Digestion in oral cavity.
Digestion in stomach, intestine and colon.
Absorption and motor function in the gastrointestinal tract.
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, digestion of the food starts by the action of mastication, a form of mechanical digestion, and the wetting contact of saliva. Saliva, a liquid 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 is 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 1-2 hours in humans, 4–6 hours in dogs, 3-4 hours in house cats)[citation needed], 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 obtaiutrients 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.
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.
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).[6] The VirB complex of Agrobacterium tumefaciens is the prototypic system.
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.
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.[10]
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.
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.
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste (gustation), as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly supplied with nerves and blood vessels. In humans a secondary function of the tongue is phonetic articulation. The tongue also serves as a natural means of cleaning one’s teeth. The ability to perceive different tastes is not localised in different parts of the tongue, as is widely believed. This error arose because of misinterpretation of some 19th-century research (see tongue map).
The word tongue derives from the Old English tunge, which comes from Proto-Germanic tungōn.It has cognates in other Germanic languages — for example tonge in West Frisian, tong in Dutch/Afrikaans, Zunge in German, tunge in Danish/Norwegian and tunga in Icelandic/Faroese/Swedish. The ue ending of the word seems to be a fourteenth-century attempt to show “proper pronunciation”, but it is “neither etymological nor phonetic”. Some used the spelling tunge and tonge as late as the sixteenth century.
It can be used as a metonym for language, as in the phrase mother tongue. Many languages have the same word for “tongue” and “language”.
Figures of speech
A common temporary failure in word retrieval from memory is referred to as the tip-of-the-tongue phenomenon. The expression tongue in cheek refers to a statement that is not to be taken entirely seriously – something said or done with subtle ironic or sarcastic humour. A tongue twister is a phrase made specifically to be very difficult to pronounce. Aside from being a medical condition, “tongue-tied” means being unable to say what you want to due to confusion or restriction. The phrase “cat got your tongue” refers to when a person is speechless. To “bite one’s tongue” is a phrase which describes holding back an opinion to avoid causing offence. A “slip of the tongue” refers to an unintentional utterance, such as a Freudian slip. Speaking in tongues is a common phrase used to describe glossolalia, which is to make smooth, language-resembling sounds that is no true spoken language itself. A deceptive person is said to have a forked tongue, and a smooth-talking person said to have a silver tongue.
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, such as enamel, dentine and cementum. Human teeth have a blood and nerve supply which enables proprioception. This is the ability of sensation when chewing, for example if you were to bite into something too hard for our teeth, such as a nectarine pip, our teeth tell our brain it cannot be chewed, stop. The shape of an animal’s teeth is related to its diet. An example of this is plant matter is difficult to digest, so herbivores have many molars for grinding.
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
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 multistage 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 digests 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.
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.
Coordination and control
Eating and swallowing are complex neuromuscular activities consisting essentially of three phases, an oral, pharyngeal and esophageal phase. Each phase is controlled by a different neurological mechanism. The oral phase, which is entirely voluntary, is mainly controlled by the medial temporal lobes and limbic system of the cerebral cortex with contributions from the motor cortex and other cortical areas. The pharyngeal swallow is started by the oral phase and subsequently is co-ordinated by the swallowing center in the medulla oblongata and pons. The reflex is initiated by touch receptors in the pharynx as a bolus of food is pushed to the back of the mouth by the tongue, or by stimulation of the palate (palatal reflex).
Oral phase
Prior to the following stages of the oral phase, the mandible depresses and the lips abduct to allow food or liquid to enter the oral cavity. Upon entering the oral cavity, the mandible elevates and the lips adduct to assist in oral containment of the food and liquid. The following stages describe the normal and necessary actions to form the bolus, which is defined as the state of the food in which it is ready to be swallowed.
1) Moistening
Food is moistened by saliva from the salivary glands (parasympathetic).
2) Mastication
Food is mechanically broken down by the action of the teeth controlled by the muscles of mastication (Vc) acting on the temporomandibular joint. This results in a bolus which is moved from one side of the oral cavity to the other by the tongue. Buccinator (VII) helps to contain the food against the occlusal surfaces of the teeth. The bolus is ready for swallowing when it is held together by (largely mucus) saliva (VII—chorda tympani and IX—lesser petrosal), sensed by the lingual nerve of the tongue (Vc). Any food that is too dry to form a bolus will not be swallowed.
3) Trough formation
A trough is then formed at the back of the tongue by the intrinsic muscles (XII). The trough obliterates against the hard palate from front to back, forcing the bolus to the back of the tongue. The intrinsic muscles of the tongue (XII) contract to make a trough (a longitudinal concave fold) at the back of the tongue. The tongue is then elevated to the roof of the mouth (by the mylohyoid (mylohyoid nerve—Vc), genioglossus, styloglossus and hyoglossus (the rest XII)) such that the tongue slopes downwards posteriorly. The contraction of the genioglossus and styloglossus (both XII) also contributes to the formation of the central trough.
4) Movement of the bolus posteriorly
At the end of the oral preparatory phase, the food bolus has been formed and is ready to be propelled posteriorly into the pharynx. In order for anterior to posterior transit of the bolus to occur, orbicularis oris contracts and adducts the lips to form a tight seal of the oral cavity. Next, the superior longitudinal muscle elevates the apex of the tongue to make contact with the hard palate and the bolus is propelled to the posterior portion of the oral cavity. Once the bolus reaches the palatoglossal arch of the oropharynx, the pharyngeal phase, which is reflex and involuntary, then begins. Receptors initiating this reflex are proprioceptive (afferent limb of reflex is IX and efferent limb is the pharyngeal plexus- IX and X). They are scattered over the base of the tongue, the palatoglossal and palatopharyngeal arches, the tonsillar fossa, uvula and posterior pharyngeal wall. Stimuli from the receptors of this phase then provoke the pharyngeal phase. In fact, it has been shown that the swallowing reflex can be initiated entirely by peripheral stimulation of the internal branch of the superior laryngeal nerve. This phase is voluntary and involves important cranial nerves: V (trigeminal), VII (facial) and XII (hypoglossal).
Pharyngeal phase
For the pharyngeal phase to work properly all other egress from the pharynx must be occluded—this includes the nasopharynx and the larynx. When the pharyngeal phase begins, other activities such as chewing, breathing, coughing and vomiting are concomitantly inhibited.
5) Closure of the nasopharynx
The soft palate is tensed by tensor palatini (Vc), and then elevated by levator palatini (pharyngeal plexus—IX, X) to close the nasopharynx. There is also the simultaneous approximation of the walls of the pharynx to the posterior free border of the soft palate, which is carried out by the palatopharyngeus (pharyngeal plexus—IX, X) and the upper part of the superior constrictor (pharyngeal plexus—IX, X).
6) The pharynx prepares to receive the bolus
The pharynx is pulled upwards and forwards by the suprahyoid and longitudinal pharyngeal muscles – stylopharyngeus (IX), salpingopharyngeus (pharyngeal plexus—IX, X) and palatopharyngeus (pharyngeal plexus—IX, X) to receive the bolus. The palatopharyngeal folds on each side of the pharynx are brought close together through the superior constrictor muscles, so that only a small bolus can pass.
7) Opening of the auditory tube
The actions of the levator palatini (pharyngeal plexus—IX, X), tensor palatini (Vc) and salpingopharyngeus (pharyngeal plexus—IX, X) in the closure of the nasopharynx and elevation of the pharynx opens the auditory tube, which equalises the pressure between the nasopharynx and the middle ear. This does not contribute to swallowing, but happens as a consequence of it.
8) Closure of the oropharynx
The oropharynx is kept closed by palatoglossus (pharyngeal plexus—IX, X), the intrinsic muscles of tongue (XII) and styloglossus (XII).
9) Laryngeal closure
It is true vocal fold closure that is the primary laryngopharyngeal protective mechanism to prevent aspiration during swallowing. The adduction of the vocal cords are effected by the contraction of the lateral cricoarytenoids and the oblique and transverse arytenoids (all recurrent laryngeal nerve of vagus). Since the true vocal folds adduct during the swallow, a finite period of apnea (swallowing apnea) must necessarily take place with each swallow. When relating swallowing to respiration, it has been demonstrated that swallowing occurs most often during expiration, even at full expiration a fine air jet is expired probably to clear the upper larynx from food remnants or liquid. The clinical significance of this finding is that patients with a baseline of compromised lung function will, over a period of time, develop respiratory distress as a meal progresses. Subsequently, false vocal fold adduction, adduction of the aryepiglottic folds and retroversion of the epiglottis take place. The aryepiglotticus (recurrent laryngeal nerve of vagus) contracts, causing the arytenoids to appose each other (closes the laryngeal aditus by bringing the aryepiglottic folds together), and draws the epiglottis down to bring its lower half into contact with arytenoids, thus closing the aditus. Retroversion of the epiglottis, while not the primary mechanism of protecting the airway from laryngeal penetration and aspiration, acts to anatomically direct the food bolus laterally towards the piriform fossa. Additionally, the larynx is pulled up with the pharynx under the tongue by stylopharyngeus (IX), salpingopharyngeus (pharyngeal plexus—IX, X), palatopharyngeus (pharyngeal plexus—IX, X) and inferior constrictor (pharyngeal plexus—IX, X).This phase is passively controlled reflexively and involves cranial nerves V, X (vagus), XI (accessory) and XII (hypoglossal). The respiratory center of the medulla is directly inhibited by the swallowing center for the very brief time that it takes to swallow. This means that it is briefly impossible to breathe during this phase of swallowing and the moment where breathing is prevented is known as deglutition apnea.
10) Hyoid elevation
The hyoid is elevated by digastric (V & VII) and stylohyoid (VII), lifting the pharynx and larynx up even further.
11) Bolus transits pharynx
The bolus moves down towards the esophagus by pharyngeal peristalsis which takes place by sequential contraction of the superior, middle and inferior pharyngeal constrictor muscles (pharyngeal plexus—IX, X). The lower part of the inferior constrictor (cricopharyngeus) is normally closed and only opens for the advancing bolus. Gravity plays only a small part in the upright position—in fact, it is possible to swallow solid food even when standing on one’s head. The velocity through the pharynx depends on a number of factors such as viscosity and volume of the bolus. In one study, bolus velocity in healthy adults was measured to be approximately 30–40 cm/s.
Esophageal phase
12) Esophageal peristalsis
Like the pharyngeal phase of swallowing, the esophageal phase of swallowing is under involuntary neuromuscular control. However, propagation of the food bolus is significantly slower than in the pharynx. The bolus enters the esophagus and is propelled downwards first by striated muscle (recurrent laryngeal, X) then by the smooth muscle (X) at a rate of 3–5 cm/s. The upper esophageal sphincter relaxes to let food pass, after which various striated constrictor muscles of the pharynx as well as peristalsis and relaxation of the lower esophageal sphincter sequentially push the bolus of food through the esophagus into the stomach.
13) Relaxation phase
Finally the larynx and pharynx move down with the hyoid mostly by elastic recoil. Then the larynx and pharynx move down from the hyoid to their relaxed positions by elastic recoil. Swallowing therefore depends on coordinated interplay between many various muscles, and although the initial part of swallowing is under voluntary control, once the deglutition process is started, it is quite hard to stop it.
Clinical significance
Swallowing becomes a great concern for the elderly since strokes and Alzheimer’s disease can interfere with the autonomic nervous system. Speech therapy is commonly used to correct this condition since the speech process uses the same neuromuscular structures as swallowing. In terminally ill patients, a failure of the reflex to swallow leads to a build-up of mucus or saliva in the throat and airways, producing a noise known as a death rattle (not to be confused with agonal respiration, which is an abnormal pattern of breathing due to cerebral ischemia or hypoxia).
Abnormalities of the pharynx and/or oral cavity may lead to oropharyngeal dysphagia. Abnormalities of the esophagus may lead to esophageal dysphagia. The failure of the lower esophagous sphincter to respond properly to swallowing is called achalasia.
Swallowing is a complex mechanism using both skeletal muscle (tongue) and smooth muscles of the pharynx and esophagus. The autonomic nervous system (ANS) coordinates this process in the pharyngeal and esophageal phases.
Pharynx
The pharynx is the part of the neck and throat situated 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.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. 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.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.
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.
Sections
The stomach is divided into four sections, each of which has different cells and functions. The sections are:
Cardia Where the contents of the esophagus empty into the stomach.
Fundus Formed by the upper curvature of the organ.
Body or Corpus The main, central region.
Pylorus The lower section of the organ that facilitates emptying the contents into the small intestine
In humans, gastrin is a peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the antrum of the stomach (the portion of the stomach adjacent the pyloric valve), duodenum, and the pancreas.
Gastrin binds to cholecystokinin B receptors to stimulate the release of histamines in enterochromaffin-like cells, and it induces the insertion of K+/H+ ATPase pumps into the apical membrane of parietal cells (which in turn increases H+ release into the stomach cavity). Its release is stimulated by peptides in the lumen of the stomach.
Gastrin is released in response to certain stimuli. These include:
stomach distension
vagal stimulation (mediated by the neurocrine bombesin, or GRP in humans)
the presence of partially digested proteins especially amino acids
hypercalcemia
Gastrin release is inhibited by:
The presence of acid (primarily the secreted HCl) in the stomach (a case of negative feedback).
Somatostatin also inhibits the release of gastrin, along with secretin, GIP (gastroinhibitory peptide), VIP (vasoactive intestinal peptide), glucagon and calcitonin.
Function
G cell is visible near bottom left, and gastrin is labeled as the two black arrows leading from it. Note: this diagram does not illustrate gastrin’s stimulatory effect on ECL cells.The presence of gastrin stimulates parietal cells of the stomach to secrete hydrochloric acid (HCl)/gastric acid. This is done both directly on the parietal cell and indirectly via binding onto CCK2/gastrin receptors on ECL cells in the stomach, which then responds by releasing histamine, which in turn acts in a paracrine manner on parietal cells stimulating them to secrete H+ ions. This is the major stimulus for acid secretion by parietal cells.
Along with the above mentioned function, gastrin has been shown to have additional functions as well:
Stimulates parietal cell maturation and fundal growth.
Causes chief cells to secrete pepsinogen, the zymogen (inactive) form of the digestive enzyme pepsin.
Increases antral muscle mobility and promotes stomach contractions.
Strengthens antral contractions against the pylorus, and relaxes the pyloric sphincter, which stimulates gastric emptying.
Plays a role in the relaxation of the ileocecal valve.
Induces pancreatic secretions and gallbladder emptying.
Impacts lower esophageal sphincter (LES) tone, causing it to contract.
Cholecystokinin (CCK or CCK-PZ; from Greek chole, “bile”; cysto, “sac”; kinin, “move”; hence, move the bile-sac (gallbladder)) is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. Cholecystokinin, previously called pancreozymin, is synthesized by I-cells in the mucosal epithelium of the small intestine and secreted in the duodenum, the first segment of the small intestine, and causes the release of digestive enzymes and bile from the pancreas and gallbladder, respectively. It also acts as a hunger suppressant. Recent evidence has suggested that it also plays a major role in inducing drug tolerance to opioids like morphine and heroin, and is partly implicated in experiences of pain hypersensitivity during opioid withdrawal.
CCK mediates a number of physiological processes, including digestion and satiety. It is released by I cells located in the mucosal epithelium of the small intestine (mostly in the duodenum and jejunum), neurons of the enteric nervous system and neurons in the brain. Release of CCK is stimulated by monitor peptide released by pancreatic acinar cells as well as CCK-releasing protein, a paracrine factor secreted by enterocytes in the gastrointestinal mucosa. In addition, release of acetylcholine by the parasympathetic nerve fibers of the vagus nerve also stimulate its secretion. The presence of fatty acids and/or certain amino acids in the chyme entering the duodenum is the greatest stimulator of CCK release.
CCK mediates digestion in the small intestine by inhibiting gastric emptying and gastric acid secretion. It stimulates the acinar cells of the pancreas to release digestive enzymes and stimulates the secretion of a juice rich in pancreatic digestive enzymes, hence the old name pancreozymin. Together these enzymes catalyze the digestion of fat, protein, and carbohydrates. Thus, as the levels of the substances that stimulated the release of CCK drop, the concentration of the hormone drops as well. The release of CCK is also inhibited by somatostatin. Trypsin, a protease released by pancreatic acinar cells hydrolyzes CCK-releasing peptide and monitor peptide effectively turning off the additional signals to secrete CCK.
CCK also causes the increased production of hepatic bile, and stimulates the contraction of the gall bladder and the relaxation of the Sphincter of Oddi (Glisson’s sphincter), resulting in the delivery of bile into the duodenal part of the small intestine. Bile salts form amphipathic micelles that emulsify fats, aiding in their digestion and absorption.
Secretin is a hormone that both controls the environment in the duodenum by regulating secretions of the stomach and pancreas, and regulates water homeostasis throughout the body. It is produced in the S cells of the duodenum, which are located in the crypts of Lieberkühn. In humans, the secretin peptide is encoded by the SCT gene. Secretin was also the first hormone to be identified.
Secretin helps in regulating the pH within the duodenum by inhibiting gastric acid secretion by the parietal cells of the stomach, and by stimulating bicarbonate production by the centroacinar cells and intercalated ducts of the pancreas.
In 2007, secretin was discovered to play a role in osmoregulation by acting on the hypothalamus, pituitary, and kidney.
Gastric inhibitory polypeptide (GIP), also known as the glucose-dependent insulinotropic peptide is a member of the secretin family of hormones
It has traditionally been called gastrointestinal inhibitory peptide or gastric inhibitory peptide and was believed to neutralize stomach acid[citatioeeded] to protect the small intestine from acid damage, reduce the rate at which food is transferred through the stomach, and inhibit the GI motility and secretion of acid. However, this is incorrect, as it was discovered that these effects are achieved only with higher-than-normal physiological level, and that these results naturally occur in the body through a similar hormone, secretin.
It is now believed that the function of GIP is to induce insulin secretion, which is stimulated primarily by hyperosmolarity of glucose in the duodenum.After this discovery, some researchers prefer the new name of glucose-dependent insulinotropic peptide, while retaining the acronym “GIP.” The amount of insulin secreted is greater when glucose is administered orally than intravenously.
GIP is also thought to have significant effects on fatty acid metabolism through stimulation of lipoprotein lipase activity in adipocytes. GIP release has been demonstrated in the ruminant animal and may play a role iutrient partitioning in milk production (lipid metabolism). GIP is secreted in response to the first maternal feed (colostrum) in goat kids—GIP being measured via umbilical vein before its closure. For ethical reasons, GIP secretion has been demonstrated in humans only at approx 10 days of age. With respect to the role of GIP in lipid metabolism, supraphysiological levels have shown a lipogenic action, however the action of collagenase in experimental protocols is known to degrade GIP/ GIP receptors. GIP is part of the diffuse endocrine system and, as a consequence, difficult to demonstrate physiological or clinical effects. In comparison to insulin its effects are very subtle.
GIP recently appeared as a major player in bone remodelling. Researchers at Universities of Angers and
Enteroglucagon is a peptide hormone derived from preproglucagon. It is a gastrointestinal hormone, secreted from mucosal cells primarily of the colon and terminal ileum. It has 37 amino acids. Enteroglucagon is released following ingestion of a mixed meal, and delays gastric emptying.
Mechanical Digestion takes place in the mouth, where the the saliva, teeth, and tongue all play an important role in this digestive process.
Salivary Glands and Saliva
Saliva is produced in and secreted from salivary glands. The basic secretory units of salivary glands are clusters of cells called an acini. These cells secrete a fluid that contains water, electrolytes, mucus and enzymes, all of which flow out of the acinus into collecting ducts.
Within the ducts, the composition of the secretion is altered. Much of the sodium is actively reabsorbed, potassium is secreted, and large quantities of bicarbonate ion are secreted. Bicarbonate secretion is of tremendous importance to ruminants because it, along with phosphate, provides a critical buffer that neutralizes the massive quantities of acid produced in the forestomachs. Small collecting ducts within salivary glands lead into larger ducts, eventually forming a single large duct that empties into the oral cavity.
Most animals have three major pairs of salivary glands that differ in the type of secretion they produce:
parotid glands – produce a serous, watery secretion.
submaxillary (mandibular) glands – produce a mixed serous and mucous secretion.
sublingual glands – secrete a saliva that is predominantly mucous in character.
The basis for different glands secreting saliva of differing composition can be seen by examining salivary glands histologically. Two basic types of acinar epithelial cells exist:
serous cells, which secrete a watery fluid, essentially devoid of mucus.
mucous cells, which produce a very mucus-rich secretion.
Acini in the parotid glands are almost exclusively of the serous type, while those in the sublingual glands are predominantly mucous cells. In the submaxillary glands, it is common to observe acini composed of both serous and mucous epithelial cells.
Secretion of saliva is under control of the autonomic nervous system, which controls both the volume and type of saliva secreted. This is actually fairly interesting: a dog fed dry dog food produces saliva that is predominantly serous, while dogs on a meat diet secrete saliva with much more mucus. Parasympathetic stimulation from the brain, as was well demonstrated by Ivan Pavlov, results in greatly enhanced secretion, as well as increased blood flow to the salivary glands.
Potent stimuli for increased salivation include the presence of food or irritating substances in the mouth, and thoughts of or the smell of food. Knowing that salivation is controlled by the brain will also help explain why many psychic stimuli also induce excessive salivation – for example, why some dogs salivate all over the house when it’s thundering.
Functions of Saliva
What then are the important functions of saliva? Actually, saliva serves many roles, some of which are important to all species, and others to only a few:
Lubrication and binding: the mucus in saliva is extremely effective in binding masticated food into a slippery bolus that (usually) slides easily through the esophagus without inflicting damage to the mucosa. Saliva also coats the oral cavity and esophagus, and food basically never directly touches the epithelial cells of those tissues.
Solubilises dry food: in order to be tasted, the molecules in food must be solubilised.
Oral hygiene: The oral cavity is almost constantly flushed with saliva, which floats away food debris and keeps the mouth relatively clean. Flow of saliva diminishes considerably during sleep, allow populations of bacteria to build up in the mouth — the result is dragon breath in the morning. Saliva also contains lysozyme, an enzyme that lyses many bacteria and prevents overgrowth of oral microbial populations.
Initiates starch digestion: in most species, the serous acinar cells secrete an alpha-amylase which can begin to digest dietary starch into maltose. Amylase does not occur in the saliva of carnivores or cattle.Provides alkaline buffering and fluid: this is of great importance in ruminants, which have non-secretory forestomachs.Evaporative cooling: clearly of importance in dogs, which have very poorly developed sweat glands – look at a dog panting after a long run and this function will be clear. Diseases of the salivary glands and ducts are not uncommon in animals and man, and excessive salivation is a symptom of almost any lesion in the oral cavity. The dripping of saliva seen in rabid animals is not actually a result of excessive salivation, but due to pharyngeal paralysis, which prevents saliva from being swallowed.
Saliva is a watery substance located in the mouths of organisms, secreted by the salivary glands. Human saliva is 99.5% water, while the other 0.5% consists of electrolytes, mucus, glycoproteins, enzymes, and antibacterial compounds such as secretory IgA and lysozyme. The enzymes found in saliva are essential in beginning the process of digestion of dietary starches and fats. These enzymes also play a role in breaking down food particles entrapped within dental crevices, protecting teeth from bacterial decay.Furthermore, saliva serves a lubricative function, wetting food and permitting the initiation of swallowing, and protecting the mucosal surfaces of the oral cavity from desiccation.
Role of saliva in vitality of human
¨ 1. Moisten of solid food;
¨ 2. Dissolving of substances;
¨ 3. Moisten of mouth;
¨ 4. Cover food;
¨ 5. To help of swallowing;
¨ 6. Primary hydrolyzing of carbohydrates;
¨ 7. Antibacterial properties;
¨ 8. Neutralized the stomach juice.
The digestive functions of saliva include moistening food and helping to create a food bolus. This lubricative function of saliva allows the food bolus to be passed easily from the mouth into the esophagus. Saliva contains the enzyme amylase, also called ptyalin, which is capable of breaking down starch into simpler sugars that can be later absorbed or further broken down in the small intestine. Salivary glands also secrete salivary lipase (a more potent form of lipase) to begin fat digestion. Salivary lipase plays a large role in fat digestion iewborn infants as their pancreatic lipase still needs some time to develop.[5] It also has a protective function, helping to prevent bacterial build-up on the teeth and washing away adhered food particles.
Disinfectants
A common belief is that saliva contained in the mouth has natural disinfectants, which leads people to believe it is beneficial to “lick their wounds”. Researchers at the University of Florida at Gainesville
The mouth of animals is the habitat of many bacteria, some pathogenic. Some diseases, such as herpes, can be transmitted through the mouth. Animal and human bites are routinely treated with systemic antibiotics because of the risk of septicemia.
Hormonal function
Saliva secretes hormone gustin, which is thought to play a role in the development of taste buds
Iodine in salivary glands and oral health
The trophic, antioxidant and apoptosis-inductor actions and the presumed anti-tumour activity of iodide might also be important for prevention of oral and salivary glands diseases.
Glue to construct bird nests
Many birds in the swift family, Apodidae, produce a viscous saliva during nesting season to glue together materials to construct a nest.[9] Two species of swifts in the genus Aerodramus build their nests using only their saliva, the base for bird’s nest soup.[10]
Stimulation
The production of saliva is stimulated both by the sympathetic nervous system and the parasympathetic.[11]
The saliva stimulated by sympathetic innervation is thicker, and saliva stimulated parasympathetically is more watery.
Sympathetic stimulation of saliva is to facilitate respiration, whereas parasympathetic stimulation is to facilitate digestion.
Parasympathetic stimulation leads to acetylcholine (ACh) release onto the salivary acinar cells. ACh binds to muscarinic receptors, specifically M3, and causes an increased intracellular calcium ion concentration (through the IP3/DAG second messenger system). Increased calcium causes vesicles within the cells to fuse with the apical cell membrane leading to secretion. ACh also causes the salivary gland to release kallikrein, an enzyme that converts kininogen to lysyl-bradykinin. Lysyl-bradykinin acts upons blood vessels and capillaries of the salivary gland to generate vasodilation and increased capillary permeability respectively. The resulting increased blood flow to the acini allows production of more saliva. In addition, Substance P can bind to Tachykinin NK-1 receptors leading to increased intracellular calcium concentrations and subsequently increased saliva secretion. Lastly, both parasympathetic and sympathetic nervous stimulation can lead to myoepitheilium contraction which causes the expulsion of secretions from the secretory acinus into the ducts and eventually to the oral cavity.
Sympathetic stimulation results in the release of norepinephrine. Norepinephrine binding to α-adrenergic receptors will cause an increase in intracellular calcium levels leading to more fluid vs. protein secretion. If norepinephrine binds β-adrenergic receptors, it will result in more protein or enzyme secretion vs. fluid secretion. Stimulation by norepinephrine initially decreases blood flow to the salivary glands due to constriction of blood vessels but this effect is overtaken by vasodilation caused by various local vasodilators.
Saliva production may also be pharmacologically stimulated by so-called sialagogues. It can also be suppressed by so-called antisialagogues.
Daily salivary output
There is much debate about the amount of saliva that is produced in a healthy person per day; estimates range from 0.75 to 1.5 liters per day while it is generally accepted that during sleep the amount drops to almost zero. In humans, the submandibular gland contributes around 70–75% of secretion, while the parotid gland secretes about 20–25% and small amounts are secreted from the other salivary glands.
Contents
Produced in salivary glands, human saliva is 99.5% water, but it contains many important substances, including electrolytes, mucus, antibacterial compounds and various enzymes.Atomar saliva
Latin saliva atomaris
Gives rise to molecular saliva
Molecular saliva
Latin saliva molecularis
Precursor atomar saliva
Gives rise to normal saliva
Normal saliva
Latin saliva normalis
Precursor molecular saliva
It is a fluid containing:
Water
Electrolytes:
2–21 mmol/L sodium (lower than blood plasma)
10–36 mmol/L potassium (higher than plasma)
1.2–2.8 mmol/L calcium (similar to plasma)
0.08–0.5 mmol/L magnesium
5–40 mmol/L chloride (lower than plasma)
25 mmol/L bicarbonate (higher than plasma)
1.4–39 mmol/L phosphate
Iodine (mmol/L usually higher than plasma, but dependent variable according to dietary iodine intake)
Mucus. Mucus in saliva mainly consists of mucopolysaccharides and glycoproteins;
Antibacterial compounds (thiocyanate, hydrogen peroxide, and secretory immunoglobulin A)
Epidermal growth factor or EGF
Various enzymes. There are three major enzymes found in saliva.
α-amylase (EC3.2.1.1). α-Amylase, or ptyalin, secreted by the acinar cells of the parotid and submandibular glands, starts the digestion of starch before the food is even swallowed. It has a pH optima of 7.4.
Lingual lipase. Lingual lipase, which is secreted by the acinar cells of the sublingual gland, has a pH optimum ~4.0 so it is not activated until entering the acidic environment of the stomach.
Kallikrein. Kallikrein is an enzyme that proteolytically cleaves high-molecular-weight kininogen to produce bradykinin, which is a vasodilator. It is secreted by the acinar cells of all three major salivary glands.
Antimicrobial enzymes that kill bacteria.
Lysozyme
Salivary lactoperoxidase
Lactoferrin
Immunoglobulin A
Proline-rich proteins (function in enamel formation, Ca2+-binding, microbe killing and lubrication)
Minor enzymes include salivary acid phosphatases A+B, N-acetylmuramoyl-L-alanine amidase, NAD(P)H dehydrogenase (quinone), superoxide dismutase, glutathione transferase, class 3 aldehyde dehydrogenase, glucose-6-phosphate isomerase, and tissue kallikrein (function unknown).
Cells: Possibly as many as 8 million human and 500 million bacterial cells per mL. The presence of bacterial products (small organic acids, amines, and thiols) causes saliva to sometimes exhibit foul odor.
Opiorphin, a newly researched pain-killing substance found in human saliva.
Haptocorrin, a protein which binds to Vitamin B12 to protect it against degradation in the stomach, before it binds to Intrinsic Factor
A building being renovated in the Carrollton section of New Orleans
Spitting
Spitting, or expectoration, is the act of forcibly ejecting saliva or other substances from the mouth. It is often considered rude and a social taboo in many parts of the world, including Western countries, where it is frequently forbidden by local laws (as it was thought to facilitate the spread of disease). These laws are generally not strictly enforced. In Singapore
Various species have special uses for saliva that go beyond predigestion. Some swifts use their gummy saliva to build nests. Aerodramus nests are prized for use in bird’s nest soup.Cobras, vipers, and certain other members of the venom clade hunt with venomous saliva injected by fangs. Some arthropods, such as spiders and caterpillars, create thread from salivary glands. The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose.
Salivary glands: #1 is Parotid gland, #2 is Submandibular gland, #3 is Sublingual gland
Latin Glandulae salivariae
Histology
The gland is internally divided into lobules. Blood vessels and nerves enter the glands at the hilum and gradually branch out into the lobules.
Acini
Secretory cells are found in a group, or acinus (plural, acini). Each acinus is located at the terminal part of the gland connected to the ductal system, with many acini within each lobule of the gland. Each acinus consists of a single layer of cuboidal epithelial cells surrounding a lumen, a central opening where the saliva is deposited after being produced by the secretory cells. The three forms of acini are classified in terms of the type of epithelial cell present and the secretory product being produced: serous, mucoserous, mucous.
Ducts
In the duct system, the lumina are formed by intercalated ducts, which in turn join to form striated ducts. These drain into ducts situated between the lobes of the gland (called interlobar ducts or secretory ducts). These are found on most major and minor glands (exception may be the sublingual gland).[1]
All of the human salivary glands terminate in the mouth, where the saliva proceeds to aid in digestion. The saliva that salivary glands release is quickly inactivated in the stomach by the acid that is present there but the saliva also contains enzymes that are actually activated by the acid.
The smallest taste, smell, and anticipation of food sends signals to the brain. The brain in turn sends messages to a system of salivary glands. Saliva is essentially made up of water and begins to soften up the food so it can pass more smoothly down the throat. Besides water there is also a very special substance, an enzyme called pytalin , whose main task is to breakdown the food into simpler forms.
An electrolyte is a compound that ionizes when dissolved in suitable ionizing solvents such as water. This includes most soluble salts, acids, and bases. Some gases, such as hydrogen chloride, under conditions of high temperature or low pressure can also function as electrolytes. Electrolyte solutions can also result from the dissolution of some biological (e.g., DNA, polypeptides) and synthetic polymers (e.g., polystyrene sulfonate), termed polyelectrolytes, which contain charged functional groups.
Electrolyte solutions are normally formed when a salt is placed into a solvent such as water and the individual components dissociate due to the thermodynamic interactions between solvent and solute molecules, in a process called solvation. For example, when table salt (sodium chloride), NaCl, is placed in water, the salt (a solid) dissolves into its component ions, according to the dissociation reaction
NaCl(s) → Na+(aq) + Cl−(aq)
It is also possible for substances to react with water producing ions, e.g., carbon dioxide gas dissolves in water to produce a solution which contains hydronium, carbonate, and hydrogen carbonate ions.
Note that molten salts can be electrolytes as well. For instance, when sodium chloride is molten, the liquid conducts electricity.
An electrolyte in a solution may be described as concentrated if it has a high concentration of ions, or dilute if it has a low concentration. If a high proportion of the solute dissociates to form free ions, the electrolyte is strong; if most of the solute does not dissociate, the electrolyte is weak. The properties of electrolytes may be exploited using electrolysis to extract constituent elements and compounds contained within the solution.
Physiological importance
In physiology, the primary ions of electrolytes are sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), chloride (Cl−), hydrogen phosphate (HPO42−), and hydrogen carbonate (HCO3−). The electric charge symbols of plus (+) and minus (−) indicate that the substance is ionic in nature and has an imbalanced distribution of electrons, the result of chemical dissociation. Sodium is the main electrolyte found in extracellular fluid and is involved in fluid balance and blood pressure control.
All known higher lifeforms require a subtle and complex electrolyte balance between the intracellular and extracellular environment. In particular, the maintenance of precise osmotic gradients of electrolytes is important. Such gradients affect and regulate the hydration of the body as well as blood pH, and are critical for nerve and muscle function. Various mechanisms exist in living species that keep the concentrations of different electrolytes under tight control.
Both muscle tissue and neurons are considered electric tissues of the body. Muscles and neurons are activated by electrolyte activity between the extracellular fluid or interstitial fluid, and intracellular fluid. Electrolytes may enter or leave the cell membrane through specialized protein structures embedded in the plasma membrane called ion channels. For example, muscle contraction is dependent upon the presence of calcium (Ca2+), sodium (Na+), and potassium (K+). Without sufficient levels of these key electrolytes, muscle weakness or severe muscle contractions may occur.
Electrolyte balance is maintained by oral, or in emergencies, intravenous (IV) intake of electrolyte-containing substances, and is regulated by hormones, generally with the kidneys flushing out excess levels. In humans, electrolyte homeostasis is regulated by hormones such as antidiuretic hormone, aldosterone and parathyroid hormone. Serious electrolyte disturbances, such as dehydration and overhydration, may lead to cardiac and neurological complications and, unless they are rapidly resolved, will result in a medical emergency.
Measurement
Measurement of electrolytes is a commonly performed diagnostic procedure, performed via blood testing with ion selective electrodes or urinalysis by medical technologists. The interpretation of these values is somewhat meaningless without analysis of the clinical history and is often impossible without parallel measurement of renal function. Electrolytes measured most often are sodium and potassium. Chloride levels are rarely measured except for arterial blood gas interpretation since they are inherently linked to sodium levels. One important test conducted on urine is the specific gravity test to determine the occurrence of electrolyte imbalance.
Rehydration
In oral rehydration therapy, electrolyte drinks containing sodium and potassium salts replenish the body’s water and electrolyte levels after dehydration caused by exercise, excessive alcohol consumption, diaphoresis, diarrhea, vomiting, intoxication or starvation. Athletes exercising in extreme conditions (for three or more hours continuously e.g. marathon or triathlon) who do not consume electrolytes risk dehydration (or hyponatremia).
A simple electrolyte drink can be home-made by using the correct proportions of water, sugar, salt, salt substitute for potassium, and baking soda.
Electrolytes are commonly found in fruit juices, coconut water, sports drinks, milk, and many fruits and vegetables (whole or in juice form) (e.g. potatoes, avocados).
In vertebrates, mucus (adjectival form: “mucous”) is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which contain both serous and mucous cells. It is a viscous colloid containing antiseptic enzymes (such as lysozyme), immunoglobulins, inorganic salts, proteins such as lactoferrin,[1] and glycoproteins known as mucins that are produced by goblet cells in the mucous membranes and submucosal glands. This mucus serves to protect epithelial cells (the lining of the tubes) in the respiratory, gastrointestinal, urogenital, visual, and auditory systems in mammals; the epidermis in amphibians; and the gills in fish. A major function of this mucus is to protect against infectious agents such as fungi, bacteria and viruses. The average human body produces about a litre of mucus per day.
Bony fish, hagfish, snails, slugs, and some other invertebrates also produce external mucus. In addition to serving a protective function against infectious agents, such mucus provides protection against toxins produced by predators, can facilitate movement and may play a role in communication.
Teeth
The aftermath of the action of the teeth in digestion results in two outcomes: havoc and devastation. The teeth are gears to demolish chunks of food by a series of actions such as clamping, slashing, piercing, grinding and crushing. The teeth do the first drastic destruction to food in the digestive system.
Tongue
The tongue consists of four types of taste buds–salty, sweet, sour, and bitter–and is a very maneuverable and pliable arrangement of muscle. It helps to remove, and dislocate food particles in the teeth and shifts food around in the mouth in order to assist with the all important act of swallowing.
The act of swallowing food, which at this place in the system is called a bolus, brings many organs into action. As the top of your tongue presses up against the hard palate , the roof of your mouth, food is shoved to the back of the mouth. This action in turn brings the soft palate and ursula (the place at the very back of the mouth where there is a teardrop shape located) into action. They keep the food from being misguided toward the nose. Once past the soft palate, the food is in the pharynx, a train station with two tracks, one leading to the trachea (windpipe), the other to the esophagus (food tube). The epiglottis projects out from the trachea side and helps to admit free movement of air as it is swallowed and at the same time restricts entrance to the esophagus. The larynx -hyper link, provides the epiglottis with most of its muscle for movement. It applies an upward force that helps to relax some tension on the esophagus, so that food enters where it is meant to go, down the esophagus and not down the windpipe. Many people have experienced at some time or another when the swallowing action did not go as it was supposed to.
Peristalsis
This mechanical action has to do with sets of muscles that cooperate to move both liquid and solid food along the digestive tract. In other word, it pushes food along your esophagus, stomach, and intestine.. Gravitational pull is lessened in a sense when food enters the esophagus because of peristalsis. Peristalsis helps a person to swallow lying down or even standing on their head. Peristalsis has another essential task besides assisting in the movement of food through the body. It also helps to knead, agitate, and pound the solid residue that is left after the teeth or those without teeth, the gums, have done their best. Digestive Sphincters
Many people spend a third of their time consciously trying to control how to get food into their digestive tracts and another third thinking about how that food is doing when it gets into their digestive tracts and another third of their time consciously trying to control how to get their food intake out of their digestive tracts. However, once food is swallowed, the conscious ability to control the passage of food is almost completely lost. When the food reaches the point of elimination some conscious control is again reestablished in the digestive system. The gastrointestinal track or as people call it, the digestive system, has the main purpose of break down food, both solid and fluid into sustenance for the various tissues and systems in the body. A normal digestive tract squeezes the utmost benefit from what it eats. Feces are the products left over when the body has selected everything that is of use from the food that has been eaten.The digestive system distance ranges from the mouth to the bottom of the trunk, which when we look at it, seems like no more than two or three feet, but is really about
From the moment the three main types of food-carbohydrates, fats and proteins-enter the mouth, they are exposed to chemical and mechanical actions that begin to break them apart so that they can be absorbed through the intestinal walls into the circulatory system.
Taste Receptors
A very large number of molecules elicit taste sensations through a rather small number of taste receptors. Furthermore, it appears that individual taste receptor cells bear receptors for one type of taste. In other words, within a taste bud, some taste receptor cells sense sweet, while others have receptors for bitter, sour, salty and umami tastes. Much of this understanding of taste receptors has derived from behavioral studies with mice engineered to lack one or more taste receptors.
The pleasant tastes (sweet and umami) are mediated by a family of three T1R receptors that assemble in pairs. Diverse molecules that lead to a sensation of sweet bind to a receptor formed from T1R2 and T1R3 subunits. Cats have a deletion in the gene for T1R2, explaining their non-responsiveness to sweet tastes. Also, mice engineered to express the human T1R2 protein have a human-like response to different sweet tastes. The receptor formed as a complex of T1R1 and T1R3 binds L-glutamate and L-amino acids, resulting the umami taste.
The bitter taste results from binding of diverse molecules to a family of about 30 T2R receptors. Sour tasting itself involves activation of a type of TRP (transient receptor potential) channel. Surprisingly, the molecular mechanisms of salt taste reception are poorly characterized relative to the other tastes.
Taste is a chemical sense which is detected by special structures called taste buds, of which we all have about 10,000, mainly on the tongue with a few at the back of the throat and on the palate. Taste buds surround pores within the protuberances on the tongue’s surface and elsewhere. There are four types of taste buds: these are sensitive to sweet, salty, sour and bitter chemicals. All tastes are formed from a mixture of these basic elements.
Many different tastes can be distinguished because of the combination of taste and the more discriminating sense of smell. The sense of smell is estimated to be about 10,000 times more sensitive than the sense of taste. The two senses are very closely related. It is usually correct to say that one smells more flavours than one tastes. When the nose fails, from a bad cold for instance, 80% of the taste ability is lost. Loss of taste without loss of smell is pretty rare, but “dry mouth” can contribute because taste buds can only detect flavour when food is dissolved in saliva. Taste can also be lost as a result of damage to the taste buds themselves or damage to the cranial nerves that carry taste sensations to the brain. Full sensory appreciation of food also involves its appearance, its consistency, and its temperature.
The picture of the tongue shows the areas where different types of taste are detected.
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Green represents the area where sweet taste is interpreted.
Blue interprets salty tastes.
Red detects sour tastes and yellow picks out the bitter tastes.
Observation of the Tongue
Observation of the tongue, also known as tongue diagnosis, is an important procedure in diagnosis by inspection. It provides primary information for the Chinese physicians to make diagnosis.
Physiology of the tongue. The tongue directly or indirectly connects with many zang – fu organs through the meridians and collaterals. The deep branch of Heart Meridian of Hand – Shaoyin goes to the root of the tongue ; the Spleen Meridian of Foot – Taiyin traverses the root of the tongue and spreads over its lower surface ; the Kidney Meridian of Foot – Shaoyin terminates at the root of the tongue. So the essential qi of the zang – fu organs can go upward to nourish the tongue, and pathological changes of the zang – fu organs can be reflected by changes in tongue conditions. This is why the observation of the tongue can determine the pathological changes of the internal organs.
1. Heart and lung
2. Spleen and Stomach
3. Kidney
4. Liver and gallbladder
Observation of the tongue includes the tongue proper and its coating. The tongue proper refers to the muscular tissue of the tongue, which is also known as the tongue body. The tongue coating refers to a layer of ” moss ” over the tongue surface, which is produced by the stomach qi. A normal tongue is of proper size, soft in quality, free in motion, slightly red in color and with a thin layer of white coating which is neither dry nor over moist.
The tongue is divided into four areas, namely, tip, central part, root and border. The tip of the tongue often reveals the pathological changes of the heart and lung ; its border reveals those of the liver and gallbladder ; its central part reveals those of the spleen and stomach ; and its root reveals those of the kidney. This method of diagnosing the pathological changes of the zang – fu organs by dividing the tongue into corresponding areas is clinically significant.
Stomach
Experimental method of studying of stomach secretion (Method of Basov – during the operation on dogs put the fistula in stomach. It connect stomach with the external environment. During eating the stomach juice go out through this fistula, but it has food and saliva. Method of Pavlov – method of “imaging eating” – during the operation on dogs put 2 fistulas: in esophagus and stomach. During eating the food go out through the esophagus fistula, that is way we have only juice. Method of Geydengine – a little stomach – to apart a little part of stomach, in which cut n.vagus. In this case we may to study humoral stimulation. Method of Pavlov – to separate little stomach from whole organ by 2 layers of mucous. In this case presents all regulatory mechanisms.)
c) Clinical method of stomach investigation (Gastroscopy, stomach sound, ultrasonic investigation, electrogastrography, pH-metry, determine helycobacter pylory.)
About
To stimulate production of duodinum gormon – secretin.)
Phases of stomach secretion
a) Cephalic phase (This phase caused by nervous system. It has conditional and unconditional reflexes. Conditional reactions caused by appearance of food, it smell and other stimulus, which are connect with food. Unconditional influences have parasympathetic. Parasympathetic components of unconditional influences beginning from receptors of tongue and other receptors of the oral cavity. From these receptors impulses pass through the fibers of nervus trigeminus, nervus facialis, nervus glossopharyngeus, nervus vagus to the medulla oblongata. Impulses return to stomach by nervus vagus. Except neuron influences this phase has humoral influences – brunch of nervus vagus produce gormon gastrin. These phase is very shortly.)
b) Stomach phase (These phase depend from quantity of food, which are present in stomach. It has vago-vagal reflexes (by mean of central nerves system) and local – peripheral reflexes, which are closed in stomach walls. Duration of these phase is longer and quantity of juice is much. It has humoral mechanisms too (production of gastrin and histamin).
c) Intestine phase (Presence of food in the upper portion of small intestine can cause the stomach to secrete small amount of gastric juice. This probably results of gastrin are also released by the duodenal mucosa in response to distension or chemical stimuli of the same type as those that stimulate the stomach gastrin mechanism.)
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.[clarificatioeeded]
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.
a) Role of duodenum in the digestive system (There are two secretor functions of pancreas – external and internal. The external secretor function of pancreas means that exsogenic cells of pancreas and ducts cells produce pancreatic juice. It helps to hydrolyzed protein to peptides and amino acids, carbohydrates to monosaccharides, lipids to the fat acids and glycerine. It neutralizes acidic chymus, which come from stomach.)
b) External secretor function of pancreas (The external secretor function of pancreas means that exsogenic cells of pancreas and ducts cells produce pancreatic juice).
c) Composition and property of pancreas juice (Quantity of pancreatic juice per day – 1,5-
d) Regulation of pancreas secretion (Regulation act by complex of neuro-humoral mechanisms. There are three phases of pancreatic secretion: cephalic, stomach and intestine. The first stage caused by act of nervous influences. Nervus vagus realizes this effect by means of conditioned and unconditioned reflexes. Secretion begins after 1-2 minutes of food. This juice consists of enzymes, small quantity of water and ions. Sympathetic influences have a trophic role. During the second phase there are two kinds of influences: nervous and humoral, for example, gastrin from stomach. The third phase caused by chymus contents. The main is humoral factors. In that time secrete 2 hormons – secretin and cholecystokinin-pancreasemin. Secretin stimulates production of a big quantity of juice with a high concentration of hydro carbonates and a small quantity of enzymes in ducts cells. Cholecystokinin-pancreasemin stimulates production of a less quantity of juice with a big concentration of enzymes in acinars cells.)
e) Bile production and bile secrete (Secretion of bile occur all time and increase by influences of bile acids, cholecystokinin-pancreasemin, secretin. Bile secretion in the duodenum depends from take food. It depends of nervus vagus and humoral influences – concentration of cholecystokinin-pancreasemin, secretin, fats.)
f) Composition of bile, their role in digestive processes (Composition: bilirubin, bile acids, cholesterol, leukocytes, some epitheliocytes, cristalls of bilirubin, calcium, cholesterol. The role of bile: 1. Neutrolyze the stomach acid; 2. Inhibit he act of stomach proteases; 3. Increase the activity of pancreatic lipase; 4. Emulgate the lipids; 5. Increase the absorption of fat acids, vitamins K, D, E; 6. Increase tone and motor function of intestines; 7. Decrease the activity of intestine microflora.)
g) Composition and properties of intestine juice (Composition of intestine juice: mucus, enzymes – peptidase, saccharase, maltase, lactase, lipase, immunoglobulins, leukocytes; epitheliocytes (
h) Cavity and membrane hydrolyses of substances (On the glicocalix of micro fibers present enzymes, which are adsorbed and digest small molecules of nutritive substances – membrane hydrolyses of substances. Cavity hydrolyses of substances provide by enzymes, which are in intestine space.)
Digestion in the 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
a) Composition of intestine juice and their properties (Composition of intestine juice: mucus, epithelial cells. Functions: protective from mechanical, chemical irritations; formed of base reaction of intestine contents.)
b) Role of the micro flora of big intestine (1. Ending decompose of all nutritive substances, which are do not digestive; synthesis of some vitamins – of B group, vitamin K; take place in metabolic processes.)
Small intestine
The small intestine is the longest organ of the digestive tract. It is divided up indiscriminately into three sections: the duodenum, the jejunum, and the ilium.
Duodenum
This is the place where the ultimate destruction of food digestion reaches its completion and where the acidity of chyme is nullified. The nutrients in the food eaten many hours ago have almost been diminished to molecules small enough to be absorbed through the intestinal walls into the bloodstream. Carbohydrates are diminished into simpler sugars; proteins to amino acids; and fats to fatty acids and glycerol. Enzymes are secreted by the walls of the duodenum and unite with the bile (essential for the digestion and absorption of tenacious fatty materials) and pancreatic enzymes in the duodenum.
Jejunum
Peristalsis pushes the nutrient liquid out of the duodenum into the first reaches of the jejunum. A greater number of villi , microscopic, hair like structures, begin to absorb amino acids , sugars, fatty acids and glycerol from the digested contents of the small intestine, and starts them on their way to other parts of the body. This part of the small intestine executes a digestive operation so that what is passed on to the large intestine is a thin watery substance almost completely devoid of nutrients.
Ilium
This is the place which is about a third of the small intestine. The greatest number of the estimated five or six million villi in the small intestine are found along the ilium making it the main absorption locale of the gastrointestinal tract. The villi here are always in a fretful movement: oscillating, pulsating, lengthening, shortening, growing narrower then wider, extorting every particle of nutrient.
The Liver, Gallbladder, and Pancreas
Legitimately, these three organs lie outside of the gastrointestinal tract. Nevertheless, digestive fluids from all three meet like intersections of a railway track at the common bile duct, and their movement from there into the duodenum is controlled by a sphincter muscle.
The pancreas is a producer of digestive enzymes. The gallbladder is a small reservoir for bile. The liver reproduces nutrients so that they can be used for cell-rebuilding and energy.
Large Intestine
There is a merger between the illium and the cecum, the first section of the large intestine. Any solid substances that flow into the large intestine through the ileocecal valve (which prevents back flow into the small intestine) are as a rule indigestible, or are bile constituents. What the cecum primarily inherits is water.
What the large intestine essentially does, other than act as a passageway for removal of body wastes, is to act as a provisional reservoir for water. There are no villi in the large intestine and peristalsis is much less forceful than in the small intestine. As water is absorbed, the contents of the large intestine change from a watery liquid and are compressed into semisolid feces. Nerve endings in the large intestine signal the brain that it is time for a bowel movement. The fecal material moves through the colon down to several remaining inches known as the rectum and out through the anus an opening controlled by the outlet valves of the large intestine.
Enzymes
|
Site of Enzyme Origin |
Enzyme |
Nutrient It Breacks Down |
Product Of Enzyme Action |
Place of Enzyme Action |
|
Salivary Glands |
Salivary Almalase |
Carbohydrates-sugars |
Simple Sugars |
Mouth |
|
Gastric glands |
Pepsin |
Proteins |
Amino Acids |
Stomach |
|
Liver |
Bile |
Fats/Lipids |
Emulsifide Fats |
Small Intestine |
|
Small Intestine |
Maltase, Lactase, Sucrase |
Carbohydrates |
Simple sugars |
Small Intestine |
|
Pancrease |
Trypsin, Lipase, Amylase |
Proteins, Fats/Lipids, Carbohydrates |
Amino acids, Glycerol/Fatty Acids, Simple Sugars |
Small Intestine |
The gastrointestinal tract is supplied with a number of muscular valves. These control and direct the quantity of food that goes through the digestive tract and inhibits the back movement of partially digested food.
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.
1. Common characteristic of absorption process
a)Determine of notion “absorption”
Absorption is a complex of processes, which are provide transport of substances from digestive tract into internal surroundings of organism (blood, lymph, intercellular substances).
Main types of transport of nutritive substances in internal surroundings of organism are: passive and active.
b)
1.Passive transport include diffusion and osmosis. This transport do not need presents of energy. In this case substances transport through the mucus shell by help of concentrative gradient. This way of transport have water, water disolved vitamins (C, B6, B2).
2.Active transport include pinocytosis and active transport by help of protein and energy. Active transport need energy of ATP. This way characteristic of amino acids, monosaccharids, vitamin B12, ions of calcium, enzymes. Pinocytosis – by help of pynocytic bulb, where secreted enzymes for proteins hydrolysis. Products of hydrolysis adsorbed by cell.
1. ANATOMICAL BASIS OF ABSORPTION
The total quantity of fluid that must be absorbed each day is equal to the ingested fluid (about
The stomach is a poor absorptive area of the gastrointestinal tract because it lacks the typical villus type of absorptive membrane and also because the junctions between the epithelial cells are tight junctions. Only a few highly lipid-soluble substances, such as alcohol and some drugs like aspirin, can be absorbed in small quantities.
The Absorptive Surface of the Intestinal Mucosa – The Villi.
The absorptive surface of the intestinal mucosa, showing many folds called valvulae conniventes (or folds of Kerckring), which increase the surface area of the absorptive mucosa about threefold. These folds extend circularly most of the way around the intestine and are especially well developed in the duodenum and jejunum, where they often protrude as much as
Located over the entire surface of the small intestine, from approximately the point at which the common bile duct empties into the duodenum down to the ileocecal valve, are literally millions of small villi, which project about
The intestinal epithelial cells are characterized by a brush border, consisting of about 600 microvilli 1 μm in length and 0,1 μm in diameter protruding from each cell. This increases the surface area exposed to the intestinal materials another 20-fold. Thus, the combination of the folds of Kerckring, the villi, and the microvilli increases the absorptive area of the mucosa about 600-fold, making a tremendous total area of about
The general organization of a villus, emphasizing especially the advantageous arrangement of the vascular system for absorption of fluid and dissolved material into the portal blood, and the arrangement of the central lacteal for absorption into the lymph. Many small pinocytic vesicles, which are pinched-off portions of infolded epithelium surrounding extracellular materials that have been entrapped inside the cells. Small amounts of substances are absorbed by this physical process of pinocytosis, though, as noted later in the chapter, most absorption occurs by means of single molecular transfer. Located near the brush border of the epithelial cell are many mitochondria, which supply the cell with oxidative energy needed for active transport of materials through the intestinal epithelium. Also, extending linearly into each microvillus of the brush border are multiple actin filaments that are believed to contract and cause continual movement of the microvilli, keeping them constantly exposed to new quantities of intestinal fluid.
BASIC MECHANISMS OF ABSORPTION
Absorption through the gastrointestinal mucosa occurs by active transport and by diffusion, as is also true for other membranes.
Briefly, active transport imparts energy to the substance as it is being transported for the purpose of concentrating it on the other side of the membrane or for moving it against an electrical potential On the other hand, the term diffusion means simply transport of substances through the membrane as a result of molecular movement along, rather than against, an electrochemical gradient.
c)Absorption in the mouth cavity and stomach
In the mouth cavity absorbed water, water soluble medicines (for example, validol, nitroglycerin, adelphan, furosemid, corinfar and others). In our oral cavity, under the tongue present a big quantity of vessels. That is why all water soluble substances absorbed in this place. They go to the bloodstream, and have immediately action on our receptors. They do not go through the liver, and do not desintoxicated, that is why may be toxic effect of some substances, for example products of food, drugs.
In esophagus do not absorbed nutritive substances as a rule.
In stomach absorbed alcohol, water and small quantity of other substances.
ABSORPTION IN THE SMALL INTESTINE
d) Absorption in intestines
Virtually all nutrients from the diet are absorbed into blood across the mucosa of the small intestine. In addition, the intestine absorbs water and electrolytes, thus playing a critical role in maintenance of body water and acid-base balance.
It’s probably fair to say that the single most important process that takes place in the small gut to make such absorption possible is establishment of an electrochemical gradient of sodium across the epithelial cell boundary of the lumen. This is a critical concept and actually quite interesting. Also, as we will see, understanding this process has undeniably resulted in the saving of millions of lives.
To remain viable, all cells are required to maintain a low intracellular concentration of sodium. In polarized epithelial cells like enterocytes, low intracellular sodium is maintained by a large number of Na+/K+ ATPases – so-called sodium pumps – embedded in the basolateral membrane. These pumps export 3 sodium ions from the cell in exchange for 2 potassium ions, thus establishing a gradient of both charge and sodium concentration across the basolateral membrane.
In rats, as a model of all mammals, there are about 150,000 sodium pumps per small intestinal enterocyte which collectively allow each cell to transport about 4.5 billion sodium ions out of each cell per minute (J Membr Biol 53:119-128, 1980). Pretty impressive! This flow and accumulation of sodium is ultimately responsible for absorption of water, amino acids and carbohydrates.
Aside from the electrochemical gradient of sodium just discussed, several other concepts are required to understand absorption in the small intestine. Also, dietary sources of protein, carbohydrate and fat must all undergo the final stages of chemical digestion just prior to absorption of, for example, amino acids, glucose and fatty acids.
At this point, its easiest to talk separately about absorption of each of the major food groups, recognizing that all of these processes take place simultaneously.
Water and electrolytes
Carbohydrates, after digestion to monosaccharides
Proteins, after digestion to small peptides and amino acids
Neutral fat, after digestion to monoglyceride and free fatty acids
Absorption in the Small Intestine:
Normally, absorption from the small intestine each day consists of several hundred grams of carbohydrates, 100 or more grams of fat, 50 to
ABSORPTION IN THE LARGE INTESTINE
Approximately 1500 ml of chyme pass through the ileocecal valve into the large intestine each day. Most of the water and electrolytes in this are absorbed in the colon, usually leaving less than 100 ml of fluid to be excreted in the feces. Also, essentially all the ions are also absorbed, leaving only about 1 mEq each of sodium and chloride ions to be lost in the feces.
Most of the absorption in the large intestine occurs in the proximal half of the colon, giving this portion the name absorbing colon, whereas the distal colon functions principally for storage and is therefore called the storage colon.
Absorption and Secretion of Electrolytes and Water.
The mucosa of the large intestine, like that of the small intestine, has a high capability for active absorption of sodium, and the electrical potential created by the absorption of the sodium causes chloride absorption as well. The „tight junctions“ between the epithelial cells of the large intestinal epithelium are much tighter than those of the small intestine. This prevents significant amounts of back-diffusion of ions through these junctions, thus allowing the large intestinal mucosa to absorb sodium ions far more completely – that is, against a much higher concentration gradient – than can occur in the small intestine.
In addition, as in the distal portion of the small intestine, the mucosa of the large intestine actively secretes bicarbonate ions while it simultaneously actively absorbs an equal amount of chloride ions in an exchange transport process. The bicarbonate helps neutralize the acidic end-products of bacterial action in the colon.
The absorption of sodium and chloride ions creates an osmotic gradient across the large intestinal mucosa, which in turn causes absorption of water.
Bacterial Action in the
Numerous bacteria, especially colon bacilli, are present in the absorbing colon. These are capable of digesting small amounts of cellulose, in this way providing a few calories of nutrition to the body each day. In herbivorous animals this source of energy is very significant, though it is of negligible importance in the human being. Other substances formed as a result of bacterial activity are vitamin K, vitamin B12, thiamin, riboflavin, and various gases that contribute to flatus in the colon – especially carbon dioxide, hydrogen gas, and methane. Vitamin K is especially important, for the amount of this vitamin in the ingested foods is normally insufficient to maintain adequate blood coagulation.
Composition of the Feces. The feces normally are about three-fourths water and one-fourth solid matter composed of about 30 per cent dead bacteria, 10 to 20 per cent fat, 10 to 20 per cent inorganic matter, 2 to 3 per cent protein, and 30 per cent undigested roughage of the food and dried constituents of digestive juices, such as bile pigment and sloughed epithelial cells. The large amount of fat derives mainly from fat formed by bacteria and fat in the sloughed epithelial cells.
The brown color of feces is caused by stercobilin and urobilin, which are derivatives of bilirubin. The odor is caused principally by the products of bacterial action; these vary from one person to another, depending on each person’s colonic bacterial flora and on the type of food eaten. The actual odoriferous products include indole, skatole, mercaptans, and hydrogen sulfide.
ABSORPTION OF WATER
Isosmotic Absorption. Water is transported through the intestinal membrane entirely by the process of diffusion. Furthermore, this diffusion obeys the usual laws of osmosis. Therefore, when the chyme is dilute, water is absorbed through the intestinal mucosa into the blood of the villi by osmosis.
On the other hand, water can also be transported in the opposite direction, from the plasma into the chyme. This occurs especially when hyperosmotic solutions are discharged from the stomach into the duodenum Usually within minutes, sufficient water is transferred by osmosis to make the chyme isosmotic with the plasma Thereafter, the chyme remains almost exactly isosmotic throughout its total passage through the small and large intestines.
As dissolved substances are absorbed from the lumen of the gut into the blood the absorption tends to decrease the osmotic pressure of the chyme, but water diffuses so readily through the intestinal membrane (because of large 7 to
e) Methods of absorptions’ investigation
1. Angiostoma (experimental method). Surgeon put stoma, aperture, on one of the gastrointestinal vessels in which absorbed nutritive substances. He add it by help of catheter of body surface. In this case he investigate absorption processes in anybody part of intestines.
In the case of angiostoma he may investigate each stage of digestion in different organs – oral cavity, esophagus, stomach, small and large intestines. He may determining the speed of absorption; quantity of different substances, which are absorbed in different part of digestive tract; components of food, which can absorbed in different part of gastro-intestinal tract; speed of bloodstream in the different part of gastro-intestinal tract, which help to absorbed some substances; mechanism of absorption in different part of gastrointestinal tract.
2. X-ray investigation (experimental or clinical method). In this case by help of different substances, for example, suspension of barium for determining motor function of gastrointestinal tract and other water-soluble substances to determining absorption. Doctor do X-ray investigation and determining place of absorption, place of increase or decrease speed of absorption, part of digestive tract, where present decrease of absorption. This method may be act on animal too, for example, if we need to determining absorption of new substances.
3. Biochemical method of investigation (experimental or clinical method). In this case laboratory assistant investigate blood, urine, saliva to content of different substances – glucose, amino acids, fat acids, lactose, mannose, sugar and others. For example, to determining pathology of carbohydrates absorption in intestines doctor laboratory assistant investigate quantity of glucose, or galactose, or lactose, or mannose in blood and urine and if he know the quantity of glucose which are coming into organism, he may value absorption of glucose, or galactose, or lactose, or mannose in digestive tract. For example, to determining pathology of sodium or potassium absorption in digestive tract doctor laboratory assistant investigate concentration of sodium or potassium in saliva, blood, urine and after that doctor may value their absorption.
4. Radioisotopic investigation (clinical method). Nurses inject intravenously radioisotop, which absorbed in digestive tract, into the organism of patient. After some time, which is necessary for investigation, doctor scan the places, where this isotop must absorbed. Then he determining the absorptive function of intestines, as he see the speed of absorption, quantity of radioisotop, which are absorbed and place of absorption of radioisotop.
Water and mineral salts
Active Transport of Sodium. Twenty to
The principles of sodium absorption from the intestine are also essentially the same as those for absorption of sodium from the renal tubules. The motive power for the sodium absorption is provided by active transport of sodium from inside the epithelial cells through the side walls of these cells into the intercellular spaces. This active transport obeys the usual laws of active transport it requires energy, and it is catalyzed by appropriate ATPase carrier enzymes in the cell membrane. Part of the sodium is transported along with chloride ions that are passively „dragged“ along by the positive electrical charges of the sodium ion. However, other sodium ions are absorbed while either potassium or hydrogen ions are transported into the lumen of the gut in exchange for the sodium ions. In the membrane of the brush border are special transport proteins that facilitate these exchanges between sodium and potassium or sodium and hydrogen.
The active transport of sodium reduces its concentration in the cell to a low value (about 50 mEq/liter). Since the sodium concentration in the chyme is normally about 142 mEq/liter (that is, approximately equal to that in the plasma), sodium moves by passive absorption from the chyme through the brush border of the epithelial cell into the epithelial cell cytoplasm. This replaces the sodium that is actively transported out of the epithelial cells into the intercellular spaces.
The next step in the transport process is osmosis of water into the intercellular spaces. This movement is caused by the osmotic gradient created by the elevated concentration of ions in the intercellular space. Most of this osmosis occurs through the „tight junctions“ between the apical borders of the epithelial cells, as discussed earlier, but a smaller proportion occurs through the cells themselves. The osmotic movement of water creates a flow of fluid into the intercellular space, then through the basement membrane of the epithelium, and finally into the circulating blood of the villi.
Absorption of Chloride Ions in the Duodenum and Jejunum. In the upper part of the small intestine chloride absorption is mainly by passive diffusion. The absorption of sodium ions through the epithelium creates electronegativity in the chyme and electropositivity on the basal side of the epithelial cells. Then chloride ions move along this electrical gradient to „follow“ the sodium ions.
„Active“ Absorption of Bicarbonate Ions in the Duodenum and Jejunum. Often, large quantities of bicarbonate ions must be reabsorbed from the upper small intestine because of the large amounts of bicarbonate ions in both the pancreatic secretion and bile. However, the bicarbonate ion is absorbed in an indirect way as follows: When sodium ions are absorbed, moderate amounts of hydrogen ions are secreted into the lumen of the gut in exchange for some of the sodium, as explained earlier. These hydrogen ions in turn combine with the bicarbonate ion to form carbonic acid (H2CO3), and this then dissociates to form H2O and CO3. The water remains part of the chyme in the intestines, but the carbon dioxide is readily absorbed into the blood and subsequently expired through the lungs. Thus, this is the so-called „active“ absorption of bicarbonate ions. It is the same mechanism that occurs in the tubules of the kidneys.
Active Absorption of Chloride Ions and Active Secretion of Bicarbonate Ions in the Ileum and Large Intestine. The epithelial cells of the ileum and of the large intestine have the special capability of actively absorbing chloride ions by means of a tightly coupled transport mechanism in which an equivalent number of bicarbonate ions are secreted. The functional role of this mechanism is to provide bicarbonate ions for neutralization of acidic products formed by bacteria – especially in the large intestine.
Various bacterial toxins, particularly those of cholera, colon bacilli, and staphylococci, can strongly stimulate this chloride-bicarbonate exchange mechanism.
Absorption of Other Ions. Calcium ions are actively absorbed, especially from the duodenum, and calcium ion absorption is exactly controlled in relation to the need of the body for calcium. One important factor controlling calcium absorption is parathyroid hormone secreted by the parathyroid glands, and another is vitamin D. The parathyroid hormone activates vitamin D in the kidneys, and the activated vitamin D in turn greatly enhances calcium absorption.
Iron ions are also actively absorbed from the small intestine. The principles of iron absorption and the regulation of its absorption in proportion to the body’s need for iron.
Potassium, magnesium, phosphate, and probably still other ions can also be actively absorbed through the mucosa. In general, the monovalent ions are absorbed with ease and in great quantities. On the other hand, the bivalent ions are normally absorbed in only small amounts; for instance, the maximum absorption of calcium ions is only 1/50 as great as the normal absorption of sodium ions. Fortunately, only small quantities of the divalent ions are normally needed by the body.
b) Products of proteins hydrolyses
Absorption of Proteins
Most proteins are absorbed in the form of amino acids. However, small quantities of dipeptides and even tripeptides are also absorbed, and extremely minute quantities of whole proteins can at times be absorbed by the process of pinocytosis, though not by the usual absorptive mechanisms.
The absorption of amino acids also obeys the principles listed above for active absorption of glucose; that is, the different types of amino acids are absorbed selectively and certain ones interfere with the absorption of others, illustrating that common carrier systems exist. Finally, metabolic poisons block the absorption of amino acids in the same way that they block the absorption of glucose.
Absorption of amino acids through the intestinal mucosa can occur far more rapidly than can protein digestion in the lumen of the intestine. As a result, the normal rate of absorption is determined not by the rate at which they can be absorbed but by the rate at which they can be released from the proteins during digestion. For these reasons, essentially no free ammo acids can be found in the intestine during digestion – that is, they are absorbed as rapidly as they are formed. Since most protein digestion occurs in the upper small intestine, most protein absorption occurs in the duodenum and jejunum.
Basic Mechanisms of Amino Acid Transport. As is true for monosaccharide absorption, very little is known about the basic mechanisms of amino acid transport. However, at least four different carrier systems transport different amino acids – one transports neutral amino acids, a second transports basic amino acids, a third transports acidic amino acids, and a fourth has specificity for the two imino acids proline and hydroxyproline. Also, the transport mechanisms have far greater affinity for transporting L-stereoisomers of amino acids than D-stereoisomers.
Amino acid transport (at least for most of the amino acids), like glucose transport, occurs only in the presence of simultaneous sodium transport. Furthermore, the carrier systems for amino acid transport, like those for glucose transport, are in the brush border of the epithelial cell. It is believed that amino acids are transported by the same sodium cotransport mechanism as that explained above for glucose transport. That is, the theory postulates that the carrier has receptor sites for both an amino acid molecule and a sodium ion. Only when both of the sites are filled will the carrier move both the sodium and the amino acid to the interior of the cell at the same time. Because of the sodium gradient across the brush border, the sodium diffusion to the cell interior pulls the amino acid to the interior where the amino acid becomes trapped. Therefore, amino acid concentration increases within the cell, and it then diffuses through the sides or base of the cell into the portal blood, probably by a facilitated diffusion process.
c)Products of carbohydrates hydrolyses
Essentially all the carbohydrates are absorbed in the form of monosaccharides, only a small fraction of a per cent being absorbed as disaccharides and almost none as larger carbohydrate compounds. Furthermore, little carbohydrate absorption results from simple diffusion, for the pores of the mucosa through which diffusion occurs are essentially impermeable to water-soluble solutes with molecular weights greater than 100.
That the transport of most monosaccharides through the intestinal membrane is an active process is demonstrated by several important experimental observations:
1. Transport of most of them, especially glucose and galactose, can be blocked by metabolic inhibitors, such as iodoacetic acid, cyanides, and phlorhizin.
2. The transport is selective, specifically transporting certain monosaccharides without transporting others. The order of preference for transporting different monosaccharides and their relative rates of transport in comparison with glucose are:
3. There is a maximum rate of transport for each type of monosaccharide. The most rapidly transported monosaccharide is galactose, with glucose running a close second. Fructose, which is also one of the three important monosaccharides for nutrition, is absorbed less than half as rapidly as either galactose or glucose; also, its mechanism of absorption is different, as will be explained below.
4. There is competition between certain sugars for the respective carrier system. For instance, if large amounts of galactose are being transported, the amount of glucose that can be transported simultaneously is considerably reduced.
Mechanism of Glucose and Galactose Absorption.
Glucose and galactose transport either ceases or is greatly reduced wherever active sodium transport is blocked. Therefore, it is assumed that the energy required for transport of these two monosaccharides is actually provided by the sodium transport system. A theory that attempts to explain this is the following: It is known that the carrier protein for transport of glucose (which is the carrier for galactose as well) is present in the brush border of the epithelial cell. However, this carrier will not transport the glucose in the absence of sodium transport. Therefore, it is believed that the carrier protein has receptor sites for both a glucose molecule and a sodium ion, and that it will not transport either of these to the interior of the epithelial cell until both receptor sites are simultaneously filled. The energy to cause movement of the carrier from the exterior of the membrane to the interior is derived from the difference in sodium concentration between the outside and inside. That is, as sodium diffuses to the inside of the cell it „drags“ the glucose along with it, thus providing the energy for transport of the glucose. For obvious reasons, this explanation is called the sodium cotransport theory for glucose transport; it is also called secondary active transport of glucose. This sodium cotransport of glucose obviously moves the glucose only to the interior of the cell. However, this increases the intracellular glucose concentration to a higher thaormal level, and the glucose then diffuses, probably by facilitated diffusion, through the basolateral membrane of the epithelial cell into the extracellular fluid.
Subsequently, we will see that sodium transport is also required for transport of many if not all amino acids, suggesting a similar „carrier-drag“ mechanism for ammo acid transport.
Absorption of Fructose.
Transport of fructose is slightly different from that of most other monosaccharides. It is not blocked by some of the same metabolic poisons – specifically, phlorhizin – and it does not require metabolic energy for transport, even though it does require a specific carrier. Therefore, it is transported by facilitated diffusion rather than active transport. Also, it is mainly converted into glucose inside the epithelial cell before entering the portal blood, the fructose first becoming phosphorylated, then converted to glucose, and finally released from the epithelial cell into the blood.
Products of fats hydrolyses
As fats are digested to form monoglycerides and free fatty acids, both of these digestive end-products become dissolved in the lipid portion of the bile acid micelles. Because of the molecular dimensions of these micelles, only 2,5 nanometers in diameter, and also because of their highly charged exterior, they are soluble in the chyme. In this form the monoglycerides and the fatty acids are transported to the surfaces of the brush border microvilli, even penetrating into the recesses among the moving, agitating microvilli. On coming in contact with these surfaces, both the monoglycerides and the fatty acids immediately diffuse through the epithelial membrane, because they are equally as soluble in this membrane as in the micelles. This leaves the bile acid micelles still in the chyme. The micelles then diffuse back through the chyme and absorb still more monoglycerides and fatty acids, and similarly transport these also to the epithelial cells. Thus, the bile acids perform a „ferrying“ function, which is highly important for fat absorption. In the presence of an abundance of bile acids, approximately 97 per cent of the fat is absorbed; in the absence of bile acids, only 50 to 60 per cent is normally absorbed.
The mechanism for absorption of the monoglycerides and fatty acids through the brush border is based entirely on the fact that both these substances are highly lipid-soluble. Therefore, they become dissolved in the membrane and simply diffuse to the interior of the cell.
The undigested triglycerides and the diglycerides are both also highly soluble in the lipid membrane of the epithelial cell. However, only small quantities of these are normally absorbed because the bile acid micelles will not dissolve either triglycerides or diglycerides and therefore will not ferry them to the epithelial membrane.
After entering the epithelial cell, the fatty acids and monoglycerides are taken up by the smooth endoplasmic reticulum, and here they are mainly recombined to form new triglycerides. However, a few of the monoglycerides are further digested into glycerol and fatty acids by an epithelial cell lipase. Then, the free fatty acids are reconstituted by the smooth endoplasmic reticulum into triglycerides. Most of the glycerol that is utilized for this purpose is synthesized de novo from alpha-glycerophosphate, this synthesis requiring both energy from ATP and a complex of enzymes to catalyze the reactions.
Once formed, the triglycerides aggregate within the endoplasmic reticulum into globules along with absorbed cholesterol, absorbed phospholipids, and small amounts of newly synthesized cholesterol and phospholipids. The phospholipids arrange themselves in these globules with the fatty portion of the phospholipid toward the center and the polar portions located on the surface. This provides an electrically charged surface that makes these globules miscible with the fluids of the cell. In addition, small amounts of beta-lipoprotein, also synthesized by the endoplasmic reticulum, coat part of the surface of each globule. In this form the globule diffuses to the side of the epithelial cell and is excreted by the process of cellular exocytosis into the space between the cells; from there it passes into the lymph in the central lacteal of the villus. These globules are then called chylomicrons.
The beta-lipoprotein is essential for cellular exocytosis of the chylomicrons to occur, because this protein provides a means for attaching the fatty globule to the cell membrane before it is extruded. In persons who have a genetic inability to form this (3-lipoprotein, the epithelial cells become engorged with fatty products that cannot proceed the rest of the way to be absorbed.
Transport of the Chylomicrons in the Lymph.
From the sides of the epithelial cells the chylomicrons wend their way into the central lacteals of the villi and from here are propelled, along with the lymph, by the lymphatic pump upward through the thoracic duct to be emptied into the great veins of the neck. Between 80 and 90 per cent of all fat absorbed from the gut is absorbed in this manner and is transported to the blood by way of the thoracic lymph in the form of chylomicrons.
Direct Absorption of Fatty Acids into the Portal Blood.
Small quantities of short chain fatty acids, such as those from butterfat, are absorbed directly into the portal blood rather than being converted into triglycerides and absorbed into the lymphatics. The cause of this difference between short and long chain fatty acid absorption is that the shorter chain fatty acids are more water-soluble and are not reconverted into triglycerides by the endoplasmic reticulum. This allows direct diffusion of these fatty acids from the epithelial cells into the capillary blood of the villus.
References:
1. Review of Medical Physiology // W.F.Ganong. – 24th edition, 2012.
2. Textbook of Medical Physiology // A.C.Guyton, J.E.Hall. – Eleventh edition, 2005.
3. Physiology // V.M.Moroz, O.A. Shandra. – Vinnytsia. – Nova khyha Publishers, 2011
