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In human anatomy, the intestine (or bowel, hose or gut) is the segment of the alimentary canal 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.
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 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.
People will have different sized intestines according to their size and age.
The gastrointestinal wall is a structure that is relatively consistent throughout the intestines and other parts of the gastrointestinal tract. The gastrointestinal wall is conventionally regarded as having four layers: a mucosa, which is exposed to the contents of the tract, a submucosa underneath the mucosa; and two outer layers, consisting of the an external muscular layer, and a serosa.
The epithelium, the most exposed part of the mucosa, is a glandular epithelium with many goblet cells. Goblet cells secrete mucus, which lubricates the passage of food along and protects it from digestive enzymes. In the small intestine, villi are folds of the mucosa that increase the surface area of the intestine. The villi contain a lacteal, a vessel connected to the lymph system that aids in the removal of lipids and tissue fluids. Microvilli are present on the epithelium of a villus and further increase the surface area over which absorption can take place. Pocket-like invaginations into the underlying tissue are termed Crypts of Lieberkühn. In the large intestines, villi are absent and a flat surface with thousands of crypts is observed. Underlying the epithelium is the lamina propria, which contains myofibroblasts, blood vessels, nerves, and several different immune cells, and the muscularis mucosa which is a layer of smooth muscle that aids in the action of continued peristalsis and catastalsis along the gut.
Surrounding this is the muscularis externa, which comprises longitudinal and circular smooth muscle that again helps with continued peristalsis and the movement of digested material out of and along the gut. In between the two layers of muscle lies Auerbach's plexus.
Lastly, there is the serosa which is made up of loose connective tissue and coated in mucus so as to prevent any friction damage from the intestine rubbing against other tissue. Holding all this in place are the mesenteries which suspend the intestine in the abdominal cavity and stop it being disturbed when a person is physically active.
The large intestine hosts several kinds of bacteria that deal with molecules the human body is not able to break down itself. This is an example of symbiosis. These bacteria also account for the production of gases 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.
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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.
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