Gastrointestinal tract/ja: Difference between revisions

=====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.

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==== 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]].

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====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.

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====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]].

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=====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.

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====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]].

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====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.

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==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]].

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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.

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==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}}

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===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.

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===Symptoms===

* [[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]]

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===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.

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===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.

* [[Bowel twist/ja|腸捻転]]（あるいは同様に腸絞扼）は比較的まれな事象である（通常、腸の大手術の後に発症する）。しかし、正しく診断するのは難しく、放置すると腸[[infarction/ja|梗塞]]を引き起こし死に至ることもある。(歌手の[[:en:Maurice Gibb|モーリス・ギブ]]はこれが原因で死亡したとされている）。