Small Intestines: the small intestines are located under the liver and the stomach. The small intestines contain millions of villi which contains microvilli so that the nutrients from the stomach and duodenum can be absorbed efficiently with such a large surface area.
Duodenum: the first portion of the small intestine is the duodenum. It is a continuation of the pyloric end of the stomach. It is a short “U” shaped tube, approximately 1 cm. long. The common bile duct and the pancreatic duct open into the duodenum. It uses enzymes to break down food in the small intestine and regulates the rate of which the stomach empties with hormonal pathways. Secretin and chloecytokinin are released from the cells in the duocanal epithelium in response to acidic and fatty stimuli when the pyloris opens and releases gastric chyme into the duodenum for further digestion.
Jejunum: The second section of the small intestine is the jejunum, which makes up about half the length of this organ. The jejunum lies between the duodenum and the ileum. The jejunum is specialized in the absorption of carbohydrates and proteins. What is left over from the stomach and the duodenum is soaked into the villi of the jejunum and is carried into the blood stream.
Ileum: the ileum is the final part of the small intestine and is connected to the jejunum and is separated from the cecum by the ileocecal valve. The ileum mainly absorbs vitamin B12 and bile salts as well as other digestion products that were not absorbed by the jejunum with the villi and microvilli that are a part of the folds in the ileum's inner wall.
The coils of the small intestine are held in place by a fine peritoneal membrane, the mesentery. It may be observed when lifting a coil of the small intestine and stretching the two ends. A fine, thin membrane, the mesentery, will be visible. It is responsible for the coiling observed.