Human gastrointestinal tract
| Human gastrointestinal tract (Digestive System) | |
|---|---|
 | |
| 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), refers to the stomach and intestine, and is divided into the upper and lower gastrointestinal tracts.[1]It can also include all the structures from the mouth to the anus.[2] (The digestive system is a broader term that includes other structures, including the digestive organs and their accessories).[3]
The whole digestive tract is about nine metres long.[4] The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment of the tract.
The GI tract constantly 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
Upper gastrointestinal tract
The upper gastrointestinal tract consists of the esophagus, stomach, and duodenum.[6] The exact demarcation between the upper and lower tracts can vary. Upon dissection, the duodenum may appear to be a unified organ, but it is often divided into two parts based upon function, arterial supply, or embryology.
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.[7]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 and all of the large intestine.[8]
- Bowel or intestine
- Small Intestine: Has three parts:
- Duodenum: Here the digestive juices from the pancreas (digestive enzymes) and hormones and the gall bladder (bile) mix. 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.
- 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.
- Ileum: Has villi similar to the jejunum, and absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients.
- Large Intestine: Has three parts:
- Caecum: The vermiform appendix is attached to the caecum.
- Colon: Includes the ascending colon, transverse colon, descending colon and sigmoid Flexure: The main function of the Colon is to absorb water, but it also contains bacteria that produce beneficial vitamins like vitamin K.
- Rectum
- Anal canal: The terminal part of the large intestine.
- Small Intestine: Has three parts:
Development
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 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.[9]
| 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
The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy.[10] The GI tract can be divided into four concentric layers in the following order:
- Mucosa
- Submucosa
- Muscularis externa (the external muscular layer)
- Adventitia or serosa
Mucosa
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
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
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
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.
Physiology
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.[11][12]
Immune function
The gastrointestinal tract is also a prominent part of the immune system.[13] 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.[14][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 pass metabolism.
Health-enhancing intestinal bacteria 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).
Clinical relevance
There are a number of diseases and conditions affecting the gastrointestinal system, including:
- Appendicitis
- Cancer
- Celiac Disease
- Cholera
- Colorectal cancer
- Diarrhoea
- Diverticulitis
- Enteric duplication cyst
- Gastroenteritis, also known as "stomach flu"; an inflammation of the stomach and intestines
- Giardiasis
- Inflammatory bowel disease (including Crohn's disease and ulcerative colitis)
- Irritable bowel syndrome
- Pancreatitis
- Peptic ulcer disease
- Yellow Fever
See also
This article uses anatomical terminology; for an overview, see anatomical terminology.
- Dysbiosis
- Gastrointestinal hormone
- Major systems of the human body
References
- ↑ "gastrointestinal tract" at Dorland's Medical Dictionary
- ↑ Gastrointestinal tract at the US National Library of Medicine Medical Subject Headings (MeSH)
- ↑ "digestive system" at Dorland's Medical Dictionary
- ↑ 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.
- ↑ Nelson RJ. 2005. Introduction to Behavioral Endocrinology. Sinauer Associates: Massachusetts. p 57.
- ↑ Upper Gastrointestinal Tract at the US National Library of Medicine Medical Subject Headings (MeSH)
- ↑ 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.
- ↑ Lower Gastrointestinal Tract at the US National Library of Medicine Medical Subject Headings (MeSH)
- ↑ Bruce M. Carlson (2004). Human Embryology and Developmental Biology (3rd ed.). Saint Louis: Mosby. ISBN 0-323-03649-X.
- ↑ Abraham L. Kierszenbaum (2002). Histology and cell biology: an introduction to pathology. St. Louis: Mosby. ISBN 0-323-01639-1.
- ↑ Kim SK. Small intestine transit time in the normal small bowel study. American Journal of Roentgenology 1968; 104(3):522-524.
- ↑ 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.
- ↑ Richard Coico, Geoffrey Sunshine, Eli Benjamini (2003). Immunology: a short course. New York: Wiley-Liss. ISBN 0-471-22689-0.
- ↑ Animal Physiology textbook
Additional images
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Illustration of Gastrointestinal Tract.
External links
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Wikimedia Commons has media related to Human gastrointestinal tract. |
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Wikimedia Commons has media related to Digestive system. |
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The Wikibook Human Physiology has a page on the topic of: The gastrointestinal system |
- Pediatric Overview of Human Digestive System
- Anatomy atlas of the Digestive System
- Overview at Colorado State University
- Your Digestive System and How It Works at National Institutes of Health
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