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{{Short description|Organ system within humans and other animals}}
{{Infobox anatomy
| Name        = Gastrointestinal tract
| Latin       = tractus digestorius ([[mouth]] to [[anus]]),<br />canalis alimentarius ([[esophagus]] to [[large intestine]]),<br />canalis gastrointestinales [[stomach]] to [[large intestine]])
| Image       = Stomach_colon_rectum_diagram-en.svg
| Caption     = Diagram of [[stomach]], intestines and [[rectum]] in the average [[human]]
| System      = [[Digestion|Digestive system]]
}}
{{Gastrointestinal tract sidebar}}
The '''gastrointestinal tract''' ('''GI tract''', '''digestive tract''', '''alimentary canal''') is the tract or passageway of the [[Digestion|digestive system]] that leads from the [[mouth]] to the [[anus]]. The GI tract contains all the major [[organ (biology)|organ]]s of the digestive system, in humans and other animals, including the [[esophagus]], [[stomach]], and [[intestines]]. Food taken in through the mouth is [[digestion|digested]] to extract [[nutrient]]s and absorb [[energy]], and the waste expelled at the anus as [[Feces|faeces]]. ''Gastrointestinal'' is an adjective meaning of or pertaining to the stomach and intestines.

[[Nephrozoa|Most animals]] have a "through-gut" or complete digestive tract. Exceptions are more primitive ones: [[sponge]]s have small pores ([[ostium (sponges)|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.

The human gastrointestinal tract consists of the [[esophagus]],  stomach, and intestines, and is divided into the upper and lower gastrointestinal tracts. The GI tract includes all structures between the [[human mouth|mouth]] and the [[Human anus|anus]], The tract may also be divided into [[foregut]], [[midgut]], and [[hindgut]], reflecting the [[embryology|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 spots to allow for local distention and [[peristalsis]].

The gastrointestinal tract contains the [[gut microbiota]], with some 1,000 different [[strain (biology)|strains]] of [[bacteria]] having diverse roles in the maintenance of [[immune system|immune health]] and [[Gut microbiota#Metabolism|metabolism]], and many other [[microorganism]]s. Cells of the GI tract release [[hormone]]s to help regulate the digestive process. These [[Digestion#Digestive hormones|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]].

==Human gastrointestinal tract==
===Structure===
{{Digestive system diagram}}
The structure and function can be described both as [[gross anatomy]] and as [[microscopic anatomy]] or [[histology]]. The tract itself is divided into upper and lower tracts, and the intestines [[small intestine|small]] and [[large intestine|large]] parts.

====Upper gastrointestinal tract====
{{Main|Mouth|Pharynx|Esophagus|Stomach|duodenum}}
The upper gastrointestinal tract consists of the [[Human mouth|mouth]], [[pharynx]], [[esophagus]], [[stomach]], and [[duodenum]].
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): [[duodenal bulb|bulb]], descending, horizontal, and ascending. The suspensory muscle attaches the superior border of the ascending duodenum to the [[thoracic diaphragm|diaphragm]].

The suspensory muscle is an important anatomical landmark that shows the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively. This is a thin muscle which is derived from the [[embryo]]nic [[mesoderm]].

====Lower gastrointestinal tract====
The lower gastrointestinal tract includes most of the [[small intestine]] and all of the [[large intestine]]. 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 [[Human anus|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]] while the large intestine is subdivided into the [[cecum]], ascending, transverse, descending, and sigmoid [[colon (anatomy)|colon]], [[rectum]], and [[anal canal]].

=====Small intestine=====
[[File:Blausen 0817 SmallIntestine Anatomy.png|thumb|240px|Illustration of the small intestine]]
{{Main|Small intestine|Duodenum|Jejunum|Ileum}}
The [[small intestine]] begins at the [[duodenum]] and is a tubular structure, usually between 6 and 7 m long. Its [[mucosal]] area in an adult human is about {{convert|30|m2|abbr=on}}. 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 {{Convert|250|sqm|sqft|abbr=on}} for the entire small intestine. Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream. There are three major divisions:
# [[Duodenum]]: A short structure (about 20–25&nbsp;cm long) 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 [[emulsion|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.
# [[Jejunum]]: This is the midsection of the small intestine, connecting the duodenum to the ileum. It is about {{Convert|2.5|m|ft|abbr=on}} long and contains the [[circular folds]] also known as plicae circulares and [[intestinal villus|villi]] that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here.
# [[Ileum]]: The final section of the small intestine. It is about 3 m long, and contains [[intestinal villus|villi]] similar to the jejunum. It absorbs mainly [[vitamin B12]] and [[bile acids]], as well as any other remaining nutrients.

=====Large intestine=====
{{Main|Large intestine}}

The [[large intestine]], also called the colon, forms an arch starting at the [[cecum]] and ending at the [[rectum]] and [[anal canal]]. It also includes the [[appendix (anatomy)|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 {{convert|2|m2|abbr=on}}. Its main function is to absorb water and salts. The colon is further divided into:
# [[Cecum]] (first portion of the colon) and [[Appendix (anatomy)|appendix]]
# [[Ascending colon]] (ascending in the back wall of the abdomen)
# [[Right colic flexure]] (flexed portion of the ascending and transverse colon apparent to the [[liver]])
# [[Transverse colon]] (passing below the diaphragm)
# [[Left colic flexure]] (flexed portion of the transverse and descending colon apparent to the [[spleen]])
# [[Descending colon]] (descending down the left side of the abdomen)
# [[Sigmoid colon]] (a loop of the colon closest to the rectum)
# [[Rectum]]
# [[Anal canal]]

====Development====
{{main|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 [[ventral]]ly (with the embryo's ventral surface becoming [[concave polygon|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]].

During [[fetus|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.

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 (anatomy)|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.

{| class="wikitable"
|-
! 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<br />(Though the spleen is supplied by the [[Celiac artery|celiac trunk]], it is derived from dorsal mesentery and therefore not a foregut derivative) || celiac trunk
 |-1
 | [[Midgut]] || lower duodenum, to the first two-thirds of the transverse colon || lower [[duodenum]], [[jejunum]], [[ileum]], [[cecum]], [[vermiform appendix|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====
{{Main|Gastrointestinal wall}}
[[File:Layers of the GI Tract numbers.svg|thumb|300px|General structure of the gut wall
{{unbulleted list
|'''1:''' ''Mucosa:'' Epithelium
|'''2:''' ''Mucosa:'' Lamina propria
|'''3:''' ''Mucosa:'' Muscularis mucosae
|'''4:''' Lumen
|'''5:''' Lymphatic tissue
|'''6:''' Duct of gland outside tract
|'''7:''' Gland in mucosa
|'''8:''' ''Submucosa''
|'''9:''' Glands in submucosa
|'''10:''' Meissner's submucosal plexus
|'''11:''' Vein
|'''12:''' ''Muscularis:'' Circular muscle
|'''13:''' ''Muscularis:'' Longitudinal muscle
|'''14:''' ''Serosa:'' Areolar connective tissue
|'''15:''' ''Serosa:'' Epithelium
|'''16:''' Auerbach's myenteric plexus
|'''17:''' Nerve
|'''18:''' Artery
|'''19:''' Mesentery
}}]]
The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy. The GI tract can be divided into four concentric layers in the following order:
* [[Mucous membrane|Mucosa]]
* [[Submucosa]]
* [[Muscular layer]]
* [[Adventitia]] or [[Serous membrane|serosa]]

=====Mucosa=====
{{See also|Oral mucosa|Gastric mucosa}}
The mucosa is the innermost layer of the gastrointestinal tract. The mucosa surrounds the [[lumen (anatomy)|lumen]], or open space within the tube. This layer comes in direct contact with digested food ([[chyme]]). The mucosa is made up of:
* [[Intestinal epithelium|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=====
{{Main|Submucosa}}
The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa and [[Muscular layer|muscularis externa]]. It contains the [[submucosal plexus]], an [[enteric nervous system|enteric nervous plexus]], situated on the inner surface of the ''muscularis externa''.

=====Muscular layer=====
The [[muscular layer]] consists of an inner circular layer and a [[Anatomical terms of location|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. 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 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]].

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 [[Human feces|faeces]].

=====Adventitia and serosa=====
{{Main|Serous membrane|Adventitia}}
The outermost layer of the gastrointestinal tract consists of several layers of [[connective tissue]].

[[Peritoneum#Classification of abdominal structures|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 [[Vermiform appendix|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]].

[[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 [[human mouth|oral cavity]] has adventitia.

==== Gene and protein expression ====
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. 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 [[Pepsin|pepsinogen PGC]] and the [[Lipase|lipase LIPF]], expressed in [[Gastric chief cell|chief cells]], and gastric [[ATPase|ATPase ATP4A]] and [[Gastric intrinsic factor|gastric intrinsic factor GIF]], expressed in [[parietal cell]]s 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]].

====Transit time====
The time taken for food to transit through the gastrointestinal tract varies on multiple factors, including age, ethnicity, and gender. Several techniques have been used to measure transit time, including radiography following a [[barium]]-labeled meal, breath [[hydrogen]] analysis, [[scintigraphic]] analysis following a [[radionuclide|radiolabeled]] meal, and simple ingestion and spotting of [[maize|corn kernel]]s. It takes 2.5 to 3 hours for 50% of the contents to leave the stomach. 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. Total emptying of the stomach takes around 4–5 hours, and transit through the colon takes 30 to 50 hours.

====Immune function====
The gastrointestinal tract forms an important part of the [[immune system]].

=====Immune barrier=====
The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court. With such a large exposure (more than three times larger than the [[Human skin|exposed surface of the skin]]), these immune components function to prevent pathogens from entering the blood and lymph circulatory systems. 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. 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 [[microorganism]]s that enter it. Similarly, [[mucus]] (containing [[IgA]] [[antibody|antibodies]]) neutralizes many pathogenic microorganisms. Other factors in the GI tract contribution to immune function include [[enzyme]]s secreted in the [[saliva]] and [[bile]].

=====Immune system homeostasis=====
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. It has been demonstrated that the intake of a high fiber diet could be responsible for the induction of [[T-regulatory cell]]s (Tregs). This is due to the production of [[short-chain fatty acid]]s 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#Protein acetylation|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.

====Intestinal microbiota====
The large intestine contains multiple types of [[bacteria]] that can break down molecules the human body cannot process alone, demonstrating a [[symbiosis|symbiotic]] relationship. These bacteria are responsible for gas production at [[host–pathogen interface]], which is released as [[flatulence]]. 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.

Beneficial [[Gut flora|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]]. An imbalanced ratio results in [[dysbiosis]].

====Detoxification and drug metabolism====
[[Enzyme]]s such as [[CYP3A4]], along with the [[antiporter]] activities, are also instrumental in the intestine's role of [[drug metabolism]] in the detoxification of [[antigen]]s and [[xenobiotic]]s.

==Other animals==
In most [[vertebrates]], including [[fish]]es, [[amphibian]]s, [[bird]]s, [[reptile]]s, and [[monotreme|egg-laying mammals]], the gastrointestinal tract ends in a [[cloaca]] and not an [[anus]]. In the cloaca, the [[urinary system]] is fused with the genito-anal pore. [[Theria]]ns (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.

During [[Development of the digestive system|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 [[Digestive system of ruminants|additional stomach compartments]].

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

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

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 (geology)|period]] about 550 million years ago.

A through-gut (one with both mouth and anus) is thought to have evolved within the [[nephrozoa]]n clade of [[Bilateria]], after their ancestral ventral orifice (single, as in [[cnidaria]]ns and [[xenacoelomorpha|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 [[Gonopore|genital opening]]). A stretched gut without the middle part closed is present in another branch of bilaterians, the extinct [[proarticulata|proarticulates]]. This and the [[embryological origins of the mouth and anus|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.

==Clinical significance==
{{hatnote|This section discusses related diseases, medical associations with the gastrointestinal tract, and use in surgery.}}
{{Main|Gastrointestinal disease|Gastroenterology}}
{{Further|Clinical significance}}

===Diseases===
There are many diseases and conditions that can affect the gastrointestinal system, including [[infection]]s, [[inflammation]] and [[cancer]].

Various [[pathogen]]s, such as [[bacteria]] that cause [[foodborne illness]]es, can induce [[gastroenteritis]] which results from [[inflammation]] of the stomach and small intestine. [[Antibiotic]]s to treat such bacterial infections can decrease the [[Microbiome of humans|microbiome]] diversity of the gastrointestinal tract, and further enable inflammatory mediators. Gastroenteritis is the most common disease of the GI tract.
* [[Gastrointestinal cancer]] may occur at any point in the gastrointestinal tract, and includes [[mouth cancer]], [[tongue cancer]], [[oesophageal cancer]], [[stomach cancer]], and [[colorectal cancer]].
* Inflammatory conditions. [[Ileitis]] is an inflammation of the [[ileum]], [[colitis]] is an inflammation of the [[large intestine]].
* [[Appendicitis]] is inflammation of the [[appendix (anatomy)|appendix]] located at the caecum. This is a potentially fatal condition if left untreated; most cases of appendicitis require surgical intervention.

[[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]].

[[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 [[autoimmunity|autoimmune disease]]. Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such.

[[Functional gastrointestinal disorder]]s 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.

===Symptoms===
Several symptoms can indicate problems with the gastrointestinal tract, including:
* [[Vomiting]], which may include [[regurgitation (digestion)|regurgitation]] of food or the [[haematemesis|vomiting of blood]]
* [[Diarrhea]], or the passage of liquid or more frequent stools
* [[Constipation]], which refers to the passage of fewer and hardened stools
* [[Blood in stool]], which includes [[Haematochezia|fresh red blood]], maroon-coloured blood, and [[melaena|tarry-coloured blood]]

===Treatment===
[[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.

===Imaging===
Various methods of imaging the gastrointestinal tract include the [[upper gastrointestinal series|upper]] and [[lower gastrointestinal series]]:
* [[Radioopaque]] dyes may be swallowed to produce a [[barium swallow]]
* Parts of the tract may be visualised by camera. This is known as [[endoscopy]] if examining the upper gastrointestinal tract and [[colonoscopy]] or [[sigmoidoscopy]] if examining the lower gastrointestinal tract. [[Capsule endoscopy]] is where a capsule containing a camera is swallowed in order to examine the tract. [[Biopsy|Biopsies]] may also be taken when examined.
* An [[abdominal x-ray]] may be used to examine the lower gastrointestinal tract.

===Other related diseases===
* [[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 (biology)|microenvironment]]: it is both [[capnophile|capnophilic]] and [[microaerophile|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.
* [[Intestinal pseudoobstruction|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.
* [[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 (medical disorder)|Intussusception]]
* [[Polyp (medicine)]] (see also [[colorectal polyp]])
* [[Pseudomembranous colitis]]
* [[Toxic megacolon]] usually a complication of ulcerative colitis

== Uses of animal guts ==
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 {{linktext|calves}}.  [[Pig]] and [[calf (animal)|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 (music)|strings]] by musicians can be traced back to the [[third dynasty of Egypt]]. In the recent past, strings were made out of [[domestic sheep|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, [[cat]]s were never used as a source for gut strings.
* Sheep gut was the original source for natural gut string used in [[racquet]]s, 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 Africa]]n [[bendir]] frame drum still uses gut for this purpose.
* "Natural" [[sausage]] hulls, or [[Casing (sausage)|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 clock]]s and for [[fusee (horology)|fusee]] movements in [[bracket clock]]s, but may be replaced by metal wire.
* The oldest known [[condom]]s, from 1640 AD, were made from animal intestine.

== See also ==
{{Anatomy-terms}}
* [[Gastrointestinal physiology]]
* [[Gut-on-a-chip]]
* {{Look from|Gastrointestinal}}
* {{In title|Gastrointestinal}}

== External links ==
{{wiktionary|gastrointestinal tract|gastrointestinal|tract}}
{{Commons category multi|Gastrointestinal tract|Digestive system}}
{{Wikibooks|Human Physiology|The gastrointestinal system}}
* [https://www.proteinatlas.org/humanproteome/gastrointestinal+tract The gastro intestinal tract in the Human Protein Atlas]
* [https://web.archive.org/web/20090507150127/http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/ Your Digestive System and How It Works at National Institutes of Health]

{{Human systems and organs}}
{{Mouth anatomy}}
{{Digestive tract}}

{{二次利用|date=20 March 2024}}
{{DEFAULTSORT:Gastrointestinal}}
[[Category:Abdomen]]
[[Category:Digestive system]]
[[Category:Endocrine system]]
[[Category:Routes of administration]]