chyme and is used to excrete waste products such as bilin and bile acids. Bile is produced by the liver and then stored in the gallbladder where it will be released to the small intestine via the bile duct. The bile in the gallbladder is much more concentrated.[clarification needed]

Pancreatic juice made by the pancreas, which secretes enzymes such as pancreatic amylase, pancreatic lipase, and trypsinogen (inactive form of protease).

Intestinal juice secreted by the intestinal glands in the small intestine. It contains enzymes such as enteropeptidase, erepsin, trypsin, chymotrypsin, maltase, lactase and sucrase (all three of which process only sugars).

 

The pH level increases in the small intestine as all three fluids are alkaline. A more basic environment causes more helpful enzymes to activate and begin to help in the breakdown of molecules such as fat globules. Small, finger-like structures called villi, and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine and enhancing speed at which nutrients are absorbed. Blood containing the absorbed nutrients is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing.

 

The small intestine and remainder of the digestive tract undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices and absorbed. The circular muscles and longitudinal muscles are antagonistic muscles, with one contracting as the other relaxes. When the circular muscles contract, the lumen becomes narrower and longer and the food is squeezed and pushed forward. When the longitudinal muscles contract, the circular muscles relax and the gut dilates to become wider and shorter to allow food to enter.

 

Large intestine

 

After the food has been passed through the small intestine, the food enters the large intestine. Within it, digestion is retained long enough to allow fermentation due to the action of gut bacteria, which breaks down some of the substances that remain after processing in the small intestine; some of the breakdown products are absorbed. In humans, these include most complex saccharides (at most three disaccharides are digestible in humans). In addition, in many vertebrates, the large intestine reabsorbs fluid; in a few, with desert lifestyles, this reabsorbtion makes continued existence possible.

 

In general, the large intestine is less vigorous in absorptive activity. It produces sacculation, renews epithelial cells, and provides protective mucus and mucosal immunity. In humans, the large intestine is roughly 1.5 meters long, with three parts: the cecum at the junction with the small intestine, the colon, and the rectum. The colon itself has four parts: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The large intestine absorbs water from the chyme and stores feces until it can be egested. Food products that cannot go through the villi, such as cellulose (dietary fiber), are mixed with other waste products from the body and become hard and concentrated feces. The feces is stored in the rectum for a certain period and then the stored feces is eliminated from the body due to the contraction and relaxation through the anus. The exit of this waste material is regulated by the anal sphincter.

 

Protein digestion

 

Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens. For example, trypsin is secreted by pancreas in the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin. Trypsin then cleaves proteins to smaller polypeptides.

 

Fat digestion

 

Digestion of some fats can begin in the mouth where lingual lipase breaks down some short chain lipids into diglycerides. The presence of fat in the small intestine produces hormones that stimulate the release of pancreatic lipase from the pancreas and bile from the liver for breakdown of fats into fatty acids. Complete digestion of one molecule of fat (a triglyceride) results in 3 fatty acid molecules and one glycerol molecule.

 

Carbohydrate digestion

 

In humans, dietary starches are composed of glucose units arranged in long chains called amylose, a polysaccharide. During digestion, bonds between glucose molecules are broken by salivary and pancreatic amylase, resulting in progressively smaller chains of glucose. This results in simple sugars glucose and maltose (2 glucose molecules) that can be absorbed by the small intestine.

 

Lactase is an enzyme that breaks down the disaccharide lactose to its component parts, glucose and galactose. Glucose and galactose can be absorbed by the small intestine. Approximately half of the adult population produce only small amounts of lactase and are unable to eat milk-based foods. This is commonly known as lactose intolerance.

 

Sucrase is an enzyme that breaks down the disaccharide sucrose, commonly known as table sugar, cane sugar, or beet sugar. Sucrose digestion yields the sugars fructose and glucose which are readily absorbed by the small intestine.

DNA and RNA digestion

 

DNA and RNA are broken down into mononucleotides by the nucleases deoxyribonuclease and ribonuclease (DNase and RNase) from the pancreas.

Digestive hormones

Action of the major digestive hormones

 

There are at least five hormones that aid and regulate the digestive system in mammals. There are variations across the vertebrates, as for instance in birds. Arrangements are complex and additional details are regularly discovered. For instance, more connections to metabolic control (largely the glucose-insulin system) have been uncovered in recent years.

Gastrin - is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH .

Secretin - is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver. This hormone responds to the acidity of the chyme.

Cholecystokinin (CCK) - is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in response to fat in chyme.

Gastric inhibitory peptide (GIP) - is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach. Another function is to induce insulin secretion.

Motilin - is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.

Significance of pH in digestion

 

Digestion is a complex process controlled by several factors. pH plays a crucial role in a normally functioning digestive tract. In the mouth, pharynx, and esophagus, pH is typically about 6.8, very weakly acidic. Saliva controls pH in this region of the digestive tract. Salivary amylase is contained in saliva and starts the breakdown of carbohydrates into monosaccharides. Most digestive enzymes are sensitive to pH and will denature in a high or low pH environment.

 

The stomach's high acidity inhibits the breakdown of carbohydrates within it. This acidity confers two benefits: it denatures proteins for further digestion in the small intestines, and provides non-specific immunity, damaging or eliminating various pathogens.

 

In the small intestines, the duodenum provides critical pH balancing to activate digestive enzymes. The liver secretes bile into the duodenum to neutralize the acidic conditions from the stomach, and the pancreatic duct empties into the duodenum, adding bicarbonate to neutralize the acidic chyme, thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline with a pH of about 8.5.

Uses of animal's internal organs by humans

The stomachs of calves have commonly been used as a source of rennet for making cheese.

The use of animal gut strings by musicians can be traced back to the third dynasty of Egypt. In the recent past, strings were made out of lamb gut. With the advent of the modern era, musicians have tended to use strings made of silk, or synthetic materials such as nylon or steel. Some instrumentalists, however, still use gut strings in order to evoke the older tone quality. Although such strings were commonly referred to as "catgut" strings, cats were never used as a source for gut strings.

Sheep gut was the original source for natural gut string used in racquets, such as for tennis. Today, synthetic strings are much more common, but the best gut strings are now made out of cow gut.

Gut cord has also been used to produce strings for the snares that provide a snare drum's characteristic buzzing timbre. While the modern snare drum almost always uses metal wire rather than gut cord, the North African bendir frame drum still uses gut for this purpose.

"Natural" sausage hulls (or casings) are made of animal gut, especially hog, beef, and lamb. Similarly, Haggis is traditionally boiled in, and served in, a sheep stomach.

Chitterlings, a kind of food, consist of thoroughly washed pig's gut.

Animal gut was used to make the cord lines in longcase clocks and for fusee movements in bracket clocks, but may be replaced by metal wire.

The oldest known condoms, from 1640 AD, were made from animal intestine.

 

The gastrointestinal tract (GI tract), also called the digestive tract, or the alimentary canal. It is the system of organs within multicellular animals that takes in food, digests it to extract energy and nutrients, and expels the remaining waste. The major functions of the GI tract are digestion and excretion.

The GI tract differs substantially from animal to animal. For instance, some animals have multi-chambered stomachs, while some animals' stomachs contain a single chamber. In a normal human adult male, the GI tract is approximately 6.5 meters (20 feet) long and consists of the upper and lower GI tracts. The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment of the tract.

Upper and Lower gastrointestinal tract

Upper and Lower gastrointestinal tract

Upper gastrointestinal tract

The upper GI tract consists of the mouth, pharynx, esophagus, and stomach.

·                     The mouth contains the buccal mucosa, which contains the openings of the salivary glands; the tongue; and the teeth.

·                     Behind the mouth lies the pharynx, which leads to a hollow muscular tube, the esophagus.

·                     Peristalsis takes place, which is the contraction of muscles to propel the food down the esophagus which extends through the chest and pierces the diaphragm to reach the stomach.

·                     The stomach, in turn, leads to the small intestine.

The upper GI tract roughly corresponds to the derivatives of the foregut, with the exception of the first part of the duodenum (see below for more details.)

Lower gastrointestinal tract

The lower GI tract comprises the intestines and anus.

·                     Bowel or intestine

o                     small intestine, which has three parts:

§                                              duodenum

§                                              jejunum

§                                              ileum

o                     large intestine, which has three parts:

§                                              cecum (the vermiform appendix is attached to the cecum).

§                                              colon (ascending colon, transverse colon, descending colon and sigmoid flexure)

§                      rectum

·                     anus

Related organs

The liver secretes bile into the small intestine via the biliary system, employing the gallbladder as a reservoir. Apart from storing and concentrating bile, the gall bladder has no other specific function. The pancreas secretes an isosmotic fluid containing bicarbonate and several enzymes, including trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes (deoxyribonuclease and ribonuclease), into the small intestine.anussecretory organs aid in digestion.

·                    Endocrine system: communication within the body using hormones made by endocrine glands such as the hypothalamus, pituitary or pituitary gland, pineal body or pineal gland, thyroid, parathyroids, and adrenals or adrenal glands

Major endocrine glands. (Male left, female on the right.) 1. Pineal gland 2. Pituitary gland 3. Thyroid gland 4. Thymus 5. Adrenal gland 6. Pancreas 7. Ovary 8. Testes

Major endocrine glands. (Male left, female on the right.) 1. Pineal gland 2. Pituitary gland 3. Thyroid gland 4. Thymus 5. Adrenal gland 6. Pancreas 7. Ovary 8. Testes

 

The endocrine system is a control system of ductless glands that secrete hormones that circulate within the body via the bloodstream to affect cells within specific organs. It is also instrumental in regulating mood, growth and development, tissue function, and metabolism, as well as sending messages and acting on them. Typical endocrine glands are pituitary, thyroid, and adrenal glands, but not exocrine glands such as salivary glands, sweat glands and glands within the gastrointestinal tract.

The field of medicine that deals with disorders of endocrine glands is endocrinology, a branch of the wider field of internal medicine.

he endocrine system is the system of glands, each of which secretes different types of hormones directly into the bloodstream (some of which are transported along nerve tracts[citation needed]) to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. The word endocrine derives from the Greek words "endo" meaning inside, within, and "crinis" for secrete. The endocrine system is an information signal system like the nervous system, yet its effects and mechanism are classifiably different. The endocrine system's effects are slow to initiate, and prolonged in their response, lasting from a few hours up to weeks. The nervous system sends information very quickly, and responses are generally short lived. Hormones are substances (chemical mediators) released from endocrine tissue into the bloodstream where they travel to target tissue and generate a response. Hormones regulate various human functions, including metabolism, growth and development, tissue function, and mood. The field of study dealing with the endocrine system and its disorders is endocrinology, a branch of internal medicine.

 

Features of endocrine glands are, in general, their ductless nature, their vascularity, and usually the presence of intracellular vacuoles or granules storing their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.

 

In addition to the specialised endocrine organs mentioned above, many other organs that are part of other body systems, such as the kidney, liver, heart and gonads, have secondary endocrine functions. For example the kidney secretes endocrine hormones such as erythropoietin and renin.

 

The endocrine system is made of a series of glands that produce chemicals called hormones. A number of glands that signal each other in sequence are usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis.

Endocrine organs and secreted hormones

 

Alimentary system

 

Reproductive

 

Calcium regulation

 

 

Miscellaneous

 

Major endocrine systems

 

The human endocrine system consists of several systems that operate via feedback loops. Several important feedback systems are mediated via the hypothalamus and pituitary.[12]

TRH - TSH - T3/T4

GnRH - LH/FSH - sex hormones

CRH - ACTH - cortisol

Renin - angiotensin - aldosterone

 

Diseases

 

Diseases of the endocrine system are common, including conditions such as diabetes mellitus, thyroid disease, and obesity. Endocrine disease is characterized by disregulated hormone release (a productive pituitary adenoma), inappropriate response to signaling (hypothyroidism), lack of a gland (diabetes mellitus type 1, diminished erythropoiesis in chronic renal failure), or structural enlargement in a critical site such as the thyroid (toxic multinodular goitre). Hypofunction of endocrine glands can occur as a result of loss of reserve, hyposecretion, agenesis, atrophy, or active destruction. Hyperfunction can occur as a result of hypersecretion, loss of suppression, hyperplastic or neoplastic change, or hyperstimulation.

 

Endocrinopathies are classified as primary, secondary, or tertiary. Primary endocrine disease inhibits the action of downstream glands. Secondary endocrine disease is indicative of a problem with the pituitary gland. Tertiary endocrine disease is associated with dysfunction of the hypothalamus and its releasing hormones.

 

As the thyroid, and hormones have been implicated in signaling distant tissues to proliferate, for example, the estrogen receptor has been shown to be involved in certain breast cancers. Endocrine, paracrine, and autocrine signaling have all been implicated in proliferation, one of the required steps of oncogenesis.

 

Other types of signaling

 

The typical mode of cell signaling in the endocrine system is endocrine signaling. However, there are also other modes, i.e., paracrine, autocrine, and neuroendocrine signaling.[16] Purely neurocrine signaling between neurons, on the other hand, belongs completely to the nervous system.

 

Autocrine

 

 

Autocrine signaling is a form of signaling in which a cell secretes a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on the same cell, leading to changes in the cells.

 

Paracrine

 

Paracrine signaling is a form of cell signaling in which the target cell is near the signal-releasing cell.

 

Juxtacrine

 

Juxtacrine signaling is a type of intercellular communication that is transmitted via oligosaccharide, lipid, or protein components of a cell membrane, and may affect either the emitting cell or the immediately adjacent cells.

 

It occurs between adjacent cells that possess broad patches of closely opposed plasma membrane linked by transmembrane channels known as connexons. The gap between the cells can usually be between only 2 and 4 nm.

 

Unlike other types of cell signaling (such as paracrine and endocrine), juxtacrine signaling requires physical contact between the two cells involved.

 

Juxtacrine signaling has been observed for some growth factors, cytokine and chemokine cellular signals.

 

 

·                    Integumentary system: skin, hair and nails

In zootomy, the integumentary system is the external covering of the body, comprised of the skin, hair, feathers, scales, nails, sweat glands and their products (sweat and mucus).

The name derives from the Latin integumentum, which means 'a covering'.

The cutaneous membrane (skin) and its accessory structures (hair, scales, feathers, nails, exocrine glands) make up the integumentary system.

There are three layers of skin:

1.                Epidermis

2.                Dermis

3.                Subcutaneous tissue

Below the dermis, the subcutis acts to protect underlying muscles, tissues, and other organs. Hair on the surface of the skin helps maintain body temperature and filter out harmful particles.

Cutaneous glands include:

·                     Sweat glands (also known as sudoriferous glands) - excrete sweat to regulate temperature

·                     Sebaceous glands - oil-producing glands that keep skin and hair moist and soft

·                     Ceruminous glands - glands of the ear canal that produce earwax

·                     Mammary glands - milk-producing glands located in the breasts.

The integumentary system has multiple roles in homeostasis, including protection, temperature regulation, sensory reception, biochemical synthesis, and absorption. All body systems work in an interconnected manner to maintain the internal conditions essential to the function of the body. The skin has an important job of protecting the body and acts somewhat as the body’s first line of defense against infection, temperature change or other challenges to homeostasis.

The integumentary system has numerous functions:

·                     Protects the body’s internal living tissues and organs

·                     Protects against invasion by infectious organisms

·                     Protects the body from dehydration

·                     Protects the body against abrupt changes in temperature

·                     Helps excrete waste materials through perspiration

·                     Acts as a receptor for touch, pressure, pain, heat, and cold (see Somatosensory system)

·                     Protects the body against sunburns

·                     Generates vitamin D through exposure to ultraviolet light

·                     Stores water, fat, and vitamin D

The integumentary system is the organ system that protects the body from damage, comprising the skin and its appendages (including hair, scales, feathers, hoofs, and nails). The integumentary system has a variety of functions; it may serve to waterproof, cushion, and protect the deeper tissues, excrete wastes, and regulate temperature, and is the attachment site for sensory receptors to detect pain, sensation, pressure, and temperature. In most terrestrial vertebrates with significant exposure to sunlight, the integumentary system also provides for vitamin D synthesis.

 

Layers of the skin

 

The integumentary system is the largest of the body's organ systems. In humans, this system accounts for about 12 to 15 percent of total body weight and covers 1.5-2m2 of surface area. It distinguishes, separates, and protects the organism from its surroundings. Small-bodied invertebrates of aquatic or continually moist habitats respire using the outer layer (integument). This gas exchange system, where gases simply diffuse into and out of the interstitial fluid, is called integumentary exchange.

 

The human skin (integument) is composed of a minimum of 3 major layers of tissue: the epidermis; dermis; and hypodermis. The epidermis forms the outermost layer, providing the initial barrier to the external environment. Beneath this, the dermis comprises two sections, the papillary and reticular layers, and contains connective tissues, vessels, glands, follicles, hair roots, sensory nerve endings, and muscular tissue. The deepest layer is the hypodermis, which is primarily made up of adipose tissue. Substantial collagen bundles anchor the dermis to the hypodermis in a way that permits most areas of the skin to move freely over the deeper tissue layers.

 

Epidermis

 

This is the top layer of skin made up of epithelial cells. It does not contain blood vessels. Its main function is protection, absorption of nutrients, and homeostasis. In structure, it consists of a keratinized stratified squamous epithelium comprising four types of cells: keratinocytes, melanocytes, Merkel cells, and Langerhans' cells. The major cell of the epidermis is the keratinocyte, which produces keratin. Keratin is a fibrous protein that aids in protection. Keratin is also a water-proofing protein. Millions of dead keratinocytes rub off daily. The majority of the skin on the body is keratinized, meaning waterproofed. The only skin on the body that is non-keratinized is the lining of skin on the inside of the mouth. Non-keratinized cells allow water to "stay" atop the structure.

 

The protein keratin stiffens epidermal tissue to form fingernails. Nails grow from thin area called the nail matrix; growth of nails is 1 mm per week on average. The lunula is the crescent-shape area at the base of the nail, this is a lighter color as it mixes with the matrix cells.

Dermis

 

The dermis is the middle layer of skin, composed of dense irregular connective tissue and areolar connective tissue such as collagen with elastin arranged in a diffusely bundled and woven pattern. The dermis has two layers. The Papillary layer which is the superficial layer and consists of the areolar connective tissue and the Reticular layer which is the deep layer of the dermis and consists of the dense irregular connective tissue. These layers serve to give elasticity to the integument, allowing stretching and conferring flexibility, while also resisting distortions, wrinkling, and sagging. The dermal layer provides a site for the endings of blood vessels and nerves. Many chromatophores are also stored in this layer, as are the bases of integumental structures such as hair, feathers, and glands.

Appendages of the Skin

Sweat (Sudoriferous) Glands

 

Eccrine sweat glands, or merocrine sweat glands, produce true sweat, are the most numerous of the sweat glands, and are particularly abundant on the palms of the hands, soles of the feet, and forehead.

 

Apocrine sweat glands are confined to the axillary and anogenital areas and produce true sweat with the addition of fatty substances and proteins.

 

Ceruminous glands are modified sweat glands found lining the ear canal that secrete ear wax, or cerumen.

 

Mammary glands are modified sweat glands found in the breasts that secrete milk.

 

Sebaceous (Oil) Glands

 

Sebaceous glands are simple alveolar glands found all over the body except the palms of the hands and soles of the feet that secrete sebum, an oily secretion.

 

The sebaceous glands function as holocrine glands, secreting their product into a hair follicle or to a pore on the surface of the skin.

 

Secretion by sebaceous glands is stimulated by hormones.

 

Hairs and Hair Follicles

 

Hairs, or pili, are flexible strands produced by hair follicles that consist of dead, keratinized cells.

 

The main regions of a hair are the shaft and the root.

 

A hair has three layers of keratinized cells: the inner core is the medulla, the middle layer is the cortex, and the outer layer is the cuticle.

 

Hair pigments (melanin of different colors) are made by melanocytes at the base of the hair follicle.

Structure of a Hair Follicle

 

Hair follicles fold down from the epidermis into the dermis and occasionally into the hypodermis.

 

The deep end of a hair follicle is expanded, forming a hair bulb, which is surrounded by a knot of sensory nerve endings called a hair follicle receptor, or root hair plexus.

 

The wall of a hair follicle is composed of an outer connective tissue root sheath, a thickened basement membrane called a glassy membrane, and an inner epithelial root sheath.

 

Associated with each hair follicle is a bundle of smooth muscle cells called an arrector pili muscle.

Types and Growth of Hair

 

Hairs come in various sizes and shapes, but can be classified as vellus or terminal.

 

Hair growth and density are influenced by many factors, such as nutrition and hormones.

 

The rate of hair growth varies from one body region to another and with sex and age.

 

Hair Thinning and Baldness

 

After age 40 hair is not replaced as quickly as it is lost, which leads to hair thinning and some degree of balding, or alopecia, in both sexes.

 

Male pattern baldness, which is a type of true, or frank, balding, is a genetically determined, sex-influenced condition.

 

Nails

 

A nail is a scalelike modification of the epidermis that forms a clear, protective covering.

 

Nails are made up of hard keratin and have a free edge, a body, and a proximal root.

Hypodermis

 

Also called the hypoderm, subcutaneous tissue, or superficial fascia and the bottom layer of the integumentary system in vertebrates (hypoderm and subcutaneous are from Greek and Latin words, respectively, for "beneath the skin"). Types of cells that are found in the hypodermis are fibroblasts, adipose cells, and macrophages. It is derived from the mesoderm, but unlike the dermis, it is not derived from the dermatome region of the mesoderm. In arthropods, the hypodermis is an epidermal layer of cells that secretes the chitinous cuticle.

 

Functions

 

The integumentary system has multiple roles in homeostasis. All body systems work in an interconnected manner to maintain the internal conditions essential to the function of the body. The skin has an important job of protecting the body and acts as the body’s first line of defense against infection, temperature change, and other challenges to homeostasis. Functions include:

Functions of the Integumentary System

 

·        Protection

 

·        Chemical barriers include skin secretions and melanin.

 

·        Physical or mechanical barriers are provided by the continuity of the skin, and the hardness of the keratinized cells.

 

·        Biological barriers include the Langerhans’ cells of the epidermis, the macrophages of the dermis, and the DNA itself.

 

·        The skin plays an important role in body temperature regulation by using the sweat glands of the skin to cool the body, and constriction of dermal capillaries to prevent heat loss.

 

·        Cutaneous sensation is made possible by the placement of cutaneous sensory receptors, which are part of the nervous system, in the layers of the skin.

 

·        The skin provides the metabolic function of making vitamin D when it is exposed to sunlight.

 

·        The skin may act as a blood reservoir by holding up to 5% of the body’s blood supply, which may be diverted to other areas of the body should the need arise.

 

·        Limited amounts of nitrogenous wastes are excreted through the skin.

·        Formation of new cells from stratum germinativum to repair minor injuries

Diseases and injuries

 

Possible diseases and injuries to the human integumentary system include:

·        Rash

·        Blister

·        Athlete's foot

·        Infection

·        Sunburn

·        Skin cancer

·        Albinism

·        Acne

·        Herpes

·        Cold Sores

·        Mosquito bites

·        Impetigo

·        Rubella

·        STD

·        Cancer

·        Psoriasis

Developmental Aspects of the Integumentary System

 

The epidermis develops from the embryonic ectoderm, and the dermis and the hypodermis develop from the mesoderm.

 

By the end of the fourth month of development the skin is fairly well formed.

 

During infancy and childhood, the skin thickens and more subcutaneous fat is deposited.

 

During adolescence, the skin and hair become oilier as sebaceous glands are activated.

 

The skin reaches its optimal appearance when we reach our 20s and 30s; after that time the skin starts to show the effects of cumulative environmental exposures.

 

As old age approaches, the rate of epidermal cell replacement slows and the skin thins, becoming more prone to bruising and other types of injuries.

 

·                    Lymphatic system: structures involved in the transfer of lymph between tissues and the blood stream, the lymph and the nodes and vessels that transport it including the Immune system: defending against disease-causing agents with leukocytes, tonsils, adenoids, thymus, and spleen

The lymphatic system is a complex network of lymphoid organs, lymph nodes, lymph ducts, lymphatic tissues, lymph capillaries and lymph vessels that produce and transport lymph fluid from tissues to the circulatory system. The lymphatic system is a major component of the immune system.

The human lymphatic system

The lymphatic system has three interrelated functions: (1) removal of excess fluids from body tissues, (2) absorption of fatty acids and subsequent transport of fat, chyle, to the circulatory system and, (3) production of immune cells (such as lymphocytes, monocytes, and antibody producing cells called plasma cells).

Lymph originates as blood plasma that leaks from the capillaries of the circulatory system, becoming interstitial fluid, and filling the space between individual cells of tissue. Plasma is forced out of the capillaries (called filtration) and forced back in (called absorption) due to interactions of hydrostatic pressure (favoring movement out of the capillaries) and oncotic pressure (favoring movement into the capillaries). While out of the capillaries, the fluid mixes with the interstitial fluid, causing a gradual increase in the volume of fluid. Most of the fluid is returned to the capillaries. The proportion of interstitial fluid that is returned to the circulatory system by osmosis is about 90% of the former plasma, with about 10% accumulating as overfill. The excess interstitial fluid is collected by the lymphatic system by diffusion into lymph capillaries, and is processed by lymph nodes prior to being returned to the circulatory system. Once within the lymphatic system the fluid is called lymph, and has almost the same composition as the original interstitial fluid.

The lymphatic system is part of the circulatory system, comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph (from Latin lympha "water goddess") unidirectionally towards the heart. The lymphatic system was first described in the seventeenth century independently by Olaus Rudbeck and Thomas Bartholin. The lymph system is not a closed system. The circulatory system processes an average of 20 liters of blood per day through capillary filtration which removes plasma while leaving the blood cells. Roughly 17 liters of the filtered plasma actually get reabsorbed directly into the blood vessels, while the remaining 3 liters are left behind in the interstitial fluid. The primary function of the lymph system is to provide an accessory route for these excess 3 liters per day to get returned to the blood. Lymph is essentially recycled blood plasma.

 

Lymphatic organs play an important part in the immune system, having a considerable overlap with the lymphoid system. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils. Lymphoid tissues contain lymphocytes, but they also contain other types of cells for support. The system also includes all the structures dedicated to the circulation and production of lymphocytes (the primary cellular component of lymph), which includes the spleen, thymus, bone marrow, and the lymphoid tissue associated with the digestive system.

 

The blood does not directly come in contact with the parenchymal cells and tissues in the body, but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body. Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system. The lymph is then moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic passages or by extrinsic compression of the lymphatic vessels via external tissue forces (e.g. the contractions of skeletal muscles). The organization of lymph nodes and drainage follows the organization of the body into external and internal regions; therefore, the lymphatic drainage of the head, limbs, and body cavity walls follows an external route, and the lymphatic drainage of the thorax, abdomen, and pelvic cavities follows an internal route. Eventually, the lymph vessels empty into the lymphatic ducts, which drain into one of the two subclavian veins (near the junctions of the subclavian veins with the internal jugular veins).

Human lymphatic system

Latin systema lymphoideum

Functions

 

The lymphatic system has multiple interrelated functions:

·        it is responsible for the removal of interstitial fluid from tissues

·        it absorbs and transports fatty acids and fats as chyle from the digestive system

·        it transports white blood cells to and from the lymph nodes into the bones

The lymph transports antigen-presenting cells (APCs), such as dendritic cells, to the lymph nodes where an immune response is stimulated.

 

Clinical significance

 

The study of lymphatic drainage of various organs is important in diagnosis, prognosis, and treatment of cancer. The lymphatic system, because of its physical proximity to many tissues of the body, is responsible for carrying cancerous cells between the various parts of the body in a process called metastasis. The intervening lymph nodes can trap the cancer cells. If they are not successful in destroying the cancer cells the nodes may become sites of secondary tumors.

 

Organization

 

The lymphatic system can be broadly divided into the conducting system and the lymphoid tissue.

The conducting system carries the lymph and consists of tubular vessels that include the lymph capillaries, the lymph vessels, and the right and left thoracic ducts.

The lymphoid tissue is primarily involved in immune responses and consists of lymphocytes and other white blood cells enmeshed in connective tissue through which the lymph passes. Regions of the lymphoid tissue that are densely packed with lymphocytes are known as lymphoid follicles. Lymphoid tissue can either be structurally well organized as lymph nodes or may consist of loosely organized lymphoid follicles known as the mucosa-associated lymphoid tissue (MALT).

 

Lymphoid tissue

 

Lymphoid tissue associated with the lymphatic system is concerned with immune functions in defending the body against the infections and spread of tumors. It consists of connective tissue with various types of white blood cells enmeshed in it, most numerous being the lymphocytes.

 

The lymphoid tissue may be primary, secondary, or tertiary depending upon the stage of lymphocyte development and maturation it is involved in. (The tertiary lymphoid tissue typically contains far fewer lymphocytes, and assumes an immune role only when challenged with antigens that result in inflammation. It achieves this by importing the lymphocytes from blood and lymph.)

 

Primary lymphoid organs

 

The central or primary lymphoid organs generate lymphocytes from immature progenitor cells.

 

The thymus and the bone marrow constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes.

 

Secondary lymphoid organs

 

Secondary or peripheral lymphoid organs maintain mature naive lymphocytes and initiate an adaptive immune response. The peripheral lymphoid organs are the sites of lymphocyte activation by antigen. Activation leads to clonal expansion and affinity maturation. Mature lymphocytes recirculate between the blood and the peripheral lymphoid organs until they encounter their specific antigen.

 

Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes. It is exemplified by the lymph nodes, and the lymphoid follicles in tonsils, Peyer's patches, spleen, adenoids, skin, etc. that are associated with the mucosa-associated lymphoid tissue (MALT).

 

Lymph nodes

A lymph node showing afferent and efferent lymphatic vessels

 

A lymph node is an organized collection of lymphoid tissue, through which the lymph passes on its way to returning to the blood. Lymph nodes are located at intervals along the lymphatic system. Several afferent lymph vessels bring in lymph, which percolates through the substance of the lymph node, and is drained out by an efferent lymph vessel.

 

The substance of a lymph node consists of lymphoid follicles in the outer portion called the "cortex," which contains the lymphoid follicles, and an inner portion called "medulla," which is surrounded by the cortex on all sides except for a portion known as the "hilum." The hilum presents as a depression on the surface of the lymph node, which makes the otherwise spherical or ovoid lymph node bean-shaped. The efferent lymph vessel directly emerges from the lymph node here. The arteries and veins supplying the lymph node with blood enter and exit through the hilum.

 

Lymph follicles are a dense collection of lymphocytes, the number, size and configuration of which change in accordance with the functional state of the lymph node. For example, the follicles expand significantly upon encountering a foreign antigen. The selection of B cells occurs in the germinal center of the lymph nodes.

 

Lymph nodes are particularly numerous in the mediastinum in the chest, neck, pelvis, axilla (armpit), inguinal (groin) region, and in association with the blood vessels of the intestines.

 

Lymphatics

lymphatic system

 

Tubular vessels transport back lymph to the blood ultimately replacing the volume lost from the blood during the formation of the interstitial fluid. These channels are the lymphatic channels or simply called lymphatics.

 

Function of the fatty acid transport system

 

Lymph vessels called lacteals are present in the lining of the gastrointestinal tract, predominantly in the small intestine. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain via the portal vein into the liver for processing, fats (lipids) are passed on to the lymphatic system to be transported to the blood circulation via the thoracic duct. (There are exceptions, for example medium-chain triglycerides (MCTs) are fatty acid esters of glycerol that passively diffuse from the GI tract to the portal system.) The enriched lymph originating in the lymphatics of the small intestine is called chyle. The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation.

 

Diseases of the lymphatic system

 

Lymphedema is the swelling caused by the accumulation of lymph fluid, which may occur if the lymphatic system is damaged or has malformations. It usually affects the limbs, though face, neck and abdomen may also be affected.

 

Some common causes of swollen lymph nodes include infections, infectious mononucleosis, and cancer, e.g. Hodgkin's and non-Hodgkin lymphoma, and metastasis of cancerous cells via the lymphatic system.

 

Hodgkin's lymphoma, which is also known in the medical profession as Hotchkin’s (or Hodgkin’s) Lymphoma, is a type of cancer. This type of cancer usually results from the white blood cells in the body becoming diseased or damaged.

 

Lymphangiomatosis is a disease involving multiple cysts or lesions formed from lymphatic vessels.

 

In elephantiasis, infection of the lymphatic vessels cause a thickening of the skin and enlargement of underlying tissues, especially in the legs and genitals. It is most commonly caused by a parasitic disease known as lymphatic filariasis. Lymphangiosarcoma is a malignant soft tissue tumor, whereas lymphangioma is a benign tumor occurring frequently in association with Turner syndrome. Lymphangioleiomyomatosis is a benign tumor of the smooth muscles of the lymphatics that occurs in the lungs.

 

Lymphoid leukemias and lymphomas are now considered to be tumors of the same type of cell lineage. They are called "leukemia" when in the blood or marrow and "lymphoma" when in lymphatic tissue. They are grouped together under the name "lymphoid malignancy".

 

Development of lymphatic tissue

 

Lymphatic tissues begin to develop by the end of the fifth week of embryonic development. Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm.

 

The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck and head. Some of the plexuses enlarge and form lymphatic vessels in their respective regions. Each jugular lymph sac retains at least one connection with its jugular vein, the left one developing into the superior portion of the thoracic duct.

 

The next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine. It develops from the primitive vena cava and mesonephric veins. Capillary plexuses and lymphatic vessels spread from the retroperitoneal lymph sac to the abdominal viscera and diaphragm. The sac establishes connections with the cisterna chyli but loses its connections with neighboring veins.

 

The last of the lymph sacs, the paired posterior lymph sacs, develop from the iliac veins. The posterior lymph sacs produce capillary plexuses and lymphatic vessels of the abdominal wall, pelvic region, and lower limbs. The posterior lymph sacs join the cisterna chyli and lose their connections with adjacent veins.

 

With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and are converted into groups of lymph nodes.

 

The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach. The thymus arises as an outgrowth of the third pharyngeal pouch.

 

Lymphatico-venous communications

 

Present research has found clues about a lymphatico-venous communication. In mammals, lymphatico-venous communications other than those at the base of the neck are not easy to demonstrate, but described in some experiments.

 

The specialists observed that the pulmonary complications following lymphangiography (a test which utilizes X ray technology, along with the injection of a contrast agent, to view lymphatic circulation and lymph nodes for diagnostic purposes) are more often severe in patients with lymphatic obstruction. In these cases, the contrast medium is thought to reach the vascular system via lymphovenous communications which shunt the material directly into the venous stream, bypassing those lymph nodes distal to the communications, Because less contrast agent is absorbed in lymph nodes, a greater portion of the injected volume passes into the vascular system. Since pulmonary complications are related to the amount of medium reaching the lungs area, the early recognition of lymphovenous communications is a great significance to the lymphangiographer. Another "hint" in proving a lymph-vein communication is offered by a Robert F Dunn experiment. The passage of radioactively tagged tracers, injected at elevated pressure, through the lymph node-venous communications coincides with the increased pressures of injection and subsequent nodal palpation in dogs. The passage of iodinated I 125 serum albumen (ISA) indicates that direct lymph node-venous communications are present, whereas passage of nucleated erythrocytes requires a communication structure the size of a capillary or larger. Moreover, the evidence suggest that in mammals under normal conditions, most of the lymph is returned to the blood stream through the lymphatico-venous communications at the base of the neck. When the thoracic duct-venous communication is blocked, however, the resultant raised intralymphatic pressure will usually cause other normal non-functioning communications to open and thereby allow the return of lymph to the blood stream.

 

History

 

Hippocrates was one of the first people to mention the lymphatic system in 5th century BC. In his work On Joints, he briefly mentioned the lymph nodes in one sentence. Rufus of Ephesus, a Roman physician, identified the axillary, inguinal and mesenteric lymph nodes as well as the thymus during the 1st to 2nd century AD. The first mention of lymphatic vessels was in 3rd century BC by Herophilos, a Greek anatomist living in Alexandria, who incorrectly concluded that the "absorptive veins of the lymphatics," by which he meant the lacteals (lymph vessels of the intestines), drained into the hepatic portal veins, and thus into the liver.[16] Findings of Ruphus and Herophilos findings were further propagated by the Greek physician Galen, who described the lacteals and mesenteric lymph nodes which he observed in his dissection of apes and pigs in the 2nd century AD.

 

In the mid 16th century, Gabriele Falloppio (discoverer of the fallopian tubes), described what are now known as the lacteals as "coursing over the intestines full of yellow matter." In about 1563 Bartolomeo Eustachi, a professor of anatomy, described the thoracic duct in horses as vena alba thoracis. The next breakthrough came when in 1622 a physician, Gaspare Aselli, identified lymphatic vessels of the intestines in dogs and termed them venae alba et lacteae, which is now known as simply the lacteals. The lacteals were termed the fourth kind of vessels (the other three being the artery, vein and nerve, which was then believed to be a type of vessel), and disproved Galen's assertion that chyle was carried by the veins. But, he still believed that the lacteals carried the chyle to the liver (as taught by Galen). He also identified the thoracic duct but failed to notice its connection with the lacteals. This connection was established by Jean Pecquet in 1651, who found a white fluid mixing with blood in a dog's heart. He suspected that fluid to be chyle as its flow increased when abdominal pressure was applied. He traced this fluid to the thoracic duct, which he then followed to a chyle-filled sac he called the chyli receptaculum, which is now known as the cisternae chyli; further investigations led him to find that lacteals' contents enter the venous system via the thoracic duct. Thus, it was proven convincingly that the lacteals did not terminate in the liver, thus disproving Galen's second idea: that the chyle flowed to the liver. Johann Veslingius drew the earliest sketches of the lacteals in humans in 1647.

 

The idea that blood recirculates through the body rather than being produced anew by the liver and the heart was first accepted as a result of works of William Harvey—a work he published in 1628. In 1652, Olaus Rudbeck (1630–1702), a Swede, discovered certain transparent vessels in the liver that contained clear fluid (and not white), and thus named them hepatico-aqueous vessels. He also learned that they emptied into the thoracic duct, and that they had valves. He announced his findings in the court of Queen Christina of Sweden, but did not publish his findings for a year, and in the interim similar findings were published by Thomas Bartholin, who additionally published that such vessels are present everywhere in the body, and not just the liver. He is also the one to have named them "lymphatic vessels." This had resulted in a bitter dispute between one of Bartholin's pupils, Martin Bogdan, and Rudbeck, whom he accused of plagiarism.

 

Galen's ideas prevailed in medicine until the 17th century. It was believed that blood was produced by the liver from chyle contaminated with ailments by the intestine and stomach, to which various spirits were added by other organs, and that this blood was consumed by all the organs of the body. This theory required that the blood be consumed and produced many times over. Even in the 17th century, his ideas were defended by some physicians.

 

·                    Muscular system: movement with muscles.

Neuromuscular junctions are the focal point where a motor neuron attaches to a muscle. Acetylcholine, (a neurotransmitter used in skeletal muscle contraction) is released from the axon terminal of the nerve cell when an action potential reaches the miscoscopic junction, called a synapse. A group of chemical messengers cross the synapse and stimulate the formation of electrical changes, which are produced in the muscle cell when the acetylcholine binds to receptors on its surface. Calcium is released from its storage area in the cell's sarcoplasmic reticulum. An impulse from a nerve cell causes calcium release and brings about a single, short muscle contraction called a muscle twitch. If there is a problem at the neuromuscular junction, a very prolonged contraction may occur, tetanus. Also, a loss of function at the junction can produce paralysis.

·                    Nervous system: collecting, transferring and processing information with brain, spinal cord, peripheral nerves, and nerves

Image:Nervous system diagram.png

Structure

 

The nervous system derives its name from nerves, which are cylindrical bundles of fibers that emanate from the brain and central cord, and branch repeatedly to innervate every part of the body. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using a microscope. A microscopic examination shows that nerves consist primarily of the axons of neurons, along with a variety of membranes that wrap around them and segregate them into fascicles. The neurons that give rise to nerves do not lie entirely within the nerves themselves—their cell bodies reside within the brain, central cord, or peripheral ganglia.

 

All animals more advanced than sponges have nervous systems. However, even sponges, unicellular animals, and non-animals such as slime molds have cell-to-cell signaling mechanisms that are precursors to those of neurons. In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a diffuse network of isolated cells. In bilaterian animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the Cambrian period, over 500 million years ago.

Cells

 

The nervous system contains two main categories or types of cells: neurons and glial

cells.

 

 

 

Structure of a typical neuronNeuron

 

The nervous system is defined by the presence of a special type of cell—the neuron (sometimes called "neurone" or "nerve cell"). Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via synapses, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neuron possess an axon, a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts. Axons frequently travel through the body in bundles called nerves.

 

Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions. These include sensory neurons that transmute physical stimuli such as light and sound into neural signals, and motor neurons that transmute neural signals into activation of muscles or glands; however in many species the great majority of neurons receive all of their input from other neurons and send their output to other neurons.

 

Glial cells

 

Glial cells (named from the Greek for "glue") are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas. Among the most important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy pathogens and remove dead neurons; and to provide guidance cues directing the axons of neurons to their targets. A very important type of glial cell (oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system) generates layers of a fatty substance called myelin that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently.

Anatomy in vertebrates This section requires expansion. (September 2010)

 

 Diagram showing the major divisions of the vertebrate nervous system.

 

The nervous system of vertebrate animals (including humans) is divided into the central nervous system (CNS) and peripheral nervous system (PNS).

 

The central nervous system (CNS) is the largest part, and includes the brain and spinal cord. The spinal cavity contains the spinal cord, while the head contains the brain. The CNS is enclosed and protected by meninges, a three-layered system of membranes, including a tough, leathery outer layer called the dura mater. The brain is also protected by the skull, and the spinal cord by the vertebrae.

 

The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS. The large majority of the axon bundles called nerves are considered to belong to the PNS, even when the cell bodies of the neurons to which they belong reside within the brain or spinal cord. The PNS is divided into somatic and visceral parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in dorsal root ganglia of the spinal cord. The visceral part, also known as the autonomic nervous system, contains neurons that innervate the internal organs, blood vessels, and glands. The autonomic nervous system itself consists of two parts: the sympathetic nervous system and the parasympathetic nervous system. Some authors also include sensory neurons whose cell bodies lie in the periphery (for senses such as hearing) as part of the PNS; others, however, omit them.

Horizontal bisection of the head of an adult man, showing skin, skull, and brain with grey matter (brown in this image) and underlying white matter

 

The vertebrate nervous system can also be divided into areas called grey matter ("gray matter" in American spelling) and white matter. Grey matter (which is only grey in preserved tissue, and is better described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons. White matter is composed mainly of myelinated axons, and takes its color from the myelin. White matter includes all of the nerves, and much of the interior of the brain and spinal cord. Grey matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces. There is an anatomical convention that a cluster of neurons in the brain or spinal cord is called a nucleus, whereas a cluster of neurons in the periphery is called a ganglion. There are, however, a few exceptions to this rule, notably including the part of the forebrain called the basal ganglia.

 

Comparative anatomy and evolution

 

Neural precursors in sponges

 

Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, and therefore no nervous system. They do, however, have homologs of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal-receiving part of a synapse). However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction.

 

Radiata

 

Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons that pick up chemical, tactile, and visual signals, motor neurons that can activate contractions of the body wall, and intermediate neurons that detect patterns of activity in the sensory neurons and send signals to groups of motor neurons as a result. In some cases groups of intermediate neurons are clustered into discrete ganglia.

 

The development of the nervous system in radiata is relatively unstructured. Unlike bilaterians, radiata only have two primordial cell layers, endoderm and ectoderm. Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type.

 

Bilateria

Nervous system of a bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front

 

The vast majority of existing animals are bilaterians, meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from a common wormlike ancestor that appeared in the Cambrian period, 550–600 million years ago. The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain".

Area of the human body surface innervated by each spinal nerve

 

Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain.

 

Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups (superphyla) called protostomes and deuterostomes. Deuterostomes include vertebrates as well as echinoderms, hemichordates (mainly acorn worms), and Xenoturbellidans. Protostomes, the more diverse group, include arthropods, molluscs, and numerous types of worms. There is a basic difference between the two groups in the placement of the nervous system within the body: protostomes possess a nerve cord on the ventral (usually bottom) side of the body, whereas in deuterostomes the nerve cord is on the dorsal (usually top) side. In fact, numerous aspects of the body are inverted between the two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Most anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of the body, while all vertebrates have spinal cords that run along the dorsal midline.

Earthworm nervous system. Top: side view of the front of the worm. Bottom: nervous system in isolation, viewed from above

 

Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by transverse nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two ganglia at the head end function similar to a simple brain. Photoreceptors on the animal's eyespots provide sensory information on light and dark.

 

The nervous system of one very small worm, the roundworm Caenorhabditis elegans, has been mapped out down to the synaptic level. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and hermaphrodites, have different numbers of neurons and groups of neurons that perform sex-specific functions. In C. elegans, males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons.

 

Arthropods

Internal anatomy of a spider, showing the nervous system in blue

 

Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly. Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia. The head segment contains the brain, also known as the supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the mouthparts, the salivary glands and certain muscles. Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. The sensory information from these organs is processed by the brain.

 

In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling. A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, most parts of the insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called neuropil, in the interior.

 

"Identified" neurons

 

A neuron is called identified if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties. In vertebrate nervous systems very few neurons are "identified" in this sense—in humans, there are believed to be none—but in simpler nervous systems, some or all neurons may be thus unique. In the roundworm C. elegans, whose nervous system is the most thoroughly described of any animal's, every neuron in the body is uniquely identifiable, with the same location and the same connections in every individual worm. One notable consequence of this fact is that the form of the C. elegans nervous system is completely specified by the genome, with no experience-dependent plasticity.

 

The brains of many molluscs and insects also contain substantial numbers of identified neurons. In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish. Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape, then straightens, thereby propelling itself rapidly forward. Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not the only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response all by itself, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response.

 

Mauthner cells have been described as command neurons. A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior all by itself. Such neurons appear most commonly in the fast escape systems of various species—the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances.

 

Function

 

At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are multiple ways that a cell can send signals to other cells. One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second.

 

At a more integrative level, the primary function of the nervous system is to control the body. It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into the central nervous system, processing the information to determine an appropriate response, and sending output signals to muscles or glands to activate the response. The evolution of a complex nervous system has made it possible for various animal species to have advanced perception abilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans, the sophistication of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain.

 

Neurons and synapses

Major elements in synaptic transmission. An electrochemical wave called an action potential travels along the axon of a neuron. When the wave reaches a synapse, it provokes release of a small amount of neurotransmitter molecules, which bind to chemical receptor molecules located in the membrane of the target cell.

 

Most neurons send signals via their axons, although some types are capable of dendrite-to-dendrite communication. (In fact, the types of neurons called amacrine cells have no axons, and communicate only via their dendrites.) Neural signals propagate along an axon in the form of electrochemical waves called action potentials, which produce cell-to-cell signals at points where axon terminals make synaptic contact with other cells.

 

Synapses may be electrical or chemical. Electrical synapses make direct electrical connections between neurons, but chemical synapses are much more common, and much more diverse in function. At a chemical synapse, the cell that sends signals is called presynaptic, and the cell that receives signals is called postsynaptic. Both the presynaptic and postsynaptic areas are full of molecular machinery that carries out the signalling process. The presynaptic area contains large numbers of tiny spherical vessels called synaptic vesicles, packed with neurotransmitter chemicals. When the presynaptic terminal is electrically stimulated, an array of molecules embedded in the membrane are activated, and cause the contents of the vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the synaptic cleft. The neurotransmitter then binds to receptors embedded in the postsynaptic membrane, causing them to enter an activated state. Depending on the type of receptor, the resulting effect on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more complex ways. For example, release of the neurotransmitter acetylcholine at a synaptic contact between a motor neuron and a muscle cell induces rapid contraction of the muscle cell. The entire synaptic transmission process takes only a fraction of a millisecond, although the effects on the postsynaptic cell may last much longer (even indefinitely, in cases where the synaptic signal leads to the formation of a memory trace).

Structure of a typical chemical synapse

 

There are literally hundreds of different types of synapses. In fact, there are over a hundred known neurotransmitters, and many of them have multiple types of receptors. Many synapses use more than one neurotransmitter—a common arrangement is for a synapse to use one fast-acting small-molecule neurotransmitter such as glutamate or GABA, along with one or more peptide neurotransmitters that play slower-acting modulatory roles. Molecular neuroscientists generally divide receptors into two broad groups: chemically gated ion channels and second messenger systems. When a chemically gated ion channel is activated, it forms a passage that allow specific types of ion to flow across the membrane. Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory. When a second messenger system is activated, it starts a cascade of molecular interactions inside the target cell, which may ultimately produce a wide variety of complex effects, such as increasing or decreasing the sensitivity of the cell to stimuli, or even altering gene transcription.

 

According to a rule called Dale's principle, which has only a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses. This does not mean, though, that a neuron exerts the same effect on all of its targets, because the effect of a synapse depends not on the neurotransmitter, but on the receptors that it activates. Because different targets can (and frequently do) use different types of receptors, it is possible for a neuron to have excitatory effects on one set of target cells, inhibitory effects on others, and complex modulatory effects on others still. Nevertheless, it happens that the two most widely used neurotransmitters, glutamate and GABA, each have largely consistent effects. Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Similarly, GABA has several widely occurring receptor types, but all of them are inhibitory. Because of this consistency, glutamatergic cells are frequently referred to as "excitatory neurons", and GABAergic cells as "inhibitory neurons". Strictly speaking this is an abuse of terminology—it is the receptors that are excitatory and inhibitory, not the neurons—but it is commonly seen even in scholarly publications.

 

One very important subset of synapses are capable of forming memory traces by means of long-lasting activity-dependent changes in synaptic strength. The best-known form of neural memory is a process called long-term potentiation (abbreviated LTP), which operates at synapses that use the neurotransmitter glutamate acting on a special type of receptor known as the NMDA receptor. The NMDA receptor has an "associative" property: if the two cells involved in the synapse are both activated at approximately the same time, a channel opens that permits calcium to flow into the target cell. The calcium entry initiates a second messenger cascade that ultimately leads to an increase in the number of glutamate receptors in the target cell, thereby increasing the effective strength of the synapse. This change in strength can last for weeks or longer. Since the discovery of LTP in 1973, many other types of synaptic memory traces have been found, involving increases or decreases in synaptic strength that are induced by varying conditions, and last for variable periods of time. Reward learning, for example, depends on a variant form of LTP that is conditioned on an extra input coming from a reward-signalling pathway that uses dopamine as neurotransmitter. All these forms of synaptic modifiability, taken collectively, give rise to neural plasticity, that is, to a capability for the nervous system to adapt itself to variations in the environment.

 

Neural circuits and systems

 

The basic neuronal function of sending signals to other cells includes a capability for neurons to exchange signals with each other. Networks formed by interconnected groups of neurons are capable of a wide variety of functions, including feature detection, pattern generation, and timing. In fact, it is difficult to assign limits to the types of information processing that can be carried out by neural networks: Warren McCulloch and Walter Pitts showed in 1943 that even networks formed from a greatly simplified mathematical abstraction of a neuron are capable of universal computation. Given that individual neurons can generate complex temporal patterns of activity all by themselves, the range of capabilities possible for even small groups of neurons are beyond current understanding.

 

Historically, for many years the predominant view of the function of the nervous system was as a stimulus-response associator. In this conception, neural processing begins with stimuli that activate sensory neurons, producing signals that propagate through chains of connections in the spinal cord and brain, giving rise eventually to activation of motor neurons and thereby to muscle contraction, i.e., to overt responses. Descartes believed that all of the behaviors of animals, and most of the behaviors of humans, could be explained in terms of stimulus-response circuits, although he also believed that higher cognitive functions such as language were not capable of being explained mechanistically. Charles Sherrington, in his influential 1906 book The Integrative Action of the Nervous System, developed the concept of stimulus-response mechanisms in much more detail, and Behaviorism, the school of thought that dominated Psychology through the middle of the 20th century, attempted to explain every aspect of human behavior in stimulus-response terms.

 

However, experimental studies of electrophysiology, beginning in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for generating patterns of activity intrinsically, without requiring an external stimulus. Neurons were found to be capable of producing regular sequences of action potentials, or sequences of bursts, even in complete isolation. When intrinsically active neurons are connected to each other in complex circuits, the possibilities for generating intricate temporal patterns become far more extensive. A modern conception views the function of the nervous system partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns—both types of activity interact with each other to generate the full repertoire of behavior.

 

Reflexes and other stimulus-response circuits

Simplified schema of basic nervous system function: signals are picked up by sensory receptors and sent to the spinal cord and brain, where processing occurs that results in signals sent back to the spinal cord and then out to motor neurons

 

The simplest type of neural circuit is a reflex arc, which begins with a sensory input and ends with a motor output, passing through a sequence of neurons in between. For example, consider the "withdrawal reflex" causing the hand to jerk back after a hot stove is touched. The circuit begins with sensory receptors in the skin that are activated by harmful levels of heat: a special type of molecular structure embedded in the membrane causes heat to change the electrical field across the membrane. If the change in electrical potential is large enough, it evokes an action potential, which is transmitted along the axon of the receptor cell, into the spinal cord. There the axon makes excitatory synaptic contacts with other cells, some of which project (send axonal output) to the same region of the spinal cord, others projecting into the brain. One target is a set of spinal interneurons that project to motor neurons controlling the arm muscles. The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which travel down their axons to the point where they make excitatory synaptic contacts with muscle cells. The excitatory signals induce contraction of the muscle cells, which causes the joint angles in the arm to change, pulling the arm away.

 

In reality, this straightforward schema is subject to numerous complications. Although for the simplest reflexes there are short neural paths from sensory neuron to motor neuron, there are also other nearby neurons that participate in the circuit and modulate the response. Furthermore, there are projections from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex.

 

Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the brain. Consider, for example, what happens when an object in the periphery of the visual field moves, and a person looks toward it. The initial sensory response, in the retina of the eye, and the final motor response, in the oculomotor nuclei of the brain stem, are not all that different from those in a simple reflex, but the intermediate stages are completely different. Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the thalamus, cerebral cortex, basal ganglia, superior colliculus, cerebellum, and several brainstem nuclei. These areas perform signal-processing functions that include feature detection, perceptual analysis, memory recall, decision-making, and motor planning.

 

Feature detection is the ability to extract biologically relevant information from combinations of sensory signals. In the visual system, for example, sensory receptors in the retina of the eye are only individually capable of detecting "points of light" in the outside world. Second-level visual neurons receive input from groups of primary receptors, higher-level neurons receive input from groups of second-level neurons, and so on, forming a hierarchy of processing stages. At each stage, important information is extracted from the signal ensemble and unimportant information is discarded. By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties. The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in the spinal cord and in peripheral sensory organs such as the retina.

 

Intrinsic pattern generation

 

Although stimulus-response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity. Because of the variety of voltage-sensitive ion channels that can be embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high-rate bursting and quiescence. When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and even chaos. A network of neurons that uses its internal structure to generate temporally structured output, without requiring a corresponding temporally structured stimulus, is called a central pattern generator.

 

Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer. One of the most important types of temporal pattern is circadian rhythmicity—that is, rhythmicity with a period of approximately 24 hours. All animals that have been studied show circadian fluctuations in neural activity, which control circadian alternations in behavior such as the sleep-wake cycle. Experimental studies dating from the 1990s have shown that circadian rhythms are generated by a "genetic clock" consisting of a special set of genes whose expression level rises and falls over the course of the day. Animals as diverse as insects and vertebrates share a similar genetic clock system. The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time-of-day cues are available. The clock genes are expressed in many parts of the nervous system as well as many peripheral organs, but in mammals all of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a tiny part of the brain called the suprachiasmatic nucleus.

 

Development

 

In vertebrates, landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory.

 

All bilaterian animals at an early stage of development form a gastrula, which is polarized, with one end called the animal pole and the other the vegetal pole. The gastrula has the shape of a disk with three layers of cells, an inner layer called the endoderm, which gives rise to the lining of most internal organs, a middle layer called the mesoderm, which gives rise to the bones and muscles, and an outer layer called the ectoderm, which gives rise to the skin and nervous system.

Human embryo, showing neural groove

Four stages in the development of the neural tube in the human embryo

 

 

In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the neural plate. The inner portion of the neural plate (along the midline) is destined to become the central nervous system (CNS), the outer portion the peripheral nervous system (PNS). As development proceeds, a fold called the neural groove appears along the midline. This fold deepens, and then closes up at the top. At this point the future CNS appears as a cylindrical structure called the neural tube, whereas the future PNS appears as two strips of tissue called the neural crest, running lengthwise above the neural tube. The sequence of stages from neural plate to neural tube and neural crest is known as neurulation.

 

In the early 20th century, a set of famous experiments by Hans Spemann and Hilde Mangold showed that the formation of nervous tissue is "induced" by signals from a group of mesodermal cells called the organizer region. For decades, though, the nature of the induction process defeated every attempt to figure it out, until finally it was resolved by genetic approaches in the 1990s. Induction of neural tissue requires inhibition of the gene for a so-called bone morphogenetic protein, or BMP. Specifically the protein BMP4 appears to be involved. Two proteins called Noggin and Chordin, both secreted by the mesoderm, are capable of inhibiting BMP4 and thereby inducing ectoderm to turn into neural tissue. It appears that a similar molecular mechanism is involved for widely disparate types of animals, including arthropods as well as vertebrates. In some animals, however, another type of molecule called Fibroblast Growth Factor or FGF may also play an important role in induction.

 

Induction of neural tissues causes formation of neural precursor cells, called neuroblasts. In drosophila, neuroblasts divide asymmetrically, so that one product is a "ganglion mother cell" (GMC), and the other is a neuroblast. A GMC divides once, to give rise to either a pair of neurons or a pair of glial cells. In all, a neuroblast is capable of generating an indefinite number of neurons or glia.

 

As shown in a 2008 study, one factor common to all bilateral organisms (including humans) is a family of secreted signaling molecules called neurotrophins which regulate the growth and survival of neurons. Zhu et al. identified DNT1, the first neurotrophin found in flies. DNT1 shares structural similarity with all known neurotrophins and is a key factor in the fate of neurons in Drosophila. Because neurotrophins have now been identified in both vertebrate and invertebrates, this evidence suggests that neurotrophins were present in an ancestor common to bilateral organisms and may represent a common mechanism for nervous system formation.

 

Pathology

 

The central nervous system is protected by major physical and chemical barriers. Physically, the brain and spinal cord are surrounded by tough meningeal membranes, and enclosed in the bones of the skull and spinal vertebrae, which combine to form a strong physical shield. Chemically, the brain and spinal cord are isolated by the so-called blood–brain barrier, which prevents most types of chemicals from moving from the bloodstream into the interior of the CNS. These protections make the CNS less susceptible in many ways than the PNS; the flip side, however, is that damage to the CNS tends to have more serious consequences.

 

Although nerves tend to lie deep under the skin except in a few places such as the ulnar nerve near the elbow joint, they are still relatively exposed to physical damage, which can cause pain, loss of sensation, or loss of muscle control. Damage to nerves can also be caused by swelling or bruises at places where a nerve passes through a tight bony channel, as happens in carpal tunnel syndrome. If a nerve is completely transected, it will often regenerate, but for long nerves this process may take months to complete. In addition to physical damage, peripheral neuropathy may be caused by many other medical problems, including genetic conditions, metabolic conditions such as diabetes, inflammatory conditions such as Guillain–Barré syndrome, vitamin deficiency, infectious diseases such as leprosy or shingles, or poisoning by toxins such as heavy metals. Many cases have no cause that can be identified, and are referred to as idiopathic. It is also possible for nerves to lose function temporarily, resulting in numbness as stiffness—common causes include mechanical pressure, a drop in temperature, or chemical interactions with local anesthetic drugs such as lidocaine.

 

Physical damage to the spinal cord may result in loss of sensation or movement. If an injury to the spine produces nothing worse than swelling, the symptoms may be transient, but if nerve fibers in the spine are actually destroyed, the loss of function is usually permanent. Experimental studies have shown that spinal nerve fibers attempt to regrow in the same way as nerve fibers, but in the spinal cord, tissue destruction usually produces scar tissue that cannot be penetrated by the regrowing nerves.

 

·                    Reproductive system: the sex organs, such as ovaries, fallopian tubes, uterus, vagina, mammary glands, testes, vas deferens, seminal vesicles, prostate, and penis.

·                    Respiratory system: the organs used for breathing, the pharynx, larynx, trachea, bronchi, lungs, and diaphragm.

Among quadrupeds, the respiratory system generally includes tubes, such as the bronchi, used to carry air to the lungs, where gas exchange takes place. A diaphragm pulls air in and pushes it out. Respiratory systems of various types are found in a wide variety of organisms. Even trees have respiratory systems.

Image:3DScience respiratory labeled.jpg

In humans and other mammals, the respiratory system consists of the airways, the lungs, and the respiratory muscles that mediate the movement of air into and out of the body. Within the alveolar system of the lungs, molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous environment and the blood. Thus, the respiratory system facilitates oxygenation of the blood with a concomitant removal of carbon dioxide and other gaseous metabolic wastes from the circulation. The system also helps to maintain the acid-base balance of the body through the efficient removal of carbon dioxide from the blood.

·                    Skeletal system: structural support and protection with bones, cartilage, ligaments, and tendons. The skeletal system is the framework of the body. It is what gives the body its basic shape, while protecting the delicate internal tissues and organs. The joints of the skeletal system act as levers that are attached to various muscles. The elbows, knees, ankles are just a few examples of these levers. Another use of the skeleton system is as a storage area for calcium in case there is an inadequate amount available in the diet. A vital role of this system is also the production of red blood cells.

A human skeleton - (endoskeleton)

·                    The skeleton is made up of 208 bones and broken up into two different parts, the axial skeleton and the appendicular skeleton. The axial skeleton is comprised of the bones of the head and torso while the appendicular skeleton makes up the framework for the extremities. These bones can be grouped in two divisions: axial skeleton and appendicular skeleton. The 80 bones of the axial skeleton form the vertical axis of the body. They include the bones of the head, vertebral column, ribs and breastbone or sternum. The appendicular skeleton consists of 126 bones and includes the free appendages and their attachments to the axial skeleton. The free appendages are the upper and lower extremities, or limbs, and their attachments which are called girdles. The named bones of the body are listed below by category.

Axial Skeleton (80 bones):

·        Parietal (2)

·        Temporal (2)

·        Frontal (1)

·        Occipital (1)

·        Ethmoid (1)

·        Sphenoid (1)

 

Facial Bones:

·        Maxilla (2)

·        Zygomatic (2)

·        Mandible (1)

·        Nasal (2)

·        Platine (2)

·        Inferior nasal concha (2)

·        Lacrimal (2)

·        Vomer (1)

Auditory Ossicles:

·        Malleus (2)

·        Incus (2)

·        Stapes (2)

Hyoid (1)

Vetebral Column

·        Cervical vertebrae (7)

·        Thoracic vertebrae (12)

·        Lumbar vertebrae (5)

·        Sacrum (1)

·        Coccyx (1)

Thoracic Cage

·        Sternum (1)

·        Ribs (24)

Appendicular Skeleton (126 bones)

Pectoral girdles

·        Clavicle (2)

·        Scapula (2)

Upper Extremity

·        Humerus (2)

·        Radius (2)

·        Ulna (2)

·        Carpals (16)

·        Metacarpals (10)

·        Phalanges (28)

Pelvic Girdle

Coxal, innominate, or hip bones (2)

·        Lower Extremity

·        Femur (2)

·        Tibia (2)

·        Fibula (2)

·        Patella (2)

·        Tarsals (14)

·        Metatarsals (10)

·        Phalanges (28)

 

 

Your Skeletal system is all of the bones in the body and the tissues such as tendons, ligaments and cartilage that connect them.

Your teeth are also considered part of your skeletal system but they are not counted as bones. Your teeth are made of enamel and dentin. Enamel is the strongest substance in your body.

Support

 The main job of the skeleton is to provide support for our body. Without your skeleton your body would collapse into a heap. Your skeleton is strong but light. Without bones you'd be just a puddle of skin and guts on the floor.

 

 

Protection

 Your skeleton also helps protect your internal organs and fragile body tissues. The brain, eyes, heart, lungs and spinal cord are all protected by your skeleton. Your cranium (skull) protects your brain and eyes, the ribs protect your heart and lungs and your vertebrae (spine, backbones) protect your spinal cord.

 

Movement

 Bones provide the structure for muscles to attach so that our bodies are able to move. Tendons are tough inelastic bands that hold attach muscle to bone.

Babies have more than adults! At birth, you have about 300 bones. As you grow older, small bones join together to make big ones. Adults end up with about 206 bones.

Old bones are dead, dry and brittle. But in the body, bones are very much alive. They have their own nerves and blood vessels, and they do various jobs, such as storing body minerals like calcium. Bones are made of a mix of hard stuff that gives them strength and tons of living cells which help them grow and repair themselves.

A typical bone has an outer layer of hard or compact bone, which is very strong, dense and tough. Inside this is a layer of spongy bone, which is like honeycomb, lighter and slightly flexible. In the middle of some bones is jelly-like bone marrow, where new cells are constantly being produced for the blood. Calcium is an important mineral that bone cells need to stay strong so keep drinking that low-fat milk!

 

·                    Urinary system: kidneys, ureters, bladder and urethra involved in fluid balance, electrolyte balance and excretion of urine.

Kidneys

Typically, every human has two kidneys. The kidneys are bean-shaped organs about the size of a bar of soap. The kidneys lie in the abdomen, posterior or retroperitoneal to the organs of digestion, around or just below the ribcage and close to the lumbar spine. The kidneys are surrounded by what is called peri-nephric fat, and situated on the superior pole of each kidney is an adrenal gland. The kidneys receive their blood supply of 1.25 L/min (25% of the cardiac output) from the renal arteries which are fed by the Abdominal aorta.

Kidneys viewed from behind with spine removed

Kidneys viewed from behind with spine removed

This is important because the kidneys' main role is to filter water soluble waste products from the blood. The other attatchment of the kidneys are at their functional endpoints the ureters, which lies more medial and runs down to the trigone of the bladder.

Functionally the kidney performs a number of tasks. In its role in the urinary system it concentrates urine, plays a crucial role in regulating electrolyes, and maintains acid-base homeostasis. The kidney excretes and re-absorbs electrolytes (e.g. sodium, potassium and calcium) under the influence of local and systemic hormones. pH balance is regulated by the excretion of bound acids and ammonium ions. In addition, they remove urea, a nitrogenous waste product from the metabolism of proteins from amino acids. The end point is a hyperosmolar solution carrying waste for storage in the bladder prior to urination.

Humans produce about 1.5 liters of urine over 24 hours, although this amount may vary according to circumstances. Because the rate of filtration at the kidney is proportional to the glomerular filtration rate, which is in turn related to the blood flow through the kidney, changes in body fluid status can effect kidney function. Hormones exogenous and endogenous to the kidney alter the amount of blood flowing through the glomerulus. Some medications interfere directly or indirectly with urine production. Diuretics achieve this by altering the amount of absorbed or excreted electrolytes or osmalites, which causes a diuresis.

Ureters

Urine is stored in the renal pelvis (or pyelum), which overlaps the ureters, which carry urine to the bladder. The ureters are about 200 to 250 mm (8 to 10 inches) long. Smooth muscular tissue in the walls of the ureters peristaltically force the urine downward. Small amounts of toxic waste is emptied into the bladder from the ureters about every 50 to 60 hours .

Bladder

The urinary bladder is a hollow muscular organ shaped like a balloon. It is located in the pelvic fossa and held in place by ligaments attached to the pelvic bones.

The bladder stores urine; it swells into a round shape when it is full and gets smaller when empty. In the absence of bladder disease, it can hold up to 500 mL (17 fl. oz.) of urine comfortably for two to five hours. The epithelial tissue associated with the bladder is called transitional epithelium. It allows the bladder to stretch to accommodate urine without rupturing the tissue.

Normally the bladder is sterile.

Sphincters (circular muscles) regulate the flow of urine from the bladder. The bladder itself has a muscular layer (detrusor muscle) that, when contracted, increases pressure on the bladder and creates urinary flow.

Urination is a conscious process, generally initiated by stretch receptors in the bladder wall which signal to the brain that the bladder is full. This is felt as an urge to urinate. When urination is initiated, the sphincter relaxes and the detrusor muscle contracts, producing urinary flow.

Urethra

The endpoint of the urinary system is the urethra. Typically the urethra in humans is colonised by commensal bacteria below the external urethral sphincter. The urethra emerges from the end of the penis in males and between the clitoris and vulva in females.

 

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Systems of the Human Body

What is a Preposition?

A preposition links nouns, pronouns and phrases to other words in a sentence. The word or phrase that the preposition introduces is called the object of the preposition.

A preposition usually indicates the temporal, spatial or logical relationship of its object to the rest of the sentence as in the following examples:

The book is on the table.

The book is beneath the table.

The book is leaning against the table.

The book is beside the table.

She held the book over the table.

She read the book during class.

In each of the preceding sentences, a preposition locates the noun "book" in space or in time.

A prepositional phrase is made up of the preposition, its object and any associated adjectives or adverbs. A prepositional phrase can function as a noun, an adjective, or an adverb. The most common prepositions are "about," "above," "across," "after," "against," "along," "among," "around," "at," "before," "behind," "below," "beneath," "beside," "between," "beyond," "but," "by," "despite," "down," "during," "except," "for," "from," "in," "inside," "into," "like," "near," "of," "off," "on," "onto," "out," "outside," "over," "past," "since," "through," "throughout," "till," "to," "toward," "under," "underneath," "until," "up," "upon," "with," "within," and "without."

Each of the highlighted words in the following sentences is a preposition:

The children climbed the mountain without fear.

In this sentence, the preposition "without" introduces the noun "fear." The prepositional phrase "without fear" functions as an adverb describing how the children climbed.

There was rejoicing throughout the land when the government was defeated.

Here, the preposition "throughout" introduces the noun phrase "the land." The prepositional phrase acts as an adverb describing the location of the rejoicing.

The spider crawled slowly along the banister.

The preposition "along" introduces the noun phrase "the banister" and the prepositional phrase "along the banister" acts as an adverb, describing where the spider crawled.

The dog is hiding under the porch because it knows it will be punished for chewing up a new pair of shoes.

Here the preposition "under" introduces the prepositional phrase "under the porch," which acts as an adverb modifying the compound verb "is hiding."

The screenwriter searched for the manuscript he was certain was somewhere in his office.

Similarly in this sentence, the preposition "in" introduces a prepositional phrase "in his office," which acts as an adverb describing the location of the missing papers.

 

Literature:

1. Адамчик М.В. Великий англо-український словник. – Київ, 2007.

2. Англійська мова за професійним спрямуванням: Медицина: навч. посіб. для студ. вищ. навч. закл. IV рівня акредитації / І. А. Прокоп, В. Я. Рахлецька, Г. Я. Павлишин ; Терноп. держ. мед. ун-т ім. І. Я. Горбачевського. –  Тернопіль: ТДМУ : Укрмедкнига, 2010. – 576 с.

3. Балла М.І., Подвезько М.Л. Англо-український словник. – Київ: Освіта, 2006. – Т. 1,2.

4. Hansen J. T. Netter’s Anatomy Coloring Book. – Saunders Elsevier, 2010. – 121 p.

5. Henderson B., Dorsey J. L. Medical Terminology for Dummies. – Willey Publishing, 2009. – P. 189-211.