Gastrointestinal tract
Gastrointestinal tract | |
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Details | |
System | Digestive system |
Identifiers | |
Latin | tractus digestorius (mouth to anus), canalis alimentarius (esophagus to large intestine), canalis gastrointestinales stomach to large intestine) |
MeSH | D041981 |
Anatomical terminology |
Major parts of the |
Gastrointestinal tract |
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The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the
The human gastrointestinal tract consists of the
The gastrointestinal tract contains the
Human gastrointestinal tract
Structure
The structure and function can be described both as
Upper gastrointestinal tract
The upper gastrointestinal tract consists of the mouth, pharynx, esophagus, stomach, and duodenum.[13] The exact demarcation between the upper and lower tracts is the
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.[14] This is a thin muscle which is derived from the embryonic mesoderm.
Lower gastrointestinal tract
The lower gastrointestinal tract includes most of the
Small intestine
The
- 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 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.
- bile acids, as well as any other remaining nutrients.
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, 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).[19] Its main function is to absorb water and salts. The colon is further divided into:
- Cecum (first portion of the colon) and 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
The gut is an
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.
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
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 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
The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy.[22] The GI tract can be divided into four concentric layers in the following order:
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:
- 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
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.
Muscular layer
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.[23] 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
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 faeces.[23]
Adventitia and serosa
The outermost layer of the gastrointestinal tract consists of several layers of connective tissue.
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.
Transit time
The time taken for food to transit through the gastrointestinal tract varies on multiple factors, including age, ethnicity, and gender.
Immune function
The gastrointestinal tract forms an important part of the immune system.[34]
Immune barrier
The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court.[19] 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.[35] 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.[36] 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
Immune system homeostasis
Beneficial bacteria also can contribute to the homeostasis of the gastrointestinal immune system. For example,
Intestinal microbiota
The large intestine contains multiple types of
Beneficial
Detoxification and drug metabolism
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.[42]
Other animals
In most
During early development the asymmetric position of the bowels and inner organs is initiated (see also axial twist theory).
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. 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
A through-gut (one with both mouth and anus) is thought to have evolved within the
Clinical significance
Diseases
There are many diseases and conditions that can affect the gastrointestinal system, including infections, inflammation and cancer.
Various
- 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 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 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 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.
Symptoms
Several symptoms can indicate problems with the gastrointestinal tract, including:
- vomiting of blood
- Diarrhea, or the passage of liquid or more frequent stools
- Constipation, which refers to the passage of fewer and hardened stools
- tarry-coloured blood
Treatment
Imaging
Various methods of imaging the gastrointestinal tract include the upper and lower gastrointestinal series:
- 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. Biopsies may also be taken when examined.
- An abdominal x-ray may be used to examine the lower gastrointestinal tract.
- Cholera
- Enteric duplication cyst
- Giardiasis
- Pancreatitis
- Peptic ulcer disease
- Yellow fever
- 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.[49][50][51][52]
- Intestinal pseudo-obstructionis 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.
- Enterovirusesare 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 Gibbis 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
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 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.[53]
- Sheep gut was the original source for natural gut string used in cowgut.
- 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" casings, are made of animal gut, especially hog, beef, and lamb.
- The wrapping of gardoubakia, and torcinellois 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.[54]
See also
- Gastrointestinal physiology
- Gut-on-a-chip
- All pages with titles beginning with Gastrointestinal
- All pages with titles containing Gastrointestinal
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