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Anatomy, Abdomen and Pelvis, Small Intestine.

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StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019-.
2019 Apr 5.

Author information

1
McLaren Greater Lansing
2
Christus Santa Rosa Hospitals

Excerpt

The small intestine is a crucial component of the digestive system that allows for the breakdown and absorption of important nutrients that permits the body to function at its peak performance. To do this, the small intestine is made up of a complex network of blood vessels, nerves, and muscles that work together to achieve this task. It is a massive organ that has an average length of 3 to 5 meters. It is divided into the duodenum, jejunum, and ileum.[1][2][3]: The duodenum is the shortest section, on average measuring from 20 cm to 25 cm in length. Its proximal end is connected to the antrum of the stomach, separated by the pylorus. The distal end is at the ligament of Treitz, in which the jejunum begins. The duodenum surrounds the pancreases, in the shape of a "C." It receives chyme from the stomach, pancreatic enzymes, and bile from the liver. The jejunum is roughly 2.5 meters in length, contains plicae circulares (muscular flaps) and villi to absorb the products of digestion. The ileum comes next, measuring around 3 m, and ends at the cecum. It absorbs any nutrients that got past the jejunum, with major absorptive products being vitamin B12 and bile acids. Layers of the Small Intestine: Serosa: The serosa is the outside layer of the small intestine and consists mesothelium and epithelium, which encircles the jejunum and ileum, and the anterior surface of the duodenum since the posterior side is retroperitoneal. The epithelial cells in the small intestine have a rapid renewal rate, with cells lasting for only 3 to 5 days. Muscularis: The muscularis consists of two smooth muscle layers, a thin outer longitudinal layer that shortens and elongates the gut, and a thicker inner circular layer of smooth muscle which causes constriction. Nerves lie between these two layers and allow these to muscle layers to work together to propagate food in a proximal to distal direction. Submucosa: The submucosa consists of a layer of connective tissue that contains the blood vessels, nerves, and lymphatics.   Mucosa: The mucosa is the innermost layer and is designed for maximal absorption by being covered with villi protruding into the lumen that increases the surface area. The crypt layer of the small bowel that is the area of continual cell renewal and proliferation. Cells move from the crypts to the villi and change into either enterocytes; goblet cells; Paneth cells; or enteroendocrine cells. Of importance is the mesentery, which is a double fold of the peritoneum that not only anchors the small intestines to the back of the abdominal wall, but also contains the blood vessels, nerves, and lymphatic vessels that supply the small intestine.[4][5]

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