Gastrointestinal tract: Difference between revisions

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{{Short description|Organ system within humans and other animals}}

{{Infobox anatomy

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]].

===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.

{{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]].

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]].

[[File:Blausen 0817 SmallIntestine Anatomy.png|thumb|240px|Illustration of the small intestine]]

{{Main|Small intestine|Duodenum|Jejunum|Ileum}}

# [[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.

{{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]]

# [[Anal canal]]

{{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.

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{{Main|Gastrointestinal wall}}

[[File:Layers of the GI Tract numbers.svg|thumb|300px|General structure of the gut wall

* [[Adventitia]] or [[Serous membrane|serosa]]

{{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:

* [[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.

{{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''.

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]].

{{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.

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]].

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.

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

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]].

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.

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]].

[[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.

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.

{{hatnote|This section discusses related diseases, medical associations with the gastrointestinal tract, and use in surgery.}}

{{Main|Gastrointestinal disease|Gastroenterology}}

{{Further|Clinical significance}}

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]].

* [[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.

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]]

* [[Blood in stool]], which includes [[Haematochezia|fresh red blood]], maroon-coloured blood, and [[melaena|tarry-coloured blood]]

[[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.

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]]

* An [[abdominal x-ray]] may be used to examine the lower gastrointestinal tract.

* [[Cholera]]

* [[Enteric duplication cyst]]

* [[Toxic megacolon]] usually a complication of ulcerative colitis

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 oldest known [[condom]]s, from 1640 AD, were made from animal intestine.

{{Anatomy-terms}}

* [[Gastrointestinal physiology]]

* {{In title|Gastrointestinal}}

{{wiktionary|gastrointestinal tract|gastrointestinal|tract}}

{{Commons category multi|Gastrointestinal tract|Digestive system}}

* [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}}