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.
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
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.
[edit]
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.
[edit]
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
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,
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
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-
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
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
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 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
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
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
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 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
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
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
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
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
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.
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.
·
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
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
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.
VIDEO
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.