Human gastrointestinal tract

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Human gastrointestinal tract (Digestive System)
Stomach colon rectum diagram.svg
Stomach colon rectum diagram
Latin Tractus digestorius (mouth to anus),
canalis alimentarius (esophagus to large Intestine),
canalis gastrointestinales (stomach to large Intestine)
System Digestive system

The human gastrointestinal tract (GI tract) is an organ system responsible for consuming and digesting foodstuffs, absorbing nutrients, and expelling waste.

The tract is commonly defined as the stomach and intestine, and is divided into the upper and lower gastrointestinal tracts.[1] However, by the broadest definition, the GI tract includes all structures between the mouth and the anus.[2] On the other hand, the digestive system is a broader term that includes other structures, including the digestive organs and their accessories.[3] The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment.

The whole digestive tract is about nine metres long.[4]

The GI tract releases hormones to help regulate the digestive process. These 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.[5]

Structure[edit]

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 and large parts.[6]

Upper gastrointestinal tract[edit]

Salivary glands Parotid gland Submandibular gland Sublingual gland pharynx Tongue Esophagus Pancreas Stomach Pancreatic duct Ileum Anus Rectum Vermiform appendix Cecum Descending colon Ascending colon Transverse colon Colon (anatomy) Bile duct Duodenum Gallbladder Liver oral cavity
Upper and Lower human gastrointestinal tract

The upper gastrointestinal tract consists of the esophagus, stomach, and duodenum.[7] The exact demarcation between the upper and lower tracts is the suspensory ligament of the duodenum (also known as the Ligament of Treitz). This delineates the embryonic borders between the foregut and midgut, and is also the division commonly used by clinicians to describe gastrointestinal bleeding as being of "upper" or "lower" origin. Upon dissection, the duodenum may appear to be a unified organ, but it is divided into four segments based upon 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 ligament attaches the superior border of the ascending duodenum to the diaphragm.

The suspensory muscle of duodenum is an important anatomical landmark which shows the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively.[8]This is a thin muscle which is derived from the embryonic mesoderm.

Lower gastrointestinal tract[edit]

The lower gastrointestinal tract includes most of the small intestine and all of the large intestine.[9] In human anatomy, the intestine (or bowel, hose or gut) is the segment of the gastrointestinal tract extending from the pyloric sphincter of the stomach to the anus and, in humans and 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 and colon.[10]

Small Intestine[edit]

The small intestine begins at the duodenum, which receives food from the stomach. The duodenum is a short structure which receives both pancreatic juices and bile. The duodenum transmits food to the jejunum and ileum. The main function of the small intestine is to absorb proteins, lipids, and vitamins, has three major divisions:

  1. Duodenum: Here the digestive juices from the pancreas (digestive enzymes) and the gall bladder (bile) mix with hormones. The digestive enzymes break down proteins and bile and emulsify fats into micelles. The duodenum contains Brunner's glands, which produce a mucus-rich alkaline secretion containing bicarbonate, which, in combination with bicarbonate from the pancreas, neutralizes HCl of the stomach.
  2. Jejunum: This is the midsection of the intestine, connecting the duodenum to the ileum. It contains the plicae circulares (also called circular folds or valves of Kerckring), and villi that increase the surface area of this part of the GI Tract. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here.
  3. Ileum: Has villi similar to the jejunum, and absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients.

Large Intestine[edit]

The large intestine consists of the colon and rectum. The colon ascends in the back wall of the abdomen, passes across the back wall, and then falls down the left side of the abdomen. The colon connects to the rectum, and finally the anus. The main function of the large intestine is to absorb water, is divided as well:

  1. Cecum: The vermiform appendix is attached to the cecum.
  2. Colon
    1. Ascending colon
    2. Transverse colon
    3. Descending colon
    4. Sigmoid Flexure
  3. Rectum
  4. Anal canal: The terminal part of the large intestine.

Development[edit]

The gut is an endoderm-derived structure. At approximately the sixteenth day of human development, the embryo begins to fold ventrally (with the embryo's ventral surface becoming 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 fetal life, the primitive gut can be divided 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 components of the definitive gut as well.

Each segment of the gut gives rise to specific gut and gut-related structures in later development. Components derived from the gut proper, including the stomach and colon, develop as swellings or dilatations namy 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.[11]

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
(Note that 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 duodenum, jejunum, ileum, cecum, appendix, ascending colon, and first two-third 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[edit]

General structure of the gut wall
  • 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.[12] The GI tract can be divided into four concentric layers in the following order:

Mucosa[edit]

The mucosa is the innermost layer of the gastrointestinal tract. that is surrounding 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[edit]

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 Meissner's plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.

Muscularis externa[edit]

The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer. The circular muscle 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.

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.

Between the two muscle layers are the myenteric or Auerbach's plexus. This controls peristalsis. Activity is initiated by the pacemaker cells (interstitial cells of Cajal). The gut has intrinsic peristaltic activity (basal electrical rhythm) due to its self-contained enteric nervous system. The rate can of course be modulated by the rest of the autonomic nervous system.

Adventitia/serosa[edit]

The outermost layer of the GI tract consists of several layers of connective tissue.

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 appendix, transverse colon, sigmoid colon and rectum. In these sections of the gut there is 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.

Function[edit]

The time taken for food or other ingested objects to transit through the gastrointestinal tract varies depending on many factors, but roughly, it takes less than an hour after a meal for 50% of stomach contents to empty into the intestines and total emptying of the stomach takes around 2 hours. Subsequently, 50% emptying of the small intestine takes 1 to 2 hours. Finally, transit through the colon takes 12 to 50 hours with wide variation between individuals.[13][14]

Immune function[edit]

The gastrointestinal tract is also a prominent part of the immune system.[15] The surface area of the digestive tract is estimated to be the surface area of a football field. With such a large exposure, the immune system must work hard to prevent pathogens from entering into blood and lymph.[16][WP:V]

The low pH (ranging from 1 to 4) of the stomach is fatal for many microorganisms that enter it. Similarly, mucus (containing IgA antibodies) neutralizes many of these microorganisms. Other factors in the GI tract help with immune function as well, including enzymes in saliva and bile. Enzymes such as Cyp3A4, along with the antiporter activities, also are instrumental in the intestine's role of detoxification of antigens and xenobiotics, such as drugs, involved in first phase metabolism.

Health-enhancing intestinal bacteria of the gut flora serve to prevent the overgrowth of potentially harmful bacteria in the gut. These two types of bacteria compete for space and "food," as there are limited resources within the intestinal tract. A ratio of 80-85% beneficial to 15-20% potentially harmful bacteria generally is considered normal within the intestines. Microorganisms also are kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue (GALT).

Intestinal flora[edit]

The large intestine hosts several kinds of bacteria that deal with molecules the human body is not able to break down itself.[17] This is an example of symbiosis. These bacteria also account for the production of gases at host-pathogen interface, inside our intestine(this gas is released as flatulence when eliminated through the anus). However the large intestine is mainly concerned with the absorption of water from digested material (which is regulated by the hypothalamus) and the re absorption of sodium, as well as any nutrients that may have escaped primary digestion in the ileum.[citation needed]

Clinical significance[edit]

Disease[edit]

There are a number of diseases and conditions affecting the gastrointestinal system, including:

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. (See List of autoimmune diseases).

Symptoms[edit]

Several symptoms are used to indicate problems with the gastrointestinal tract:

Imaging[edit]

Various methods of imaging the gastrointestinal tract are used:

Other[edit]

  • 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.

In other animals[edit]

Animal intestines have multiple uses. 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.

See also[edit]

This article uses anatomical terminology; for an overview, see anatomical terminology.

References[edit]

  1. ^ "gastrointestinal tract" at Dorland's Medical Dictionary
  2. ^ Gastrointestinal tract at the US National Library of Medicine Medical Subject Headings (MeSH)
  3. ^ "digestive system" at Dorland's Medical Dictionary
  4. ^ Kong F, Singh RP (June 2008). "Disintegration of solid foods in human stomach". J. Food Sci. 73 (5): R67–80. doi:10.1111/j.1750-3841.2008.00766.x. PMID 18577009. 
  5. ^ Nelson RJ. 2005. Introduction to Behavioral Endocrinology. Sinauer Associates: Massachusetts. p 57.
  6. ^ "Length of a Human Intestine". Retrieved 2 September 2069. 
  7. ^ Upper Gastrointestinal Tract at the US National Library of Medicine Medical Subject Headings (MeSH)
  8. ^ David A. Warrell (2005). Oxford textbook of medicine: Sections 18-33. Oxford University Press. pp. 511–. ISBN 978-0-19-856978-7. Retrieved 1 July 2010. 
  9. ^ Lower Gastrointestinal Tract at the US National Library of Medicine Medical Subject Headings (MeSH)
  10. ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1969). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1. 
  11. ^ Bruce M. Carlson (2004). Human Embryology and Developmental Biology (3rd ed.). Saint Louis: Mosby. ISBN 0-323-03649-X. 
  12. ^ Abraham L. Kierszenbaum (2002). Histology and cell biology: an introduction to pathology. St. Louis: Mosby. ISBN 0-323-01639-1. 
  13. ^ Kim SK. Small intestine transit time in the normal small bowel study. American Journal of Roentgenology 1968; 104(3):522-524.
  14. ^ [1] Uday C Ghoshal, Vikas Sengar, and Deepakshi Srivastava. Colonic Transit Study Technique and Interpretation: Can These Be Uniform Globally in Different Populations With Non-uniform Colon Transit Time? J Neurogastroenterol Motil. 2012 April; 18(2): 227–228.
  15. ^ Richard Coico, Geoffrey Sunshine, Eli Benjamini (2003). Immunology: a short course. New York: Wiley-Liss. ISBN 0-471-22689-0. 
  16. ^ Animal Physiology textbook
  17. ^ Judson Knight. Science of everyday things: Real-life earth science. Vol. 4. Gale Group; 2002. ISBN 978-0-7876-5634-8.
  18. ^ http://www.irregularbowelsyndrome.info

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External links[edit]