Gastrointestinal tract

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Intestines
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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

The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the

intestines. Food taken in through the mouth is digested to extract nutrients and absorb energy, and the waste expelled at the anus as faeces
. Gastrointestinal is an adjective meaning of or pertaining to the stomach and intestines.

acoels have a single pore for both digestion and excretion.[1][2]

The human gastrointestinal tract consists of the

smooth muscle tissue, maintain constant muscle tone in a halfway-tense state but can relax in spots to allow for local distention and peristalsis.[6][7]

The gastrointestinal tract contains the

autocrine mechanisms, indicating that the cells releasing these hormones are conserved structures throughout evolution.[11]

Human gastrointestinal tract

Structure

The structure and function can be described both as

microscopic anatomy or histology. The tract itself is divided into upper and lower tracts, and the intestines small and large parts.[12]

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

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 attaches the superior border of the ascending duodenum to the 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.[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

mucosal area in an adult human is about 30 m2 (320 sq ft).[19] 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.[20]
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. 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. 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:

  1. Cecum (first portion of the colon) and appendix
  2. Ascending colon (ascending in the back wall of the abdomen)
  3. Right colic flexure (flexed portion of the ascending and transverse colon apparent to the liver
    )
  4. Transverse colon (passing below the diaphragm)
  5. Left colic flexure (flexed portion of the transverse and descending colon apparent to the spleen
    )
  6. Descending colon (descending down the left side of the abdomen)
  7. Sigmoid colon (a loop of the colon closest to the rectum)
  8. Rectum
  9. Anal canal

Development

The gut is an

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 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

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.[21]

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

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.[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

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.[23]

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.

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 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.

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.[26]

Transit time

The time taken for food to transit through the gastrointestinal tract varies on multiple factors, including age, ethnicity, and gender.

scintigraphic analysis following a radiolabeled meal,[29] and simple ingestion and spotting of corn kernels.[30] 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. [31] Total emptying of the stomach takes around 4–5 hours, and transit through the colon takes 30 to 50 hours.[29][32][33]

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

IgA antibodies) neutralizes many pathogenic microorganisms.[38] Other factors in the GI tract contribution to immune function include enzymes secreted in the saliva and bile
.

Immune system homeostasis

Beneficial bacteria also can contribute to the homeostasis of the gastrointestinal immune system. For example,

T cells
, resulting in the reduction of the inflammatory response and allergies.

Intestinal microbiota

The large intestine contains multiple types of

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. [41]

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

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

vertebrates, including fishes, amphibians, birds, reptiles, and 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. 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.

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

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. 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

period about 550 million years ago.[43][44]

A through-gut (one with both mouth and anus) is thought to have evolved within the

roundworms) are considered to support this hypothesis.[45][46]

Clinical significance

Diseases

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

Various

microbiome diversity of the gastrointestinal tract, and further enable inflammatory mediators.[47]
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.

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

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.[48]

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.

Other related diseases

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:

See also

References

  1. ^ "Overview of Invertebrates". www.ck12.org. 6 October 2015. Retrieved 25 June 2021.
  2. .
  3. ^ "gastrointestinal tract" at Dorland's Medical Dictionary
  4. ^ Gastrointestinal+tract at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
  5. ^ "digestive system" at Dorland's Medical Dictionary
  6. S2CID 33366428
    .
  7. .
  8. .
  9. .
  10. .
  11. ^ Nelson RJ. 2005. Introduction to Behavioral Endocrinology. Sinauer Associates: Massachusetts. p 57.
  12. ^ Thomasino, Anne Marie (2001). "Length of a Human Intestine". The Physics Factbook.
  13. ^ Upper+Gastrointestinal+Tract at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
  14. . Retrieved 1 July 2010.
  15. ^ Lower+Gastrointestinal+Tract at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
  16. ^ Kapoor, Vinay Kumar (13 Jul 2011). Gest, Thomas R. (ed.). "Large Intestine Anatomy". Medscape. WebMD LLC. Retrieved 2013-08-20.
  17. ^ Gray, Henry (1918). Gray's Anatomy. Philadelphia: Lea & Febiger.
  18. ^ .
  19. ^ .
  20. .
  21. .
  22. .
  23. ^ .
  24. ^ "The human proteome in gastrointestinal tract - The Human Protein Atlas". www.proteinatlas.org. Retrieved 2017-09-21.
  25. S2CID 802377
    .
  26. .
  27. ^ Degen, L.P.; Phillips, S.F. (August 1996), "Variability of gastrointestinal transit in healthy women and men", Gut, 39: 299–305
  28. ^ Madsen, MD, Jan Lysgard (1992), "Effects of gender, age, and body mass index on gastrointestinal transit times", Digestive Diseases and Sciences, 37: 1548–1553
  29. ^ a b Bowen, Richard. "Gastrointestinal Transit: How Long Does It Take?". Colorado State University.
  30. S2CID 231817664
    .
  31. ^ Wilson, Malcom J.; Dickson, W.H.; Singleton, A.C. (1929), "Rate of evacuation of various foods from the normal stomach: a preliminary communication", Arch Intern Med, 44: 787–796
  32. PMID 5687899
    .
  33. .
  34. .
  35. .
  36. .
  37. .
  38. .
  39. .
  40. .
  41. ^ Azzouz, Laura L.; Sharma, Sandeep (31 July 2023). "Physiology, Large Intestine". National Library of Medicine. StatPearls Publishing. Retrieved 24 March 2024.
  42. PMID 2249981
    .
  43. ^ Joel, Lucas (10 January 2020). "Fossil Reveals Earth's Oldest Known Animal Guts - The find in a Nevada desert revealed an intestine inside a creature that looks like a worm made of a stack of ice cream cones". The New York Times. Retrieved 10 January 2020.
  44. PMID 31924764
    .
  45. ^ Nielsen, C., Brunet, T. & Arendt, D. Evolution of the bilaterian mouth and anus. Nat Ecol Evol 2, 1358–1376 (2018). https://doi.org/10.1038/s41559-018-0641-0
  46. ^ De Robertis, E. M., & Tejeda-Muñoz, N. (2022). Evo-Devo of urbilateria and its larval forms. Developmental Biology, 487, 10–20. https://doi.org/10.1016/j.ydbio.2022.04.003
  47. PMID 26811648
    .
  48. ^ Wier LM, Steiner CA, Owens PL (February 2015). "Surgeries in Hospital-Owned Outpatient Facilities, 2012". HCUP Statistical Brief #188. Rockville, MD: Agency for Healthcare Research and Quality.
  49. PMID 17200707
    .
  50. .
  51. .
  52. .
  53. ^ Hiskey, Daven (12 November 2010). "Violin strings were never made out of actual cat guts". TodayIFoundOut.com. Retrieved 15 December 2015.
  54. ^ "World's oldest condom". Ananova. 2008. Retrieved 2008-04-11.

External links