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What is the mechanism of antiperistalsis that occurs during vomiting? Why the peristaltic waves normally don't propagate in anal to oral direction? Please give logical explanation with authentic sources. :)
What is the mechanism of antiperistalsis that occurs during vomiting?
- Emetic agents in the bloodstream stimulate chemoreceptors in area postrema and nucleus tractus solitarius .
- Nucleus tractus solitarius is connected to motor neurons in ventral medulla and hypothalamus .
- Esophageal and gastric peristalsis is inhibited. Small bowel peristalsis is reversed. Esophagus and stomach don't have antiperistaltic movements .
Why the peristaltic waves normally don't propagate in anal to oral direction?
Because of the mechanism of descending inhibition. Distension of the esophagus and intestine causes a reflex action that leads to circular muscular contraction just above the bolus/chymus. Longitudinal muscle also contracts (shortens) .
- Hornby PJ. Central neurocircuitry associated with emesis. Am. J. Med. 2001 Dec 3;111 Suppl 8A:106S-112S. PubMed PMID: 11749934.
Brizzee KR. Mechanics of vomiting: a minireview. Can. J. Physiol. Pharmacol. 1990 Feb;68(2):221-9. PubMed PMID: 2178746.
AUMSA. Digestive Physiology. May 2012. CC-BY-SA-3.0
During peristalsis ( Fig. 1 ), the longitudinally oriented muscle in the segment ahead of the advancing intraluminal contents contracts while the circumferentially oriented muscle layer relaxes in the same segment. The esophagus and intestine are tubes that behave physically like a cylinder with constant surface area. Shortening of the longitudinal axis of the cylinder is accompanied by widening of the cross-sectional diameter. Simultaneous shortening of the longitudinal axis and relaxation of the circular muscle result in expansion of the lumen. This prepares a receiving segment for the forward-moving intraluminal contents during peristalsis.
FIGURE 1 . The circumferential and longitudinal muscle layers of the intestines behave in a stereotypical pattern during peristaltic propulsion. A “hardwired” reflex circuit in the enteric nervous system determines the pattern of behavior of the two muscle layers. During peristaltic propulsion, the longitudinal muscle layer in the segment ahead of the advancing intraluminal contents contracts while the circumferential muscle layer relaxes simultaneously. Simultaneous shortening of the longitudinal intestinal axis and relaxation of the circumferential muscle in the same segment result in expansion of the lumen, which becomes a receiving segment for the forward-moving contents. The second component of the reflex is contraction of the circular muscle in the segment behind the advancing intraluminal contents. The longitudinal muscle layer in the same segment relaxes simultaneously with contraction of the circular muscle, which results in conversion of this region to a propulsive segment that propels the luminal contents ahead into the receiving segment. The reflex circuits are coupled in series along the intestine, such that receiving segments convert to propulsive segments as the next segment in line becomes a receiving segment. Propulsive segments then return to their previous state of physiologic ileus. The distance over which the peristaltic reflex circuit for the formation of propulsive and receiving segments is activated in sequence down the bowel determines the length of bowel over which propulsion occurs in one or the other of the intestinal motility patterns.
The second component of stereotypic peristaltic behavior is contraction of the circumferentially oriented muscle layer in the segment behind the advancing intraluminal contents. The longitudinally oriented muscle layer in this segment relaxes simultaneously with contraction of the circular muscle, resulting in conversion of this region to a propulsive segment that propels the luminal contents ahead, into the receiving segment. Intestinal segments ahead of the advancing front become receiving segments and then propulsive segments in succession as the complex of propulsive and receiving segments travels along the intestine.
The Changes When Food Enters The Colon
The passage of food through the ileocecal valve seems to stimulate the colon to activity. As food is nearing the ileocecal valve the large intestine is usually quiet and relaxed (Fig. 6, 4.00), though occasionally indefinite movements are to be observed and sometimes just before the food reaches the end of the ileum the circular fibres of the colon in the region of the valve contract strongly, so that a deep indentation is present there. The indentation may persist several minutes it disappears as the muscles relax just previous to the entrance of the food. The food is moved slowly along the ileum and is pushed through the valve into the colon. The moment it has entered a strong contraction takes place all along the caecum and the beginning of the ascending colon, pressing some of the food onward, and a moment later deep antiperistaltic waves (Fig. 6, 4.03) sweep down from the transverse colon and continue running until the caecum is again normally full, i. e., for two or three minutes.
ANIMAL MODELS OF HUMAN NUTRITION
Digestion and Absorption
The relative influence of gastrointestinal microorganisms on metabolism of digesta varies widely between species. Extensive fermentation in the pregastric chambers of ruminants and the hindgut of the horse makes these animals poor models for human nutrition. Dogs and pigs are similar to humans in many aspects of gastrointestinal morphology and physiology, including the relative length of small and large bowels, ingesta transit times, influence of dietary factors on gastric emptying, glucose or xylose absorption, fecal fat excretion, activities of intestinal brush border and pancreatic enzymes at maturity, and colonic volatile fatty acid concentration. However, humans, dogs and pigs differ in the developmental patterns and primary structure of some digestive enzymes. The composition of bile and the structure of bile acids differs substantially between humans and pigs. Colonic volatile fatty acids may be an important energy source for the pig, but not for the dog the importance of colonic fermentation for humans probably falls between these two species. Numerous strains of miniature pigs have been developed and used in experiments related to digestion and absorption.
There are over 285 different species of chelonians in the world. 1 Chelonians represent one of the most unique and recognizable groups of animals in the world. Characterized by their bony shell, no other tetrapod has both its pectoral and pelvic girdles encased in bone. 1 The chelonians are the longest lived of the reptiles, with some animals living over 100 years. Over the years, chelonians have been classified using a number of different taxonomic classifications, including morphologic and molecular features. However, for the purpose of this text, we will classify them based on habitat selection. For the captive chelonian, this is a preferred method because it is useful for defining the captive needs of the chelonian. The two primary classifications based on this system are the turtle and the tortoise. Turtles include species that are aquatic. These animals generally leave the water only to move to another body of water or to lay their eggs. Examples of this group can be found in Box 9-1 and Figures 9-1 and 9-2. For this group it is important to determine what type of aquatic system the animals originate from. There are three different types of aquatic systems: freshwater, brackish, and marine systems. The provision of the most appropriate system is critical to the long-term care of these animals in captivity. The tortoises include animals that are completely terrestrial. These animals are generally considered poor swimmers, and if provided a deep body of water in captivity, could drown. Common examples of these animals can be found in Box 9-1 and Figures 9-3 and 9-4.
|Alligator snapping turtles||Macroclemys temminickii|
|Snapping turtles||Chelydra serpentina|
|Red-eared slider||Trachemys scripta elegans|
|Diamond-backed terrapins||Malaclemys terrapin|
|Soft-shelled turtles||Apalone spp.|
|Mata mata||Chelus fimbriatus|
|Sulcata tortoise||Geochelone sulcata|
|Russian tortoise||Testudo horsfieldii|
|Red-footed tortoise||Geochelone carbonaria|
|Box turtle (tortoise)||Terrapene spp.|
|Leopard tortoise||Geochelone pardalis|
Venous return from the pelvic limbs drains into the kidneys to form the renal portal system in reptiles. 3 The primary function of the renal portal vein is to maintain adequate blood flow to the renal tubules when the glomerular blood flow is low. This large vessel arises near the junction of the epigastric and external iliac veins and enters the kidney centrally. 2 The chelonian renal portal vein contains valves that effectively shunt blood drained from the caudal half of the body directly into either the kidney or the liver and central venous reserve. 4 – 6
Although avoidance of the hindlimbs for intramuscular and intravenous injections of drugs has traditionally been advocated, research has demonstrated that this may be an unsubstantiated theory. Results of research conducted by Holtz et al. demonstrated that it is unlikely that the injection site has any influence over the activity of a drug and that the caudal half of the reptile body is suitable for drug administration. 5 Both Beck et al. and Holtz et al. found no significant difference in drug metabolism when gentamicin (excreted by glomerular filtration) was administered into the hindlimb, as opposed to the forelimb. 5, 7 Studies conducted by Malley revealed that adrenaline released during caudal intramuscular injections may reduce perfusion of the renal portal system, thereby increasing hepatic circulation. 8 It is important to consider that these studies were conducted on a limited number of species. Additional research is required to further elucidate if there is an effect based on species or an animal’s physiologic status.
Chelonians are obligate nasal breathers. 3 The nares open into a keratinized vestibule, which is divided cranially by a cartilaginous septum into right and left nasal chambers. The nasal chambers do not contain either turbinates or sinuses and extend caudally until converging into a single passageway dorsal to the hard palate. The passageway terminates at the choana. A soft palate is not present in chelonians. 10, 11
The chelonian glottis is easily identified at the back of the tongue. The tracheal rings are complete 3 and the trachea, bronchi, and lungs are all covered by a ciliated glandular epithelium that is ineffective at eliminating particulate foreign material and excessive respiratory secretions. 10 The trachea divides into paired bronchi relatively cranially in most chelonians. 12 Because of this, it is important to resist passing an endotracheal tube far into the lower respiratory tract. Doing so may result in intubating a single lung and lead to inconsistent anesthesia.
Chelonian lungs occupy a large volume in the dorsal half of the coelomic cavity. The borders of the lungs include the periosteum of the carapace dorsally, the pectoral girdle cranially, and the pelvic girdle caudally. 3 The lungs are multicameral, with a single intrapulmonary bronchus that divides into a complicated network of bronchioles and faveoli. 13 Chelonian faveoli differ from mammalian alveoli in that they are compartmentalized, and oxygen exchange occurs on the reticulated surface of these compartments. 2 The lungs are separated from the other coelomic viscera by a thin, nonmuscular, postpulmonary septum or pleuroperitoneal membrane (also called the septum horizonatale or the pseudodiaphragm). 2, 13, 14 Chelonians do not have a true diaphragm and are therefore not dependent on negative pressure for breathing. The volume of the lungs is reduced to one fifth when the head and limbs are retracted. 15
To compensate for the absence of an expandable chest, chelonians have developed strong trunk muscles that help to expand and contract the lungs with active inspiration and expiration. 15 – 17 It is the movement of the septum horizontale, powered by the movements of the trunk muscles, the viscera, and the limb girdles, that alters the intrapulmonary pressure and effectively draws air in and out of the lungs. 2, 3 When the septum horizontale is pulled taut and downward by the trunk muscles (e.g., limb movement), the area occupied by the lungs increases. This increase in lung volume thereby facilitates inspiration. 2 For terrestrial chelonians, inspiration is passive and expiration is active, but this scenario is reversed for aquatic chelonians due to the effect of hydrostatic pressure on visceral volume. 18, 19
The simple chelonian stomach is the proximal boundary of the lower digestive tract. The small intestine of chelonians is not well divided into a duodenum, jejunum, or ileum, and the tract is located in the caudal coelomic cavity. 21 The mucosa of the small intestine is composed of single columnar epithelium, 2 and regeneration of the intestinal epithelium takes an estimated 8 weeks in Chrysemys picta at a temperature of 20° C to 24° C (68° F to 75° F). 22
The large intestine begins with the cecum, which is not a distinct organ but is rather a widening of the distal colonic wall. 23, 24 The cecum is located in the right caudal quadrant of the coelomic cavity. The large intestine is typically divided into ascending, transverse, and descending portions. The mucosal epithelium is comprised of a large number of glandular cells. 2 Passage of food through the chelonian digestive tract is quite slow when compared to mammalian transit times (up to 2-4 weeks), but this allows for maximal nutritional absorption. 25
Chelonian digestion is dependent on environmental temperature. More specifically, the amount and activity of secreted enzymes 26, 27 and the absorptive capability of the intestinal mucosa are directly related to temperature. 21 Maximum digestion occurs within the animal’s appropriate temperature range (ATR), and the degree of digestion decreases when either above or below the ATR. 2 At high temperatures, gastric hydrochloric acid production is reduced, thereby altering the pH of the stomach. Consequently, pepsinogen activity is reduced and digestion is slowed. 2 Cell membrane permeability of the gut mucosa is also altered at different temperatures, thereby changing the absorptive processes of glucose and amino acids and the substrate affinity to trypsin. 21 Seasonal fluctuations in digestive rates of chelonians also exist, with rates in summer being higher than those in spring. 27
It has been determined that extremely slow digestion occurs between 10° C and 15° C (50° F to 59° F) and absolutely no digestion occurs below 7° C (45° F). 21 When temperatures are below this critical level, putrefaction of the ingesta occurs. This occurs often in captive, hibernating chelonians and in captive, imported chelonians that rely on supplemental heat to maintain normal physiologic activities. The absorption of toxic putrefaction products may affect the normal functioning of the nervous or other body systems. 2 Chelonian gastrointestinal contractions have been found to be temperature dependent as well. 2 Chelonian gut contractions are not continuous but rather occur in a series. 28, 29 Gastric contractions begin at the cardia, run down the gastric wall in intervals of 21 to 31 seconds, and then terminate at the pylorus. These gastrointestinal contractions are controlled via vagal impulses, and the process is temperature dependent. 30 Contractions of the small intestine occur at intervals of approximately 45 seconds. 31 The first type of large intestinal peristalsis begins at the cecum and ends at the coprodeum. The speed of this type of contraction ranges from 0.15 to 0.5 mm/sec and typically results in defecation. The second type of contraction is an antiperistalsis, which starts at the coprodeum and pushes ingesta cranially for 2 to 3 cm. This type of antiperistalsis occurs at intervals of 18 to 25 seconds and allows urine to be shunted into the caudal parts of the colon where water and ions can be reabsorbed. 21 Gastrointestinal transit time appears to be shortest in omnivorous species and longest in herbivorous species. 21 The large intestines of herbivorous species tend to have a greater volume than those of omnivorous species. 31
Chelonians have two large, flat, lobulated kidneys that are located in the retrocoleomic cavity, ventral to the caudal lung fields and cranial to the pelvic girdle. The chelonian kidney lacks both a loop of Henle and a renal pelvis. 2 The renal nephron is comprised of a glomerulus, a proximal tubule, an intermediate segment, and a distal tubule. 32 The chelonian kidney produces urine that is hypotonic to isotonic to blood and actively excretes uric acid. Bilateral ureters enter the urodeum of the cloaca at approximately the 10 and 2 o’clock positions. As stated previously, antiperistalsis of the urodeum allows urine to be transported caudally into the coprodeum and colon to maximize water and ion absorption. Urine may also travel cranially from the urodeum to the bladder for water reabsorption. The bladder wall is lined with ciliated cells and secretes mucus. The urethra is relatively short in chelonians and enters the bladder through the midventral floor of the urodeum. 2
The chelonian kidney serves many important functions, including osmoregulation, fluid balance regulation, excretion of metabolic waste products, and the production of various hormones and vitamin D metabolites. 31, 33 – 36 Research has shown that the urine in the renal tubules and ureters of Gopherus agassizii is always hypo-osmotic to blood until passage through the urinary bladder, where it becomes isosmotic via presumptive electrolyte and fluid exchange. 34
Urinary nitrogen is excreted in chelonians as a balance of ammonia, urea, uric acid, amino acids, allantoin, guanine, xanthine, and creatinine. 32 – 34 37 There are four described chelonian excretion patterns, each largely reflecting the environment in which an animal lives. Uricotelic and ureo-uricotelic chelonians tend to live in arid or desert environments, where water must be conserved. Uricotelic chelonians excrete predominately uric acid and urates, whereas ureo-uricotelic chelonians produce uric acid and urea as their major urinary excretory products. These chelonians are able to conserve water because uric acid is poorly soluble and can be excreted with minimal associated water. 2 In uricotelic chelonians, hypo-osmotic tubular and ureteral urine is thought to decrease urate precipitation within the renal tubules during periods of dehydration. 35 With uricotelism, urates precipitate out of solution in the bladder and do not need to be actively reabsorbed. Ureotelic and amino-ureotelic chelonians live in environments where water is plentiful. Ureotelism occurs when the major urinary excretory product is urea, whereas amino-ureotelism occurs when the major urinary excretory products are both ammonia and urea. Urea is highly water soluble and difficult to concentrate therefore, it must be excreted with significant amounts of water. 2
Movements of Human Small Intestine: 2 Types | Digestive System | Biology
The following types of movements are found in the human small intestine: 1. Rhythmic segmentation or Ludwig’s pendulum 2. Peristalsis.
Type # 1. Rhythmic Segmentation:
These are rings of contraction occurring at regular space of intervals in which a portion of the intestine is divided into segments. The contraction is followed by relaxation. The contraction takes place at the site of maximum distention. It can be studied under X-ray after barium meal. The opaque column of barium meal becomes broken into several small segments.
At the next mo­ment each of these segments is subdivided by a fresh batch of contractions, the previous group having disappeared in the meantime. The halves of the adjacent segments so di­vided run together and form new segments. These are again subdivided and thus the process goes on (Fig. 9.50).
Accord­ing to Friedman, there are two types of segmenting contrac­tions in the duodenum:
(1) One type consisted of a contrac­tion localised in a segment less than 2 cm and was eccentric in appearance.
(2) The other was concentric and consisted of a local contraction involving a segment usually longer than 2 cm and uniform circumferentially.
In animals the groups of contractions succeed at the rate of 20 – 30 per minute. In man the rate is slower. The frequency is inversely proportional to the distance from the stomach. A cyclic changes in the electrical potential occurred in the duo­denum known as basic electrical rhythm (B.E.R.) originates near the entrance of the bile duct and moves down the duodenum.
In the duodenum it is about 17 per minute, in the ileum it is about 12 per minute. In addition, ir­regular bursts of spike potential superimposed on B.E.R. appear in an electrical record. So the contraction is segmental and not peristaltic since spike potential and contractions do not precede more than a few centimeters. The duration of electrical cycle is about 3.5 sec. and hence the rhythm is 17 -18 per minute.
The frequency variation at different region is due to gradient in the physiological properties, viz., rhythmicity, irritability, variation in latent period and drug susceptibility. Muscular contraction occurs at in­tervals of some multiple of 3.4 seconds and do not travels vary far along the intestine. The vagi (Fig. 9.51) and the splanchnic nerves (Fig. 9.52) regulate the activity of the intestine, and adrenal medulla in psychic condi­tions, but action of these nerves is reversed in the control of ileocaecal sphincter.
These are the most fundamental movements of the intestine and are due to outstanding property of smooth muscle that is rhythmicity. Circular muscle is responsible for the most visible movement. They are myogenic in nature and are intendant of all nerves.
Segmentation movement does not cause forward pas­sage of food materials. It helps- (1) in digestion due to proper mixing of food with enzymes of digestive juic­es, (2) in absorption due to- (a) constantly changing the layer of fluid in contact with mucosa, (b) change in pressure, (3) in the improvement of intestinal cir­culation.
Type # 2. Peristalsis:
Peristalsis is described to be a composite wave, consist­ing of a wave of relaxation followed by a wave of con­striction. It is a translatory movement and travels down the gut in an aboral direction (away from the mouth). Bayliss and Starling have demonstrated that a stim­ulus applied to a given point on the intestinal wall causes contraction above and relaxation below the stimulated point (Fig. 9.53 & 9.54). This is a local reflex of smooth muscles and their intrinsic plex­uses. This is called The Law of Intestine or Myenteric Reflex. It is suggested that peristalsis depends on this reflex.
Usually two types of contraction, viz., peristaltic and rhythmic segmenting are present simultaneously, the former is superimposed on the latter and responsible for rise into the tone level of intestinal muscle without any interruption in the rhythm of the segmenting contraction. Peristaltic wave travels for varying distances—some few cms and other a few metres depending on the intensity of stimulus.
Segmental contractions sometimes recur frequently maintaining its character and travel aborally as peristaltic movements. A peristaltic wave induced by strong stimulus may sweep over the entire length of small intestine what is called rush wave or peristaltic rush.
The peristaltic waves move aborally and not orally and are due to the gradient of rhythmicity, conductivity and irritability. The impulse arises in the most irritable point and travels in the less irritable, i.e., the aboral side and not in the oral side due to the long refractoriness.
Type of Peristalsis:
Three types of peristaltic movements are present in the small gut:
It is a slow gentle wave moving at the rate of 1 – 2 cm per second which dies out easily after travelling a short distance.
It is a very swift wave travelling the entire length of small gut at the rate of 2 – 25 cm (average about 10 cm) per second. According to Alvarez the latter is true peristalsis. Due to its rapid speed it is also known as rush peristalsis.
iii. Third Type (Antiperistalsis):
In every respect it is same as peristalsis excepting that its direction is opposite. It moves in the oral direction. It is present in the second and third parts of duodenum only in man. Weak antiperi­stalsis also occurs in the terminal part of the ileum, thus preventing a rapid passage of the ileal contents into caecum.
In the duodenum it helps through admixture, as well as causes duodenal regurgitation into the stomach. These antiperistaltic movements occur due to presence of sensitive receptor area in this re­gion which responds to the qualities of chyme and concerned with delaying the passage of chyme into lower portion of the gut facilitating more scope of digestion and absorption.
Peristalsis depends on both nervous and chemical factors. The vagi and sympathetic have got influence on peristaltic movements. Stimulation of vagus increases and that of sympathetic inhibits peristalsis. Vagotomy on the other hand decreases the peristaltic activity only to a minor extent. The local nerve plexus (Auerbach’s plexus) helps in the co-ordination of peristaltic movements.
Distention of the intestine, normally caused by presence of food, causes peristaltic movements due to a stretch reflex called myenteric reflex. Reflex inhibition of whole of the small intestine may take place due to stretching of lower part of the small intestine (such as intestine and intestinal reflex) or stretching of gall-bladder and urinary bladder, etc.
These inhibitions may be removed by stimulation of splanchnic nerves (sympathetic). Presences of local nerve plexus (myenteric plexus) are required for this and the afferent receptors of which are present in the mucous membrane of the intestine. Liberation of 5-hydroxytryptamine (serotonin) from the enterochromaffin cells is a possible mediator in this reflex action. Role of a basic polypeptide, substance P as a mediator has also been suggested.
Role of Endocrines:
Hormones also exert great influence. Pituitrin excites the movements, as also thyroxine. Adrenaline inhibits the movements.
This is a special manifestation of peristaltic movements in the ileum. Peristalsis is generally very sluggish in the last part of the ileum. But after a meal, brisk peristalsis is set up reflexly in this region. This is called gastro-ileal reflex. The purpose is to drive out the ileal contents into caceum and thus making room for fresh supply.
i. Chief function is the propagation of the food onwards.
ii. Other functions are same as of segmentation movement.
The metabolic gradient theory of Alvarez is important in this connection. He observes that excitability fre­quency of movement, strength of contraction, tone of the intestine gradually diminish from above downwards along the intestinal canal. Even the normal direction of peristalsis is from above downwards.
The latent period of the intestinal muscles gradually becomes longer in the lower parts of the small intestine. This peculiarity, according to Alvarez, is due to the difference in the degree of metabolic activity between the upper and lower parts of intestine. The metabolic rate is much higher in the upper part than in the lower part and it is this gradi­ent upon which this difference depends.
In certain pathological conditions of intestine, such as, inflammation, obstruction, etc., the metabolic rate of the diseased part may be higher than those above it. So that under such conditions, antiperistalsis will start at the site of lesion and proceed towards the stomach. This theory explains the phenomenon of faecal vomiting during intestinal obstruction.
Questions for Discovery Learning
Describe how the image was formed that Cannon subsequently traced, what the image represented, and how Cannon controlled for movements of the cat. Answer: the “shadow” image was formed by bismuth absorbing X-rays more than the surrounding tissue. The image seen by Cannon represented only that part of the GI tract contents to which the bismuth had reached. He only took traces from the same point in respiration from an immobilized cat and made three traces of each time point to averaged them.
What were some of the other basic controls that Cannon performed to ensure that ”normal“ digestion was observed? Answer: he used different doses of bismuth and applied the bismuth at different times. He used different types of food. He palpated the stomach to simulate movement. He only studied calm cats.
The stomach pylorus. Answer: gentle peristalsis/strong peristalsis followed by division.
The stomach fundus. Answer: gentle squeezing/flaccid.
What were some of the controversies in the literature that Cannon was able to settle? Answer: was there mixing between the fundus and pylorus? What were the normal movements of the stomach? How did the pyloric sphincter function? Is there antiperistalsis during vomiting?
The study of emotions on physiological function played a large role in Cannon's later research. What were some of the basic observations made by Cannon on emotional state and digestion? Answer: digestion stopped immediately whenever the cat displayed strong emotions. The effect took time to wear off, even after the cat had calmed.
Mechanism of antiperistalsis - Biology
The avian cuisine varies as much as in mammals, leading to classification of individuals as carnivores, insectivores, seed-eaters and the like. As a consequence of these behavioral and dietary adaptations, a number of variations are seen in digestive anatomy of different birds. Having recognized that, however, common features of the avian digestive tract can be described.
The Pregastric System
The mouth of birds distinctly different from mammals. They have no teeth and their jaws are covered by a beak, which is seen in remarkably different forms. Birds do not really masticate, and mechanical disruption of food is accomplished by the beak and gizzard.
The esophagus is large in diameter, particularly in birds that swallow large meals. Swallowing is accomplished by esophageal peristalsis, and in most birds appears to be aided by extension of the neck. Most but not all birds have a crop , which varies from a simple expansion of the esophagus to one or two esophageal pouches. Depending on the state of contraction of the stomach, food being swallowed is diverted into the crop, then later propelled into the stomach by waves of peristalsis in the crop.
Birds have a glandular stomach, or proventriculus, and muscular stomach or gizzard. The glandular stomach receives food from the esophagus, and secretes mucus, HCl and pepsinogen, similar to what is seen in the mammalian stomach. The gizzard is a disk shaped, very muscular and in many birds contains small stones that facilitate grinding of foodstuffs. One of the gizzard's two orifices receives ingesta from the glandular stomach and the other empties into the duodenum.
A complex cycle of contractions involving the two stomachs force feed back and forth between the two, grinding it and increasing exposure to digestive enzymes. There is also periodic retropulsion of duodenal contents back into the stomachs, again presumably facilitating mixing of ingesta with enzymes. A final type of motility is seen in the regurgitation of pellets of bones, hair and feathers from the stomach of raptors.
Birds have a small intestine that seems very similar to the small intestine of mammals. A duodenum, jejunum and ileum are defined, although these segments are not as histologically distinct as in mammals. The proximal small intestine receives bile from the liver and digestive enzymes from the pancreas, and the absorptive epithelial cells are decorated with essentially the same battery of enzymes and transporters as in mammals.
The large intestine consists of a short colon and, typically, a pair of ceca. Short villi extend into the lumen of the colon, unlike what is seen in mammals. The cloaca is an expanded, tubular structure that serves as the common opening of the digestive, reproductive and urinary systems, which opens to the outside of the bird as the vent.
As in mammals, the large intestine's primary function is absorption of water and electrolytes. Antiperistalsis that originates in the cloaca is a prominent pattern of motility in the avian colon and has been suggested to assist not only in filling the ceca, but to flush urine from the cloaca into the large gut for absorption of water. In some birds, the ceca appear dispensible and can be removed without apparent harm. In other species, the ceca are important sites for fermentation, and the volatile fatty acids generated from microbial digestion of cellulose contributes significantly to energy demands.