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The gastrointestinal tract (GI tract), also called the digestive tract, or the alimentary canal. It is the system of organs within multicellular animals that takes in food, digests it to extract energy and nutrients, and expels the remaining waste. The major functions of the GI tract are digestion and excretion.
The GI tract differs substantially from animal to animal. For instance, some animals have multi-chambered stomachs, while some animals' stomachs contain a single chamber. In a normal human adult male, the GI tract is approximately 6.5 meters (20 feet) long and consists of the upper and lower GI tracts. The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment of the tract.
Upper gastrointestinal tract
- The mouth contains the buccal mucosa, which contains the openings of the salivary glands; the tongue; and the teeth.
- Behind the mouth lies the pharynx, which leads to a hollow muscular tube, the esophagus.
- Peristalsis takes place, which is the contraction of muscles to propel the food down the esophagus which extends through the chest and pierces the diaphragm to reach the stomach.
- The stomach, in turn, leads to the small intestine.
The upper GI tract roughly corresponds to the derivatives of the foregut, with the exception of the first part of the duodenum (see below for more details.)
Lower gastrointestinal tract
The lower GI tract comprises the intestines and anus.
- Bowel or intestine
The liver secretes bile into the small intestine via the biliary system, employing the gallbladder as a reservoir. Apart from storing and concentrating bile, the gall bladder has no other specific function. The pancreas secretes an isosmotic fluid containing bicarbonate and several enzymes, including trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes (deoxyribonuclease and ribonuclease), into the small intestine. Both these secretory organs aid in digestion.
The gut is an endoderm-derived structure. At approximately the 16th 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 towards 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 nevertheless used regularly to describe components of the definitive gut as well.
Each segment of the primitive gut gives rise to specific gut and gut-related structures in the adult. 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 outpouchings of the primitive gut. The blood vessels supplying these structures remain constant throughout development.
|Part||Range in adult||Gives rise to||Arterial supply|
|foregut||the pharynx, to the upper duodenum||pharynx, esophagus, stomach, upper duodenum, respiratory tract (including the lungs), liver, gallbladder, and pancreas||branches of the celiac artery|
|midgut||lower duodenum, to the first half of the transverse colon||lower duodenum, jejunum, ileum, cecum, appendix, ascending colon, and first half of the transverse colon||branches of the superior mesenteric artery|
|hindgut||second half of the transverse colon, to the upper part of the anal canal||remaining half of the transverse colon, descending colon, rectum, and upper part of the anal canal||branches of the inferior mesenteric artery|
Specialization of organs
Four organs are subject to specialization in the kingdom Animalia.
- The first organ is the tongue which is only present in the phylum Chordata.
- The second organ is the esophagus. The crop is an enlargement of the esophagus in birds, insects and other invertebrates that is used to store food temporarily.
- The third organ is the stomach. In addition to a glandular stomach (proventriculus), birds have a muscular "stomach" called the ventriculus or "gizzard." The gizzard is used to mechanically grind up food.
- The fourth organ is the large intestine. An outpouching of the large intestine called the cecum is present in non-ruminant herbivores such as rabbits. It aids in digestion of plant material such as cellulose
The gastrointestinal tract is also a prominent part of the immune system. 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 the saliva and bile. Enzymes such as Cyp3A4, along with the antiporter activities, are also 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. Microorganisms are also kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue (GALT).
The GI tract has a uniform general histology with some differences which reflect the specialization in functional anatomy. The GI tract can be divided into 4 concentric layers:
- muscularis externa (the external muscle layer)
- adventitia or serosa
The mucosa is the innermost layer of the GI tract, surrounding the lumen, or space within the tube. This layer comes in direct contact with the food (or bolus), and is responsible for absorption and secretion, important processes in digestion.
The mucosa can be divided into:
The mucosae are highly specialized in each organ of the GI tract, facing a low pH in the stomach, absorbing a multitude of different substances in the small intestine, and also absorbing specific quantities of water in the large intestine. Reflecting the varying needs of these organs, the structure of the mucosa can consist of invaginations of secretory glands (eg, gastric pits), or it can be folded in order to increase surface area (examples include villi and plicae circulares).
The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics and nerves branching into the mucosa and muscularis. It contains Meissner's plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.
The muscularis externa consists of a circular inner muscular layer and a longitudinal outer muscular layer. The circular muscle layer prevents the food from going backwards and the longitudinal layer shortens the tract. The coordinated contractions of these layers is called peristalsis and propels the bolus, or balled-up food, through the GI tract. Between the two muscle layers are the myenteric or Auerbach's plexa.
The adventitia consists of several layers of epithelia. When the adventitia is facing the mesentery or peritoneal fold, the adventitia is covered by a mesothelium supported by a thin connective tissue layer, together forming a serosa, or serous membrane.
Auerbach's plexus is the main autonomic ganglia controlling the GI tract. It arises from cells in the nucleus ala cinerea, the parasympathetic nucleus of origin for the tenth cranial nerve (vagus), located in the medulla oblongata. The fibers are carried by both the anterior and posterior vagal nerves. The myenteric plexus is the major nerve supply to the gastrointestinal tract and controls GI tract motility.
- Bruce M. Carlson (2004). Human Embryology and Developmental Biology, 3rd edition, Saint Louis: Mosby. ISBN 0-323-03649-X.
- Richard Coico, Geoffrey Sunshine, Eli Benjamini (2003). Immunology: a short course, New York: Wiley-Liss. ISBN 0-471-22689-0.
- Abraham L. Kierszenbaum (2002). Histology and cell biology: an introduction to pathology, St. Louis: Mosby. ISBN 0-323-01639-1.
- Human Anatomy and Physiology, Marieb & Hoehn, seventh edition
- National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health.
- Digestive system
- Digestive system disorders
- Gastrointestinal disorders
- Gastrointestinal hormone
- Gastrointestinal ulcers
- Major systems of the human body
- REDIRECT Template:Digestive tract
|Human organ systems|
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