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Gastrointestinal tract
Gastrointestinal tract
Gastrointestinal tract
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Gastrointestinal tract
Diagram of the gastrointestinal tract in the average human
Details
SystemDigestive system
Identifiers
Latintractus digestorius (mouth to anus),
canalis alimentarius (esophagus to large intestine),
canalis gastrointestinales (stomach to large intestine)
MeSHD041981
Anatomical terminology
Major parts of the
Gastrointestinal tract
Upper gastrointestinal tract
Lower gastrointestinal tract
See also

The gastrointestinal tract (also called the GI tract, digestive tract, and the alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is one of the largest of the body's systems.[1] The GI tract contains all the major organs of the digestive system, in humans and other animals, including the esophagus, stomach, and intestines. Food taken in through the mouth is digested to extract nutrients and absorb energy, and the waste expelled at the anus as feces. Gastrointestinal is an adjective meaning of or pertaining to the stomach and intestines.

Most animals have a "through-gut" or complete digestive tract. Exceptions are more primitive ones: sponges have small pores (ostia) throughout their body for digestion and a larger dorsal pore (osculum) for excretion, comb jellies have both a ventral mouth and dorsal anal pores, while cnidarians and acoels have a single pore for both digestion and excretion.[2][3]

The human gastrointestinal tract consists of the esophagus, stomach, and intestines, and is divided into the upper and lower gastrointestinal tracts.[4] The GI tract includes all structures between the mouth and the anus,[5] forming a continuous passageway that includes the main organs of digestion, namely, the stomach, small intestine, and large intestine. The complete human digestive system is made up of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver and gallbladder).[6] The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment. The whole human GI tract is about nine meters (30 feet) long at autopsy. It is considerably shorter in the living body because the intestines, which are tubes of smooth muscle tissue, maintain constant muscle tone in a halfway-tense state but can relax in different areas to allow for local distension and peristalsis.[7][8]

The human gut microbiota, is made up of around 4,000 different strains of bacteria, archaea, viruses and eukaryotes, with diverse roles in the maintenance of immune health and metabolism.[9][10][11] Enteroendocrine cells of the GI tract release hormones to help regulate the digestive process. These digestive hormones, including gastrin, secretin, cholecystokinin, and ghrelin, are mediated through either intracrine or autocrine mechanisms, indicating that the cells releasing these hormones are conserved structures throughout evolution.[12]

Human gastrointestinal tract

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Structure

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Illustration of digestive system

The structure and function of the GI tract can be described both by gross anatomy and microscopic anatomy (histology). The tract itself is divided into upper and lower tracts, and the intestines into small and large intestines.[13]

Upper gastrointestinal tract

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Main articles: Mouth, Pharynx, Esophagus, Stomach, and duodenum

The upper gastrointestinal tract consists of the mouth, pharynx, esophagus, stomach, and duodenum.[14] The exact demarcation between the upper and lower tracts is the suspensory muscle of the duodenum. This differentiates the embryonic borders between the foregut and midgut, and is also the division commonly used by clinicians to describe gastrointestinal bleeding as being of either "upper" or "lower" origin. Upon dissection, the duodenum may appear to be a unified organ, but it is divided into four segments based on function, location, and internal anatomy. The four segments of the duodenum are as follows (starting at the stomach, and moving toward the jejunum): bulb, descending, horizontal, and ascending.

The suspensory muscle of the duodenum suspends the superior border of the ascending duodenum from the diaphragm, and serves as an important anatomical landmark showing the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively.[15] This is a thin muscle which is derived from the embryonic mesoderm.

Lower gastrointestinal tract

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The lower gastrointestinal tract includes most of the small intestine and all of the large intestine.[16] In human anatomy, the intestine (bowel or gut; Greek: éntera) is the segment of the gastrointestinal tract extending from the pyloric sphincter of the stomach to the anus and as in other mammals, consists of two segments: the small intestine and the large intestine. In humans, the small intestine is further subdivided into the duodenum, jejunum, and ileum. The large intestine is subdivided into the cecum, and ascending, transverse, descending, and sigmoid colons, rectum, and anal canal.[17][18]

Small intestine
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Main articles: Small intestine, Duodenum, Jejunum, and Ileum

The small intestine is a tubular structure around 6 to 7 m long, that begins at the duodenum, and ends at the ileum.[1][19] Its mucosal area in an adult human is about 30 m2 (320 sq ft).[20] The combination of the circular folds, the villi, and the microvilli increases the absorptive area of the mucosa about 600-fold, making a total area of about 250 m2 (2,700 sq ft) for the entire small intestine.[21] Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream. There are three major divisions:

  1. Duodenum: A short structure (about 20–25 cm long[19]) that receives chyme from the stomach, together with pancreatic juice containing digestive enzymes and bile from the gall bladder. The digestive enzymes break down proteins, and bile emulsifies fats into micelles. The duodenum contains Brunner's glands which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in the chyme.
  2. Jejunum: This is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m (8.2 ft) long and contains the circular folds also known as plicae circulares and villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here.
  3. Ileum: The final section of the small intestine. It is about 3 m long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients.
Large intestine
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Main article: Large intestine

The large intestine forms an arch starting at the cecum and ending at the rectum and anal canal. It also includes the appendix, which is attached to the cecum. Its length is about 1.5 m, and the area of the mucosa in an adult human is about 2 m2 (22 sq ft).[20] The longest part of the large intestine is the colon whose main function is to absorb water and salts.[22]

The large intestine begins at the cecum, where the appendix is located. This is also the start of the colon as the ascending colon in the back wall of the abdomen. At the right colic flexure (hepatic flexure) (the flexed portion of the ascending and transverse colon) it runs across the abdomen in the transverse colon, passing below the diaphragm. At the left colic flexure (splenic flexure) the flexed portion of the transverse and descending colon, it descends down the left side of the abdomen. It reaches the sigmoid colon which is a loop of the colon closest to the rectum and continues to the rectum and anal canal.

Development

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Main article: Development of the digestive system

The gut is an endoderm-derived structure. At approximately the sixteenth day of human development, the embryo begins to fold ventrally (with the embryo's ventral surface becoming concave) in two directions: the sides of the embryo fold in on each other and the head and tail fold toward one another. The result is that a piece of the yolk sac, an endoderm-lined structure in contact with the ventral aspect of the embryo, begins to be pinched off to become the primitive gut. The yolk sac remains connected to the gut tube via the vitelline duct. Usually, this structure regresses during development; in cases where it does not, it is known as Meckel's diverticulum.[citation needed]

During fetal life, the primitive gut is gradually patterned into three segments: foregut, midgut, and hindgut. Although these terms are often used in reference to segments of the primitive gut, they are also used regularly to describe regions of the definitive gut as well.[citation needed]

Each segment of the gut is further specified and gives rise to specific gut and gut-related structures in later development. Components derived from the gut proper, including the stomach and colon, develop as swellings or dilatations in the cells of the primitive gut. In contrast, gut-related derivatives — that is, those structures that derive from the primitive gut but are not part of the gut proper, in general, develop as out-pouchings of the primitive gut. The blood vessels supplying these structures remain constant throughout development.[23]

Part Part in adult Gives rise to Arterial supply
Foregut esophagus to first 2 sections of the duodenum Esophagus, stomach, duodenum (1st and 2nd parts), liver, gallbladder, pancreas, superior portion of pancreas
(Though the spleen is supplied by the celiac trunk, it is derived from dorsal mesentery and therefore not a foregut derivative)		 celiac trunk
Midgut lower duodenum, to the first two-thirds of the transverse colon lower duodenum, jejunum, ileum, cecum, appendix, ascending colon, and first two-thirds of the transverse colon branches of the superior mesenteric artery
Hindgut last third of the transverse colon, to the upper part of the anal canal last third of the transverse colon, descending colon, rectum, and upper part of the anal canal branches of the inferior mesenteric artery

Histology

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Main article: Gastrointestinal wall
General structure of the gut wall

The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy.[24] The GI tract can be divided into four concentric layers in the following order:

Mucosa
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See also: Oral mucosa and Gastric mucosa

The mucosa is the innermost layer of the gastrointestinal tract. The mucosa surrounds the lumen, or open space within the tube. This layer comes in direct contact with digested food (chyme). The mucosa is made up of:[citation needed]

  • Epithelium – innermost layer. Responsible for most digestive, absorptive and secretory processes.
  • Lamina propria – a layer of connective tissue. Unusually cellular compared to most connective tissue
  • Muscularis mucosae – a thin layer of smooth muscle that aids the passing of material and enhances the interaction between the epithelial layer and the contents of the lumen by agitation and peristalsis

The mucosae are highly specialized in each organ of the gastrointestinal tract to deal with the different conditions. The most variation is seen in the epithelium.

Submucosa
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Main article: Submucosa

The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa and muscularis externa. It contains the submucosal plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.[citation needed]

Muscular layer
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The muscular layer consists of an inner circular layer and a longitudinal outer layer. The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract. The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch.[25] Whilst the muscularis externa is similar throughout the entire gastrointestinal tract, an exception is the stomach which has an additional inner oblique muscular layer to aid with grinding and mixing of food. The muscularis externa of the stomach is composed of the inner oblique layer, middle circular layer, and the outer longitudinal layer.

Between the circular and longitudinal muscle layers is the myenteric plexus. This controls peristalsis. Activity is initiated by the pacemaker cells, (myenteric interstitial cells of Cajal). The gut has intrinsic peristaltic activity (basal electrical rhythm) due to its self-contained enteric nervous system. The rate can be modulated by the rest of the autonomic nervous system.[25]

The coordinated contractions of these layers is called peristalsis and propels the food through the tract. Food in the GI tract is called a bolus (ball of food) from the mouth down to the stomach. After the stomach, the food is partially digested and semi-liquid, and is referred to as chyme. In the large intestine, the remaining semi-solid substance is referred to as feces.[25]

Adventitia and serosa
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Main articles: Serous membrane and Adventitia

The outermost layer of the gastrointestinal tract consists of several layers of connective tissue.[citation needed]

Intraperitoneal parts of the GI tract are covered with serosa. These include most of the stomach, first part of the duodenum, all of the small intestine, caecum and appendix, transverse colon, sigmoid colon and rectum. In these sections of the gut, there is a clear boundary between the gut and the surrounding tissue. These parts of the tract have a mesentery.[citation needed]

Retroperitoneal parts are covered with adventitia. They blend into the surrounding tissue and are fixed in position. For example, the retroperitoneal section of the duodenum usually passes through the transpyloric plane. These include the esophagus, pylorus of the stomach, distal duodenum, ascending colon, descending colon and anal canal. In addition, the oral cavity has adventitia.[citation needed]

Gene and protein expression

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Approximately 20,000 protein coding genes are expressed in human cells and 75% of these genes are expressed in at least one of the different parts of the digestive organ system.[26][27] Over 600 of these genes are more specifically expressed in one or more parts of the GI tract and the corresponding proteins have functions related to digestion of food and uptake of nutrients. Examples of specific proteins with such functions are pepsinogen PGC and the lipase LIPF, expressed in chief cells, and gastric ATPase ATP4A and gastric intrinsic factor GIF, expressed in parietal cells of the stomach mucosa. Specific proteins expressed in the stomach and duodenum involved in defence include mucin proteins, such as mucin 6 and intelectin-1.[28]

Transit time

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The time taken for food to transit through the gastrointestinal tract varies on multiple factors, including age, ethnicity, and gender.[29][30] Several techniques have been used to measure transit time, including radiography following a barium-labeled meal, breath hydrogen analysis, scintigraphic analysis following a radiolabeled meal,[31] and simple ingestion and spotting of corn kernels.[32] It takes 2.5 to 3 hours for 50% of the contents to leave the stomach.[medical citation needed] The rate of digestion is also dependent of the material being digested, as food composition from the same meal may leave the stomach at different rates.[33] Total emptying of the stomach takes around 4–5 hours, and transit through the colon takes 30 to 50 hours.[31][34][35]

Immune function

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The gastrointestinal tract forms an important part of the immune system.[36]

Immune barrier
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The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court.[20] With such a large exposure (more than three times larger than the exposed surface of the skin), these immune components function to prevent pathogens from entering the blood and lymph circulatory systems.[37] Fundamental components of this protection are provided by the intestinal mucosal barrier, which is composed of physical, biochemical, and immune elements elaborated by the intestinal mucosa.[38] Microorganisms also are kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue (GALT).

There are additional factors contributing to protection from pathogen invasion. For example, low pH (ranging from 1 to 4) of the stomach is fatal for many microorganisms that enter it.[39] Similarly, mucus (containing IgA antibodies) neutralizes many pathogenic microorganisms.[40] Other factors in the GI tract contribution to immune function include enzymes secreted in the saliva and bile.

Immune system homeostasis
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Beneficial bacteria also can contribute to the homeostasis of the gastrointestinal immune system. For example, Clostridia, one of the most predominant bacterial groups in the GI tract, play an important role in influencing the dynamics of the gut's immune system.[41] It has been demonstrated that the intake of a high fiber diet could be responsible for the induction of T-regulatory cells (Tregs). This is due to the production of short-chain fatty acids during the fermentation of plant-derived nutrients such as butyrate and propionate. Basically, the butyrate induces the differentiation of Treg cells by enhancing histone H3 acetylation in the promoter and conserved non-coding sequence regions of the FOXP3 locus, thus regulating the T cells, resulting in the reduction of the inflammatory response and allergies.[citation needed]

Gastrointestinal microbiota

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Main article: Gut microbiota
Diagram of human microbiota depicted in various regions of the gastrointestinal tract

The large intestine contains multiple types of bacteria, and other microorganisms that can break down molecules the human body cannot process alone,[42][43] demonstrating a symbiotic relationship. These microbes are responsible for gas production at host–pathogen interface, which is released as flatulence. Intestinal bacteria can also participate in biosynthesis reactions. For example, certain strains in the large intestine produce vitamin B12;[44] an essential compound in humans for things like DNA synthesis and red blood cell production.[45] However, the primary function of the large intestine is water absorption from digested material (regulated by the hypothalamus) and the reabsorption of sodium and nutrients.[46]

Beneficial intestinal bacteria compete with potentially harmful bacteria for space and "food", as the intestinal tract has limited resources. A ratio of 80–85% beneficial to 15–20% potentially harmful bacteria is proposed for maintaining homeostasis.[citation needed] An imbalanced ratio results in dysbiosis.

Detoxification and drug metabolism

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Enzymes such as CYP3A4, along with the antiporter activities, are also instrumental in the intestine's role of drug metabolism in the detoxification of antigens and xenobiotics.[47]

Other animals

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See also: Equine_anatomy § Digestive_system, and Ruminant § Digestive_system_of_ruminants

In most vertebrates, including amphibians, birds, reptiles, egg-laying mammals, and some fish, the gastrointestinal tract ends in a cloaca and not an anus. In the cloaca, the urinary system is fused with the genito-anal pore. Therians (all mammals that do not lay eggs, including humans) possess separate anal and uro-genital openings. The females of the subgroup Placentalia have even separate urinary and genital openings.[citation needed]

During early development, the asymmetric position of the bowels and inner organs is initiated (see also axial twist theory).

Ruminants show many specializations for digesting and fermenting tough plant material, consisting of additional stomach compartments, and the ability to regurgitate partially digested food material for further chewing (aka "chewing cud").[48]

Many birds and other animals have a specialised stomach in the digestive tract called a gizzard used for grinding up food.[49]

Another feature found in a range of animals is the crop. In birds this is found as a pouch alongside the esophagus.[49]

In 2020, the oldest known fossil digestive tract, of an extinct wormlike organism in the Cloudinidae was discovered; it lived during the late Ediacaran period about 550 million years ago.[50][51]

A through-gut (one with both mouth and anus) is thought to have evolved within the nephrozoan clade of Bilateria, after their ancestral ventral orifice (single, as in cnidarians and acoels; re-evolved in nephrozoans like flatworms) stretched antero-posteriorly, before the middle part of the stretch would get narrower and closed fully, leaving an anterior orifice (mouth) and a posterior orifice (anus plus genital opening). A stretched gut without the middle part closed is present in another branch of bilaterians, the extinct proarticulates. This and the amphistomic development (when both mouth and anus develop from the gut stretch in the embryo) present in some nephrozoans (e.g. roundworms) are considered to support this hypothesis.[52][53]

Clinical significance

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This section discusses related diseases, medical associations with the gastrointestinal tract, and use in surgery.
Main articles: Gastrointestinal disease and Gastroenterology
Further information: Clinical significance

Diseases

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There are many diseases and conditions that can affect the gastrointestinal system, including infections, inflammation and cancer.[citation needed]

Various pathogens, such as bacteria that cause foodborne illnesses, can induce gastroenteritis which results from inflammation of the stomach and small intestine. Antibiotics to treat such bacterial infections can decrease the microbiome diversity of the gastrointestinal tract, and further enable inflammatory mediators.[54] Gastroenteritis is the most common disease of the GI tract.

Diverticular disease is a condition that is very common in older people in industrialized countries. It usually affects the large intestine but has been known to affect the small intestine as well. Diverticulosis occurs when pouches form on the intestinal wall. Once the pouches become inflamed it is known as diverticulitis.[citation needed]

Inflammatory bowel disease is an inflammatory condition affecting the bowel walls, and includes the subtypes Crohn's disease and ulcerative colitis. While Crohn's can affect the entire gastrointestinal tract, ulcerative colitis is limited to the large intestine. Crohn's disease is widely regarded as an autoimmune disease. Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such.[citation needed]

Functional gastrointestinal disorders the most common of which is irritable bowel syndrome. Functional constipation and chronic functional abdominal pain are other functional disorders of the intestine that have physiological causes but do not have identifiable structural, chemical, or infectious pathologies.[citation needed]

Symptoms

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Several symptoms can indicate problems with the gastrointestinal tract, including:[citation needed]

Treatment

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Gastrointestinal surgery can often be performed in the outpatient setting. In the United States in 2012, operations on the digestive system accounted for 3 of the 25 most common ambulatory surgery procedures and constituted 9.1 percent of all outpatient ambulatory surgeries.[56]

Imaging

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Various methods of imaging the gastrointestinal tract include the upper and lower gastrointestinal series:

[edit]
  • Cholera
  • Enteric duplication cyst
  • Giardiasis
  • Pancreatitis
  • Peptic ulcer disease
  • Yellow fever
  • Helicobacter pylori is a gram-negative spiral bacterium. Over half the world's population is infected with it, mainly during childhood; it is not certain how the disease is transmitted. It colonizes the gastrointestinal system, predominantly the stomach. The bacterium has specific survival conditions that are specific to the human gastric microenvironment: it is both capnophilic and microaerophilic. Helicobacter also exhibits a tropism for gastric epithelial lining and the gastric mucosal layer about it. Gastric colonization of this bacterium triggers a robust immune response leading to moderate to severe inflammation, known as gastritis. Signs and symptoms of infection are gastritis, burning abdominal pain, weight loss, loss of appetite, bloating, burping, nausea, bloody vomit, and black tarry stools. Infection can be detected in a number of ways: GI X-rays, endoscopy, blood tests for anti-Helicobacter antibodies, a stool test, and a urease breath test (which is a by-product of the bacteria). If caught soon enough, it can be treated with three doses of different proton pump inhibitors as well as two antibiotics, taking about a week to cure. If not caught soon enough, surgery may be required.[57][58][59][60]
  • Intestinal pseudo-obstruction is a syndrome caused by a malformation of the digestive system, characterized by a severe impairment in the ability of the intestines to push and assimilate. Symptoms include daily abdominal and stomach pain, nausea, severe distension, vomiting, heartburn, dysphagia, diarrhea, constipation, dehydration and malnutrition. There is no cure for intestinal pseudo-obstruction. Different types of surgery and treatment managing life-threatening complications such as ileus and volvulus, intestinal stasis which lead to bacterial overgrowth, and resection of affected or dead parts of the gut may be needed. Many patients require parenteral nutrition.[citation needed]
  • Ileus is a blockage of the intestines.
  • Coeliac disease is a common form of malabsorption, affecting up to 1% of people of northern European descent. An autoimmune response is triggered in intestinal cells by digestion of gluten proteins. Ingestion of proteins found in wheat, barley and rye, causes villous atrophy in the small intestine. Lifelong dietary avoidance of these foodstuffs in a gluten-free diet is the only treatment.
  • Enteroviruses are named by their transmission-route through the intestine (enteric meaning intestinal), but their symptoms are not mainly associated with the intestine.
  • Endometriosis can affect the intestines, with similar symptoms to IBS.
  • Bowel twist (or similarly, bowel strangulation) is a comparatively rare event (usually developing sometime after major bowel surgery). It is, however, hard to diagnose correctly, and if left uncorrected can lead to bowel infarction and death. (The singer Maurice Gibb is understood to have died from this.)
  • Angiodysplasia of the colon
  • Constipation
  • Diarrhea
  • Hirschsprung's disease (aganglionosis)
  • Intussusception
  • Polyp (medicine) (see also colorectal polyp)
  • Pseudomembranous colitis
  • Toxic megacolon usually a complication of ulcerative colitis

Uses of animal guts

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Intestines from animals other than humans are used in a number of ways. From each species of livestock that is a source of milk, a corresponding rennet is obtained from the intestines of milk-fed calves. Pig and calf intestines are eaten, and pig intestines are used as sausage casings. Calf intestines supply calf-intestinal alkaline phosphatase (CIP), and are used to make goldbeater's skin. Other uses are:

  • The use of animal gut strings by musicians can be traced back to the third dynasty of Egypt. In the recent past, strings were made out of lamb gut. With the advent of the modern era, musicians have tended to use strings made of silk, or synthetic materials such as nylon or steel. Some instrumentalists, however, still use gut strings in order to evoke the older tone quality. Although such strings were commonly referred to as "catgut" strings, cats were never used as a source for gut strings.[61]
  • Sheep gut was the original source for natural gut string used in racquets, such as for tennis. Today, synthetic strings are much more common, but the best gut strings are now made out of cow gut.
  • Gut cord has also been used to produce strings for the snares that provide a snare drum's characteristic buzzing timbre. While the modern snare drum almost always uses metal wire rather than gut cord, the North African bendir frame drum still uses gut for this purpose.
  • "Natural" sausage hulls, or casings, are made of animal gut, especially hog, beef, and lamb.
  • The wrapping of kokoretsi, gardoubakia, and torcinello is made of lamb (or goat) gut.
  • Haggis is traditionally boiled in, and served in, a sheep stomach.
  • Chitterlings, a kind of food, consist of thoroughly washed pig's gut.
  • Animal gut was used to make the cord lines in longcase clocks and for fusee movements in bracket clocks, but may be replaced by metal wire.
  • The oldest known condoms, from 1640 AD, were made from animal intestine.[62]

See also

[edit]
This article uses anatomical terminology.

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Gastrointestinal tract

View on Grokipedia
from Grokipedia
The gastrointestinal tract (GI tract), also referred to as the digestive tract or alimentary canal, is a continuous series of hollow organs forming a long, twisting tube that runs from the to the , measuring approximately 9 meters (30 feet) in length in adults. It comprises the , , , (including the , , and ), (including the , colon, , and appendix), and , supported by accessory organs such as the salivary glands, liver, , and . The primary functions of the GI tract include the ingestion of food, mechanical and chemical digestion to break down nutrients (proteins into , fats into fatty acids and , and carbohydrates into simple sugars), , electrolytes, and essential nutrients (with about 90% occurring in the via specialized villi and microvilli), secretion of digestive juices like , , , and enzymes, and the excretion of indigestible waste as . The GI tract operates through coordinated processes driven by contractions known as , which propel food along the tract, and segmentation, which mixes contents for optimal . begins in the with mechanical and salivary enzymes, continues in the where (maintaining a of 1.5–2.0) and initiate protein breakdown, and reaches completion in the with contributions from pancreatic enzymes and from the liver and to emulsify fats. The primarily absorbs remaining water and electrolytes, compacts waste, and hosts a diverse of trillions of that ferment undigested fibers, produce vitamins like , and support immune function. Neural control via the (with myenteric and submucosal plexuses) and hormonal regulation ensure efficient motility and secretion throughout the tract. Beyond nutrient processing, the GI tract plays critical roles in immune defense, as its mucosal lining contains lymphoid tissues that monitor for pathogens, and in metabolic , influencing overall energy balance and even brain function through the gut-brain axis. Disruptions in GI tract function can lead to common disorders such as , , and , highlighting its essential contribution to health.

Overview

Definition and Functions

The gastrointestinal tract (GI tract), also referred to as the alimentary or digestive tract, is a continuous muscular tube that runs from the to the , spanning approximately 9 meters (30 feet) in length in adults. This elongated structure forms the core of the digestive system, facilitating the processing of ingested materials through coordinated mechanical and chemical actions. The primary functions of the GI tract encompass the mechanical breakdown of food via and mixing, chemical digestion through enzymatic and acid-mediated , selective absorption of and electrolytes into the bloodstream, of water to maintain hydration, and elimination of indigestible residues as . Additionally, it serves as a protective barrier, preventing the entry of harmful pathogens and toxins into the systemic circulation while permitting the passage of beneficial substances. These roles collectively support , energy regulation, and overall metabolic balance essential for survival. The digestive process mediated by the GI tract unfolds in four key stages: , where enters the ; , involving breakdown into simpler molecules; absorption, primarily in the for uptake into circulation; and egestion, the expulsion of waste via the . Daily operations involve handling 8–10 liters of , comprising 1–2 liters from dietary intake and 7–9 liters of endogenous secretions, including 0.5–1.5 liters of for initial lubrication and enzymatic action, and 2–3 liters of gastric secretions for protein denaturation. The GI tract's metabolic demands account for about 10% of total daily energy expenditure, largely through the thermic effect of processing nutrients.

Evolutionary and Comparative Context

The gastrointestinal tract originated in early metazoans as a simple tubular structure derived from endodermal tissues, enabling directional and marking a key innovation for beyond intracellular seen in precursors like and Porifera. In bilaterians, this evolved into regionally specialized guts using conserved genetic pathways from the cnidarian-bilaterian ancestor, allowing adaptation to diverse diets through variations in length, compartmentalization, and microbial . Dietary pressures drove diversification, with herbivores developing longer intestines relative to body size to facilitate microbial of fibrous , while carnivores evolved shorter, smooth tracts typically 3–6 times body length for rapid absorption of nutrient-dense animal proteins. Human intestines are relatively longer and more complex, resembling those of herbivores in this aspect. For instance, ruminants like cows adapted around 50 million years ago during the Eocene, originating in small omnivorous ancestors and enabling efficient breakdown of via rumen microbes in a multi-chambered . In contrast, birds lack a true glandular , instead using a proventriculus for and a muscular for mechanical grinding, an avian-style system that emerged over 160 million years ago in early paravians to unmasticated efficiently. In humans, the enlarged for enhanced , coinciding with AMY1 duplications approximately 800,000 years ago that increased salivary production, likely adapting to cooked, -rich tubers and predating . This genetic shift supported higher energy extraction from carbohydrates, reflecting selective pressures from use and dietary expansion in hominins. Throughout metazoan , the GI tract co-evolved with symbiotic microbes, forming mutualistic relationships that optimize nutrient extraction; in mammals, including humans, gut ferment indigestible fibers into , with microbial communities diversifying in response to host diet and phylogeny over millions of years. This enhanced digestive efficiency, as seen in herbivores' reliance on rumen and humans' adaptation to varied diets through microbial metabolic contributions.

Human Anatomy

Upper Gastrointestinal Tract

The upper gastrointestinal tract comprises the , , , and , which collectively prepare ingested food for further processing through mechanical breakdown, , and initial transport. The , or oral cavity, serves as the entry point for food and features specialized structures for initial mechanical digestion. Teeth are arranged in four types: incisors for cutting, canines for tearing, premolars for crushing, and molars for grinding, enabling the breakdown of food into smaller particles. The , composed of intrinsic and extrinsic skeletal muscles such as the and hyoglossus, facilitates mastication by manipulating food against the teeth and . Three pairs of salivary glands contribute to moistening and enzymatic preparation: the parotid glands, located below the , produce serous saliva rich in for breakdown; the submandibular glands, situated in the mandibular groove, secrete a mixed containing and mucins; and the sublingual glands, in the of the , provide mucous . The , a muscular funnel extending from the to the , is divided into three regions: the nasopharynx posterior to the , the oropharynx behind the oral cavity, and the laryngopharynx inferiorly near the . It initiates by coordinating the bolus transfer from the to the , with the preventing aspiration into the airway. The is a approximately 25 cm long muscular tube that propels food from the to the via peristaltic waves. It features an upper esophageal of at its proximal end and a lower esophageal of approximately 3 cm above the junction, both regulating bolus passage. The mucosal lining consists of stratified squamous non-keratinized , supported by a containing glands for lubrication. The stomach is a J-shaped organ with four anatomical regions: the cardia surrounding the esophageal opening, the fundus above the cardia, the body as the main central portion, and the at the distal end leading to the via the pyloric sphincter. Its internal surface is folded into longitudinal that allow expansion, with a resting capacity of 1 to 1.5 liters. in the mucosa include parietal cells secreting , chief cells producing pepsinogen, and mucous cells providing protective . Blood supply to the upper gastrointestinal tract arises primarily from branches of the celiac trunk for the , including the along the lesser curvature and the right gastric and gastroepiploic arteries; the receives arterial supply from proximally, bronchial and esophageal branches of the in the middle, and distally. Innervation is provided by the (cranial nerve X) for parasympathetic control of and throughout the and , supplemented by the in the tract wall.

Lower Gastrointestinal Tract

The lower gastrointestinal tract comprises the small and large intestines, which collectively process nutrients from ingested material and form waste for elimination. The small intestine, extending from the pylorus of the stomach to the ileocecal junction, is the primary site for nutrient breakdown and uptake preparation, while the large intestine absorbs water and electrolytes to consolidate residues into feces. The small intestine measures approximately 6 to 7 meters in length and is divided into three segments: the duodenum, jejunum, and ileum. The duodenum, the shortest segment at about 25 centimeters, curves in a C-shape around the head of the pancreas and receives chyme from the upper gastrointestinal tract via the pyloric sphincter. The jejunum, roughly 2.5 meters long, occupies the upper left quadrant of the abdomen, and the ileum, about 3.5 meters, extends to the right lower quadrant, terminating at the ileocecal valve. To maximize contact with digestive contents, the small intestinal mucosa features permanent circular folds known as plicae circulares, finger-like projections called villi, and apical microvilli on enterocytes, collectively amplifying the absorptive surface area to approximately 200 square meters. In the duodenum specifically, submucosal Brunner's glands secrete alkaline mucus to neutralize acidic chyme, protecting the mucosa from gastric acid exposure. The , approximately 1.5 meters long, encircles the and consists of the , colon (, , , and sigmoid segments), , and . The blind-ended , about 6 centimeters in diameter, lies in the right iliac fossa and connects to the via the ileocecal , a muscular valve that prevents reflux of colonic contents. The rises along the right wall, the spans horizontally across the , the descends along the left wall, and the forms an S-shaped curve in the before joining the . The , a dilated chamber about 12 to 15 centimeters long, stores , while the , the terminal 3 to 4 centimeters, opens externally. Structurally, the large intestinal wall includes haustra (sac-like pouches formed by contractions of the circular muscle layer) and three longitudinal muscle bands called taeniae coli, which gather the excess longitudinal musculature and contribute to the formation of haustra. Unlike the , the large intestinal mucosa lacks villi but contains straight tubular glands known as crypts of Lieberkühn, lined with goblet cells that secrete mucus for lubrication. At the anorectal junction, the (, involuntary) and (, voluntary) maintain continence, while vertical folds in the called anal columns aid in sensory discrimination during . The vascular supply to the lower gastrointestinal tract derives from the superior and inferior mesenteric arteries, branches of the abdominal aorta. The superior mesenteric artery supplies the small intestine entirely, as well as the cecum, ascending colon, and proximal two-thirds of the transverse colon via its jejunal, ileal, ileocolic, right colic, and middle colic branches. The inferior mesenteric artery perfuses the distal third of the transverse colon, descending colon, sigmoid colon, rectum, and upper anal canal through the left colic, sigmoid, and superior rectal arteries, with anastomoses forming the marginal artery along the colon for collateral flow. Venous drainage parallels the arterial supply, converging into the superior and inferior mesenteric veins that join the portal system. Lymphatic drainage follows a similar pattern: lacteals within small intestinal villi collect interstitial fluid and chyle, draining to mesenteric lymph nodes and then the cisterna chyli; in the large intestine, superficial and deep mucosal lymphatics converge to regional colic nodes, ultimately reaching the thoracic duct via mesenteric chains.

Accessory Digestive Organs

The accessory digestive organs include the liver, , and , which support the gastrointestinal tract by producing and storing secretions that facilitate , though they are not part of the tubular tract itself. These organs connect to the tract via ducts and ligaments, receiving dual blood supplies and autonomic innervation to coordinate their functions. The liver, the largest solid organ in the body, is located in the upper right and consists of functional units called hepatic lobules, which are hexagonal arrangements of plates of hepatocytes separated by sinusoids. Hepatocytes, the primary parenchymal cells comprising about 80% of the liver's , perform essential roles including the production of at a rate of approximately 800 to 1,000 mL per day and of blood-borne toxins through metabolic processes like conjugation and . The liver receives a dual blood supply: 75% to 80% from the nutrient-rich , which drains the gastrointestinal tract and , and 20% to 25% from the oxygenated ; this ensures efficient processing of absorbed substances. Innervation of the liver involves sympathetic fibers from the along the and , as well as parasympathetic vagal fibers that modulate blood flow and metabolic activities. The , a pear-shaped sac attached to the underside of the liver, stores and concentrates delivered from the hepatic ducts. It has a capacity of 30 to 50 mL, where is concentrated up to 10-fold through active absorption of water and electrolytes by the gallbladder mucosa, preparing it for release during . The , measuring about 2 to 3 cm in length with spiral mucosal folds (valves of Heister), connects the gallbladder neck to the , forming part of the biliary tree. Blood supply to the gallbladder primarily comes from the , a branch of the right hepatic artery, while innervation derives from sympathetic and parasympathetic fibers of the celiac and vagus nerves, respectively, which regulate contraction and tone. The , situated retroperitoneally across the posterior , has both exocrine and endocrine components. The exocrine portion, comprising about 85% of the , consists of acinar cells clustered in acini that synthesize such as and , which are secreted via a ductal system into the . The endocrine portion includes islets of Langerhans, clusters of cells that produce hormones like insulin from beta cells. The main pancreatic duct (duct of Wirsung) merges with the at the , a dilation in the duodenal wall, while an accessory duct (duct of Santorini) may drain separately. Blood supply to the arises from branches of the celiac trunk (splenic and gastroduodenal arteries) and (pancreaticoduodenal arteries), forming an anastomotic network; innervation includes sympathetic input from the and parasympathetic from the vagus, with an intrinsic coordinating secretion. These organs are interconnected structurally: the hepatoduodenal ligament, a portion of the , suspends the liver from the and transmits the , hepatic artery, and . The , a ring of surrounding the , regulates the flow of and pancreatic secretions into the . Collectively, the accessory organs contribute essential secretions to the of the lower gastrointestinal tract for nutrient breakdown.

Development and Histology

Embryonic Development

The embryonic development of the gastrointestinal (GI) tract begins during the third week of , when the of the trilaminar disc forms a primitive gut tube through cephalocaudal and lateral folding of the , incorporating portions of the . By the fourth week, this tube differentiates into three distinct regions: the , which extends from the to the distal and gives rise to the , , proximal , liver, , and ; the , spanning from the distal to the distal and forming the distal , , , , appendix, , and proximal two-thirds of the ; and the , from the distal to the , which develops into the distal one-third of the , descending and , , and upper . The of the GI tract derives from , while surrounding contributes to , muscle layers, and blood vessels. Key developmental processes include the and herniation of the , as well as septation of the . Between weeks 6 and 10, the rapidly growing herniates through the umbilical ring into the extraembryonic due to space constraints in the , then undergoes a 270° counterclockwise around the axis before reducing back into the by week 10. This positions the posteriorly, the and in the left upper quadrant, and the in the right lower quadrant. Concurrently, the , a common chamber for and urogenital derivatives, is divided by the urorectal starting in week 6, fully separating the anorectal canal from the by week 7, with the anal membrane perforating to form the around week 8. The oral (buccopharyngeal) membrane ruptures by week 5 to open the . Organogenesis involves vacuolization and recanalization of the gut lumen, which temporarily occludes around week 6 before reopening by week 8, with incomplete processes leading to congenital anomalies. For instance, failure of recanalization in the can result in , often associated with due to incomplete separation by the tracheoesophageal septum; arises from incomplete canalization or excessive muscular proliferation in the ; and persistence of the vitelline duct may cause or ileal . rotation defects, such as non-rotation or malrotation, can lead to intestinal or obstruction. Genetic regulation is crucial for anterior-posterior patterning and regional specification of the gut tube. , a family of transcription factors, establish the craniocaudal identity of segments, with specific clusters like Hoxa and Hoxc directing , , and differentiation—for example, Hoxa3 influences foregut structures and Hoxc8-9 the midgut. Sonic hedgehog (Shh), secreted by the , acts as a to induce mesenchymal expression of targets like Bmp-4 and Abd-B-related , promoting gut tube specification, villus formation, and epithelial-mesenchymal interactions; disruptions in Shh signaling are linked to anomalies such as anorectal malformations and .

Microscopic Structure

The wall of the gastrointestinal (GI) tract consists of four principal histological layers, organized from the lumen outward: the mucosa, submucosa, muscularis externa, and serosa or adventitia. The mucosa, the innermost layer, comprises the epithelium, which lines the lumen and varies by region; the lamina propria, a loose connective tissue layer containing blood vessels, lymphatics, and immune cells; and the muscularis mucosae, a thin sheet of smooth muscle that facilitates local mucosal folding and movement. The submucosa, beneath the mucosa, is a dense irregular connective tissue layer rich in large blood vessels, lymphatics, nerves, and in some regions, exocrine glands such as Brunner's glands in the duodenum. The muscularis externa, responsible for peristalsis and segmentation, consists of an inner circular smooth muscle layer and an outer longitudinal smooth muscle layer, with the myenteric (Auerbach's) plexus of neurons situated between them to coordinate motility. The outermost serosa, a serous membrane of simple squamous epithelium over connective tissue, covers intraperitoneally located portions of the tract, while the adventitia, a fibrous connective tissue layer, envelops retroperitoneal or intrathoracic segments like the esophagus. Regional histological variations adapt the tract to specific functions, particularly in the epithelium and glandular components. In the esophagus, the epithelium is non-keratinized stratified squamous to withstand abrasion from food passage, transitioning abruptly to in the stomach and intestines. The stomach features with leading to glands secreting acid, enzymes, and mucus, while the small intestine's epithelium forms villi and microvilli for enhanced absorption, and the emphasizes mucus production with abundant goblet cells but lacks villi. Goblet cells, unicellular mucus-secreting glands, are sparse in the esophagus and stomach but increase progressively in the intestines to lubricate contents and protect the epithelium. Key cellular components within the epithelium include enterocytes, Paneth cells, and enteroendocrine cells, primarily in the small intestine. Enterocytes, the predominant absorptive cells, feature apical microvilli that form the brush border for nutrient uptake via transport proteins. Paneth cells, located at the base of crypts of Lieberkühn, secrete antimicrobial peptides such as defensins and lysozyme to regulate the local microbiota. Enteroendocrine cells, scattered throughout the epithelium, release hormones like gastrin, secretin, and cholecystokinin in response to luminal stimuli, influencing digestion and motility. Neural elements, including the submucosal (Meissner's) plexus embedded in the , provide local control over glandular secretion, blood flow, and epithelial transport without direct central input. This plexus integrates sensory information from the mucosa to modulate local functions like fluid absorption. Wall thickness varies regionally, with the measuring approximately 2-4 mm overall due to its simpler structure, while the colon's wall is thicker, up to 3-5 mm, owing to prominent haustra and a robust muscularis externa.

Physiology

Digestion and Secretion

Digestion in the gastrointestinal tract involves the enzymatic breakdown of macronutrients into absorbable units, facilitated by secretions from various glands that provide enzymes, acids, and emulsifiers. This process begins in the and continues through the and , where specialized enzymes target carbohydrates, proteins, and . Secretory mechanisms ensure the delivery of these agents in response to luminal and nutrient presence, optimizing breakdown efficiency. Carbohydrate digestion initiates in the with salivary , secreted by the parotid glands, which hydrolyzes starches into and at an optimal pH of around 6.7. This partial breakdown halts in the acidic but resumes in the upon neutralization, where pancreatic further cleaves starches and into , , and dextrins. Final hydrolysis occurs at the of the , with enzymes such as (breaking to glucose), sucrase (hydrolyzing to glucose and ), and (converting to glucose and ) completing the process to monosaccharides. Protein digestion commences in the , where , activated from pepsinogen by , cleaves peptide bonds in proteins at a low pH of 1.5-3.5, producing large polypeptides and some free . In the , pancreatic secretions provide , , and carboxypeptidase, which are initially released as inactive zymogens to prevent autodigestion; enterokinase from the duodenal mucosa activates to , which in turn activates the others. These endopeptidases ( and ) hydrolyze internal peptide bonds, while exopeptidases like carboxypeptidase remove terminal , yielding smaller and . Lipid digestion primarily occurs in the , beginning with emulsification by salts from the liver and stored in the , which disperses large fat globules into smaller droplets, increasing surface area for enzymatic action. Pancreatic then hydrolyzes triglycerides into free fatty acids and monoglycerides, with colipase aiding enzyme attachment to interfaces. The resulting products, along with salts, form micelles—small aggregates that solubilize lipids for transport to the mucosal surface. Secretory glands throughout the tract produce essential fluids: the secretes approximately 2 liters of gastric juice daily, containing , pepsinogen, and , with output regulated by luminal to maintain acidity for pepsin activity. The exocrine portion delivers about 1.5 liters of bicarbonate-rich juice per day, neutralizing duodenal contents and providing , while secretion volume adjusts based on duodenal and nutrient signals. Salivary glands contribute 1-1.5 liters daily, including and mucins, and the liver produces 0.5-1 liter of , aiding processing. briefly enhances enzyme-substrate contact during these processes. Nucleotide digestion involves pancreatic nucleases, including deoxyribonuclease (targeting DNA) and ribonuclease (targeting RNA), which hydrolyze nucleic acids into nucleotides in the small intestine following initial gastric exposure to pepsin. Brush border phosphatases then further break nucleotides into nucleosides and inorganic phosphate. Vitamin processing basics align with solubility: fat-soluble vitamins (A, D, E, K) incorporate into micelles via bile emulsification for breakdown facilitation, while water-soluble vitamins (e.g., B vitamins, C) undergo minimal enzymatic alteration, relying on luminal conditions for release from food matrices.

Absorption and Motility

The small intestine serves as the principal site for nutrient absorption, where carbohydrates, proteins, and are primarily taken up following their breakdown into absorbable forms provided by digestive secretions. Glucose and are absorbed across the apical membrane of enterocytes via the sodium-glucose cotransporter 1 (SGLT1), which facilitates secondary by harnessing the sodium . Di- and tripeptides are absorbed through the proton-coupled transporter PEPT1, enabling efficient uptake of protein-derived products throughout the and . Dietary fats, emulsified into micelles, diffuse into enterocytes where they are re-esterified into triglycerides and packaged into chylomicrons for lymphatic transport, ensuring delivery to systemic circulation. In the large intestine, the primary function shifts to the reabsorption of water and electrolytes, reclaiming approximately 90% of the fluid volume that enters the colon from the ileum to form solid feces. This process maintains fluid and electrolyte homeostasis, with sodium actively transported across colonic epithelia via epithelial sodium channels and coupled mechanisms. Absorption in the gastrointestinal tract employs diverse transport mechanisms to handle a range of substrates. Active transport predominates for polar nutrients, powered by the basolateral Na+/K+ ATPase pump that maintains the sodium gradient essential for apical cotransporters like SGLT1. Passive diffusion facilitates the movement of lipophilic molecules such as fatty acids and vitamins A, D, E, and K directly across the lipid bilayer. For certain micronutrients, receptor-mediated endocytosis is utilized; vitamin B12, bound to intrinsic factor, is internalized via cubilin-amnionless receptors in the terminal ileum, followed by lysosomal release and basolateral export. Ion balances underpin much of this absorptive efficiency, with sodium-coupled transport driving nutrient uptake in the small intestine and electroneutral sodium chloride absorption in the colon mediated by Na+/H+ and Cl-/HCO3- exchangers. In the ileum and colon, the downregulated in adenoma (DRA) exchanger facilitates Cl-/HCO3- exchange, coupling with NHE3 to enable vectorial NaCl absorption without net charge movement. Gastrointestinal motility encompasses coordinated muscular contractions that propel and mix luminal contents to optimize exposure to absorptive surfaces. involves propagating waves of circular that advance distally, ensuring progressive transit from to colon. , characterized by localized ring-like constrictions, rhythmically mix contents in the to enhance nutrient contact with the mucosa without net propulsion. During fasting, the (MMC) generates cyclical bursts of low-amplitude contractions that sweep residual debris and bacteria aborally, preventing stagnation in the . In the colon, mass movements—infrequent, high-amplitude peristaltic waves occurring 2-3 times daily—propel fecal matter toward the for storage and eventual . Overall gastrointestinal transit time typically ranges from 24 to 72 hours for a solid meal, with the majority spent in the colon; this duration is influenced by dietary factors, where increased intake accelerates colonic transit by adding bulk and stimulating water retention.

Neural and Hormonal Regulation

The (ENS), often referred to as the "second ," comprises an intrinsic network of approximately 200–600 million neurons embedded within the gastrointestinal tract wall, enabling semi-autonomous control of digestive functions. This system includes two primary plexuses: the myenteric (Auerbach's) plexus, located between the longitudinal and circular muscle layers, which primarily regulates gastrointestinal motility and ; and the submucosal (Meissner's) plexus, situated in the , which modulates glandular secretion, local blood flow, and mucosal absorption. The ENS operates largely independently but receives modulatory input from extrinsic innervation, including parasympathetic fibers from the (cranial nerve X), which provide excitatory effects on motility and secretion via preganglionic synapses in the myenteric and submucosal plexuses, and sympathetic fibers from originating in the thoracic , which generally exert inhibitory control over motility while enhancing . Hormonal complements neural control through enteroendocrine cells scattered across the gastrointestinal mucosa, releasing peptides in response to luminal contents to coordinate and . , secreted by G cells in the gastric antrum and , stimulates (HCl) from parietal cells to facilitate protein and promotes mucosal growth. , produced by S cells in the duodenal mucosa, is triggered by low pH (below 4.5) and fatty acids, inhibiting gastric acid release while stimulating bicarbonate from the and bile ducts to neutralize duodenal . Cholecystokinin (CCK), released by I cells in the and upon detection of fats and proteins, induces gallbladder contraction for release, pancreatic enzyme for lipid and protein breakdown, and slows gastric emptying to optimize duodenal processing. , originating from endocrine cells in the upper small intestine, enhances gastrointestinal by initiating the during fasting, clearing residual contents to prepare for the next meal. Several reflexes and feedback loops integrate neural and hormonal signals to maintain coordinated gastrointestinal activity. The , activated by gastric distension following a meal, increases colonic motility through enteric neural pathways involving muscarinic receptors and hormones like CCK and serotonin, promoting mass movements that propel contents toward the and often eliciting the urge to defecate. The enterogastric reflex provides inhibitory feedback, where duodenal acidification ( 3–4) or hypertonicity from triggers local enteric neurons and hormonal signals to reduce gastric motility, acid secretion, and emptying, preventing duodenal overload. This pH-sensitive mechanism in the exemplifies a key feedback loop, where acid-sensing receptors on enterocytes release and CCK to further inhibit gastric activity via both local and vagal pathways. Integration with the (CNS) occurs primarily through , which constitute about 80–90% of traffic and relay signals from gastrointestinal mechanoreceptors and chemoreceptors to the nucleus tractus solitarius in the . These afferents detect presence, distension, and hormones like CCK and , transmitting inhibitory signals to hypothalamic centers to suppress and modulate feeding behavior, thereby linking peripheral gut status to whole-body .

Microbiota and Immunity

Gastrointestinal Microbiome

The human gastrointestinal comprises approximately 101410^{14} microbial cells, primarily , that inhabit the digestive tract and play a pivotal role in host . This microbial community is dominated by two major bacterial phyla, Firmicutes and Bacteroidetes, which together constitute over 90% of the total bacterial population in healthy adults. The composition of this microbiome is dynamic and influenced by host factors such as age and diet; for instance, microbial diversity typically increases from infancy to adulthood but may decline in older age, while dietary patterns like high-fiber intake promote Bacteroidetes abundance. Other phyla, including Actinobacteria, Proteobacteria, and Verrucomicrobia, are present in lower proportions but contribute to overall functional diversity. Microbial density varies significantly along the gastrointestinal tract, reflecting differences in , oxygen levels, and transit time. In the and proximal , populations are sparse at 10310^3 to 10410^4 bacteria per milliliter due to acidic conditions and rapid flow. Density escalates in the distal to around 10710^7 to 10810^8 per gram, but the colon harbors the highest concentrations, with up to 101110^{11} to 101210^{12} per gram of content, and fecal samples typically containing about 101110^{11} per gram. This supports specialized microbial niches, where colonic thrive anaerobically on undigested substrates. The gut microbiome exerts profound metabolic influences, fermenting indigestible dietary fibers into (SCFAs) such as , propionate, and butyrate, which collectively account for over 95% of microbial-derived SCFAs in the colon. Butyrate, produced mainly by Firmicutes species like prausnitzii, serves as the primary energy source for colonocytes, supporting epithelial barrier integrity and reducing inflammation. Additionally, certain gut synthesize essential nutrients, including (via species like and ) and B vitamins such as B1, B6, B12, and , which are absorbed by the host despite limited from colonic production. The also confers protection against pathogens through colonization resistance, wherein commensal outcompete invaders for nutrients and adhesion sites, thereby preventing overgrowth of harmful species like or difficile. Dysbiosis, characterized by shifts in microbial composition, is implicated in metabolic disorders; for example, is associated with an elevated Firmicutes/Bacteroidetes ratio, which enhances energy harvest from diet and correlates with increased adiposity. Antibiotics disrupt this balance by selectively depleting susceptible taxa, significantly reducing overall diversity and promoting pathogen susceptibility for weeks to months post-treatment. Recent advances since 2020 underscore therapeutic potential: the gut modulates responses to , with higher alpha-diversity and taxa like linked to improved efficacy of inhibitors in and patients. As of 2025, studies continue to link higher abundance of with improved responses to PD-1 blockade in non-small cell . Fecal microbiota transplantation (FMT) has emerged as a highly effective intervention for recurrent infections, achieving clinical resolution rates of approximately 75-85%, as per 2024 clinical guidelines.

Immune Roles

The gastrointestinal tract plays a pivotal role in mucosal immunity by establishing physical and chemical barriers that prevent invasion while promoting tolerance to harmless antigens. These defenses are orchestrated through the (GALT), which includes organized structures like Peyer's patches and diffuse immune cells that sample luminal contents and initiate adaptive responses. The mucosal thus balances protection against pathogens and maintenance of , preventing excessive . Mucosal barriers form the first line of defense, comprising a layer secreted by goblet cells that physically separates the from luminal contents, trapping potential pathogens and facilitating their removal. Tight junctions between epithelial cells, involving proteins such as claudins, occludins, and zonula occludens-1, regulate paracellular permeability to block unauthorized entry of antigens while permitting nutrient absorption. Additionally, Paneth cells in the crypts secrete , including α-defensins and β-defensins, which exhibit broad-spectrum bactericidal activity to limit microbial colonization near the . Key immune cells reside within the GI tract to monitor and respond to threats. Intraepithelial lymphocytes (IELs), primarily T cells expressing CD8αα or TCRγδ, patrol the epithelial layer at a density of 10–20 per 100 epithelial cells, contributing to barrier integrity and rapid cytotoxicity against infected cells. Peyer's patches, multifollicular lymphoid aggregates in the small intestine submucosa, feature follicle-associated epithelium with M cells that sample antigens via pinocytosis and receptor-mediated endocytosis, such as GP2 for specific bacterial types, delivering them to underlying dendritic cells and lymphocytes for activation. These structures organize GALT, including B-cell follicles with germinal centers for antibody production and T-cell-rich interfollicular regions, enabling localized adaptive immunity. Secretory immunoglobulin A (IgA), produced as dimers by plasma cells in the lamina propria, is transported across the epithelium via the polymeric immunoglobulin receptor, neutralizing luminal pathogens and toxins without triggering inflammation. This process is supported by dendritic cells and factors like APRIL and BAFF, ensuring high concentrations of IgA in mucosal secretions. Tolerance mechanisms prevent aberrant responses to food antigens and commensals, primarily through regulatory T cells (Tregs) expressing FOXP3, which secrete anti-inflammatory cytokines like IL-10 and TGF-β to suppress effector T cells. CD103+ dendritic cells in the lamina propria educate naive T cells toward tolerance by producing retinoic acid and TGF-β, promoting Treg differentiation and inhibiting pro-inflammatory Th17 cells. These interactions occur within GALT sites, fostering immune hyporesponsiveness. Inflammation pathways are tightly regulated to maintain barrier function. Toll-like receptors (TLRs) on epithelial and immune cells recognize pathogen-associated patterns, activating NF-κB signaling to induce antimicrobial defenses or cytokines such as TNF-α and IL-12, which reinforce epithelial integrity during threats. Cytokines like IL-23 further modulate responses, ensuring controlled inflammation that supports homeostasis without chronic activation.

Variations in Other Animals

Structural Adaptations

The gastrointestinal tract exhibits diverse structural adaptations across animal taxa, shaped by dietary habits and environmental demands to optimize extraction and processing efficiency. In carnivores, the tract is typically short and simple, facilitating rapid digestion of protein-rich, easily digestible prey. For instance, in domestic cats, the combined length of the small and large intestines is approximately 3 to 4 times the body length, allowing quick transit of meat-based meals to minimize bacterial overgrowth from nutrient-dense . Carnivores generally possess short, smooth intestines measuring 3–6 times body length for rapid processing of meat, whereas human intestines are longer and more complex, with relative lengths resembling those of herbivores to accommodate a mixed diet. Their stomachs are highly acidic, with levels often below 2, enabling effective breakdown of proteins and killing of pathogens ingested from raw animal tissues. Herbivores, in contrast, possess elongated colons and enlarged ceca to accommodate the slow breakdown of fibrous material through microbial . In , the large colon features a complex, looped structure with haustral pouches formed by teniae coli, enhancing surface area for of in the , where up to 70% of dietary energy is derived from volatile fatty acids produced by gut microbes. Rabbits exemplify with a massively enlarged , comprising about 60% of the total gastrointestinal volume, which serves as a fermentation chamber for digestion and nutrient recycling via caecotrophy. Ruminants display one of the most specialized adaptations in the form of a four-chambered , enabling pregastric of matter. The , the largest chamber, functions as a microbial vat for initial breakdown of ; the aids in mixing and regurgitation; the absorbs water and volatile fatty acids; and the acts as the true glandular for enzymatic digestion. This configuration allows efficient utilization of low-quality , with the alone holding 80-90% of the 's volume in adult . In birds, the gastrointestinal tract includes a for temporary food storage and moistening, compensating for the absence of cheeks or prolonged chewing. The , a muscular organ lined with koilin and often containing grit, mechanically grinds tough and insects, while the serves as a unified chamber for digestive, urinary, and reproductive waste elimination, streamlining in flight-adapted . Aquatic vertebrates show streamlined adaptations suited to their medium and diet. Many possess relatively short gastrointestinal tracts compared to terrestrial counterparts, with carnivorous species like teleosts having intestines 1-3 times body length to expedite processing of sporadic, high-protein meals. and rays compensate for their short intestines—often less than half the body length—with a , a helical fold of mucosa that significantly increases absorptive surface area and regulates digesta flow unidirectionally. These variations reflect evolutionary pressures for efficient energy capture in nutrient-variable aquatic environments.

Functional Differences

Functional differences in the gastrointestinal tract across non-human animals reflect adaptations to diverse diets, environments, and lifestyles, particularly in , absorption, , and interactions. Foregut fermenters, such as ruminants like cows, conduct microbial in the prior to the , allowing them to derive approximately 80% of their energy from microbial breakdown of fibrous plant material into volatile fatty acids. In contrast, hindgut fermenters, including and rabbits, perform in the and colon after initial enzymatic , which limits direct access to microbial proteins but enables efficient processing of cellulose-rich diets post-absorption of simple nutrients. Koalas exemplify specialized hindgut , where in the enlarged detoxify and break down toxic leaves, enabling these marsupials to extract nutrients from a diet that is otherwise indigestible to most herbivores. Absorption efficiencies vary markedly to conserve resources in challenging habitats. Desert-adapted , such as rats, exhibit enhanced in the colon, producing extremely dry with minimal loss—often reabsorbing over 90% of ingested through aquaporin-mediated in the —to survive without free intake. This adaptation contrasts with more mesic species, where lower rates suffice due to abundant availability. Motility patterns are tailored to nutritional needs and feeding behaviors. Rabbits engage in coprophagy, selectively reingesting soft cecotropes produced in the , which allows recycling of bacterial-synthesized and contributing approximately 20% to total intake, thereby maximizing nutrient extraction from low-quality . Seed-eating birds, such as , demonstrate rapid gut transit times of 12–15 minutes for seeds, facilitating quick passage to avoid toxin accumulation while still allowing partial enzymatic breakdown of husks for energy. Microbiota play pivotal roles in enabling unique digestive capabilities. In termites, flagellate protozoa in the hindgut form mutualistic symbioses that produce cellulases and lignases, essential for degrading lignocellulose in wood and extracting fermentable sugars that the host cannot produce alone. Cetaceans, adapted to piscivorous diets high in lipids, harbor gut microbiota that facilitate the hydrolysis and absorption of fish oils and wax esters, with bacterial communities in species like bowhead whales aiding the microbial breakdown of wax esters, which comprise over 80% of their prey's lipids and are reduced by more than 50% during digestion in the midgut. Environmental factors further modulate gastrointestinal function. During hibernation, bears experience a profound slowdown in and microbial activity in the gut, reducing and transit to conserve energy over months without food intake, accompanied by shifts in composition that support metabolic suppression.

Clinical Aspects

Common Disorders

The gastrointestinal tract is afflicted by numerous common disorders that range from inflammatory and functional conditions to infectious, neoplastic, and motility-related pathologies, collectively imposing a substantial burden on through symptoms such as and altered bowel habits. Inflammatory bowel diseases (IBD) primarily comprise and , both characterized by chronic inflammation driven by genetic, environmental, and immunological factors. features transmural inflammation that may involve any segment of the gastrointestinal tract, with the terminal being the most frequent site, leading to complications like strictures and fistulas. , in contrast, is confined to continuous mucosal inflammation starting from the and extending proximally in the colon. Key risk factors include genetic susceptibility, with over 200 associated loci identified, and modifiable elements like cigarette smoking, which exacerbates while paradoxically offering protection against . Irritable bowel syndrome (IBS) represents a prevalent without structural abnormalities, diagnosed according to the Rome IV criteria: recurrent , on average at least 1 day per week in the last 3 months, associated with two or more of the following—pain related to , a change in stool frequency, or a change in stool form (as per the ). This condition affects approximately 10-15% of the global population, with higher rates in Western countries and variations by subtype (e.g., IBS with or ), contributing to significant reductions in . Infectious disorders of the gastrointestinal tract include acute , the leading cause of which is among viral pathogens, responsible for the majority of outbreaks worldwide, while enterotoxigenic Escherichia coli is a primary bacterial culprit, particularly in traveler's diarrhea and foodborne illnesses. Another major infectious entity is Helicobacter pylori-associated , where infection accounts for 90-95% of duodenal ulcers through mechanisms involving production, mucosal damage, and hypergastrinemia leading to acid hypersecretion. Neoplastic disorders prominently feature , which progresses via the adenoma-carcinoma sequence—a multistep process initiated by inactivating mutations in the , promoting polyp formation and eventual . This pathway underscores the role of genetic instability in , with environmental factors like diet amplifying risk; routine screening is advised beginning at age 45 for average-risk adults to detect precancerous adenomas early. Motility disorders disrupt normal propulsion and sphincter function; gastroesophageal reflux disease (GERD) arises from incompetence of the lower esophageal sphincter, often due to transient relaxations allowing acid reflux into the , affecting up to 20% of the population in developed nations. Achalasia, a rarer primary , results from degeneration of inhibitory neurons in the , causing failure of lower esophageal sphincter relaxation and aperistalsis, which leads to progressive esophageal dilation and food retention. Epidemiological trends highlight the rising incidence of metabolic dysfunction-associated steatotic liver disease (MASLD; formerly known as non-alcoholic fatty liver disease [NAFLD]), closely tied to the global epidemic, with prevalence estimates reaching 30-38% among adults as of 2025. This is driven by components like and visceral adiposity.

Diagnostic and Therapeutic Approaches

Diagnostic approaches to gastrointestinal () tract disorders encompass a range of invasive and non-invasive methods to evaluate structural, functional, and inflammatory abnormalities. , including upper gastrointestinal endoscopy and , remains a cornerstone for direct visualization and biopsy of mucosal lesions, with the American Society for Gastrointestinal Endoscopy (ASGE) recommending careful evaluation and photo-documentation for conditions like (GERD). Biopsies obtained during these procedures allow histopathological analysis to confirm diagnoses such as (IBD) or malignancy. Imaging modalities, including computed tomography (CT) and (), are utilized to detect strictures, masses, or complications like abscesses, while is particularly effective for assessing . Stool-based tests, such as fecal calprotectin measurement, provide a non-invasive marker for intestinal inflammation, with elevated levels indicating active IBD or other organic . Fecal immunochemical testing (FIT) detects occult blood to screen for , offering high sensitivity for early detection in at-risk populations. Functional testing evaluates motility and absorption disorders through specialized techniques. Esophageal, anorectal, or colonic manometry measures pressure and coordination of muscle contractions to diagnose conditions like achalasia or dyssynergic defecation. Hydrogen breath tests assess carbohydrate malabsorption, such as , or (SIBO) by detecting elevated hydrogen or levels post-ingestion of substrates like or glucose. These tests are non-invasive and guide targeted interventions by quantifying transit times and bacterial activity. Therapeutic strategies for GI disorders integrate pharmacological, surgical, and nutritional interventions tailored to the underlying . inhibitors (PPIs), such as omeprazole, suppress production to manage acid-related conditions like GERD and peptic ulcers, with long-term use requiring monitoring for complications. For IBD, anti-tumor necrosis factor (TNF) biologics like induce and maintain remission by neutralizing inflammatory cytokines, demonstrating mucosal healing in moderate-to-severe cases. Surgical options include for resection, often performed laparoscopically to remove affected bowel segments while preserving function, and for refractory GERD, which wraps the gastric fundus around the to reinforce the lower esophageal sphincter. Nutritional , particularly enteral feeding via nasogastric or tubes, supports patients with , , or severe IBD flares by delivering calories directly to the GI tract, improving outcomes over when gut integrity allows. Emerging advancements as of 2025 enhance precision in both and treatment. AI-assisted automates lesion detection in the small bowel, achieving high accuracies such as over 97% for certain abnormalities like protruding lesions, reducing reading times and improving polyp identification during screening. Microbiome-targeted therapies, including fecal microbiota transplantation (FMT), restore gut in recurrent and IBD, with post-FMT protocols emphasizing donor screening and follow-up to sustain engraftment and remission rates exceeding 80% in select cases. Gene editing technologies, such as /Cas9, are being tested in early-phase trials for advanced GI cancers, demonstrating potential to halt tumor growth in some patients. Professional guidelines from organizations like the American Gastroenterological Association (AGA) and ASGE standardize these approaches, recommending starting at age 45 for average-risk screening and annual FIT for non-responders to improve adherence and equity. For IBD management, AGA endorses early biologic initiation and to optimize outcomes, while ASGE emphasizes quality metrics for units to ensure procedural safety and efficacy. These frameworks integrate diagnostics and therapeutics to address GI disorders holistically.

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