- Gastrointestinal tract
Digestive system diagram The digestive tract (also known as the alimentary canal) 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 ingestion, digestion, absorption, and defecation.The picture to the right doesn't show the Jejunum. The GI tract differs substantially from animal to animal. Some animals have multi-chambered stomachs, while some animals' stomachs contain a single chamber. In a normal humanadult 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. [cite book
last = Maton
first = Anthea
coauthors = Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright
title = Human Biology and Health
publisher = Prentice Hall
date = 1993
location = Englewood Cliffs, New Jersey, USA
isbn = 0-13-981176-1]
Upper gastrointestinal tract
The upper GI tract consists of the mouth,
pharynx, esophagus, and stomach.
* The mouth contains the
buccal mucosa, which contains the openings of the salivary glands; the tongue; and the tooth.
* Behind the mouth lies the
pharynx, which leads to a hollow muscular tube, the esophagus.
Peristalsistakes place, which is the contraction of muscles to propel the food down the esophagus which extends through the chest and pierces the diaphragmto reach the stomach.
Lower gastrointestinal tract
The lower GI tract comprises the intestines and anus.
Small intestine, which has three parts:
Large intestine, which has three parts:
Cecum(the vermiform appendixis attached to the cecum).
ascending colon, transverse colon, descending colonand sigmoid flexure)
Accessory organs to the alimentary canal include the liver, gallbladder, and pancreas. The
liversecretes bileinto the small intestine via the biliary system, employing the gallbladderas a reservoir. Apart from storing and concentrating bile, the gallbladder has no other specific function. The pancreassecretes an isosmotic fluid containing bicarbonateand several enzymes, including trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes ( deoxyribonucleaseand ribonuclease), into the small intestine. Both of 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
stomachand 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. [cite book | author = Bruce M. Carlson | title = Human Embryology and Developmental Biology | publisher = Mosby | location = Saint Louis | edition = 3rd edition | year = 2004 | id = ISBN 0-323-03649-X]
pecialization of organs
Four organs are subject to specialization in the
*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 esophagusin 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"
There are a number of diseases and conditions affecting the gastrointestinal system, including:
Gastroenteritis, also known as "stomach flu";an inflammation of the stomach and intestines
Inflammatory bowel disease( Crohn's diseaseand ulcerative colitis)
Irritable bowel syndrome
The gastrointestinal tract is also a prominent part of the
immune system. [cite book | author = Richard Coico, Geoffrey Sunshine, Eli Benjamini | title = Immunology: a short course | publisher = Wiley-Liss | location = New York | year = 2003 | id = ISBN 0-471-22689-0] 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 salivaand 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 bacteriaserve to prevent the overgrowth of potentially harmful bacteriain the gut. Microorganisms are also kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue(GALT).
The gastrointestinal tract has a uniform general histology with some differences which reflect the specialization in functional anatomy. [cite book | author = Abraham L. Kierszenbaum | title = Histology and cell biology: an introduction to pathology | publisher = Mosby | location = St. Louis | year = 2002 | id = ISBN 0-323-01639-1] The GI tract can be divided into 4 concentric layers:
Muscularis externa" (the external muscle layer)
The mucosa is the innermost layer of the gastrointestinal tract that is 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 gastrointestinal 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 (e.g.,
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 an inner circular layer and a
longitudinalouter 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 peristalsisand propels the bolus, or balled-up food, through the GI tract. Between the two muscle layers are the myenteric or Auerbach's plexus.
adventitiaconsists of several layers of epithelia.When the adventitia is facing the mesenteryor peritoneal fold, the adventitia is covered by a mesothelium supported by a thin connective tissuelayer, together forming a serosa, or serious membrane.
Uses of animal gut by humans
* The stomachs of calves have commonly been used as a source of
rennetfor making cheese.
* The use of animal gut strings by musicians can be traced back to the
third dynasty of Egypt. In the recent past, strings were made out of lamb gut. With the advent of the modern era, musicians have tended to use strings made of silk, or synthetic materials such as nylonor 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.
* Sheep gut was the original source for natural gut string used in
racquets, such as for tennis. Today, synthetic strings are much more common, but the best strings are now made out of cowgut.
* Gut cord has also been used to produce strings for the snares which provide the
snare drum's characteristic buzzing timbre. While the snare drum currently almost always uses metal wire rather than gut cord, the North African bendirframe drum still uses gut for this purpose.
* "Natural" sausage hulls (or casings) are made of animal gut, especially hog, beef, and lamb.
* 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 CE, were made from animal intestine. [cite web | url = http://www.ananova.com/news/story/sm_1870958.html?menu=news.quirkies.sexlife | title = World's oldest condom | accessdate = 2008-04-11 | date = 2008| publisher = Ananova]
Dorland's Illustrated Medical Dictionary
Major systems of the human body
* National Institute of Diabetes and Digestive and Kidney Diseases,
National Institutes of Health.
* [http://www.innerbody.com/image/digeov.html Anatomy atlas of the Digestive System]
* [http://www.vivo.colostate.edu/hbooks/pathphys/digestion/ Overview] at
Colorado State University
* [http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/ Your Digestive System and How It Works] at [http://digestive.niddk.nih.gov/ National Digestive Diseases Information Clearinghouse (NDDIC)]
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