부산대학교 도서관

The small intestine consists of the duodenum, jejunum, and ileum. The combined length of the jejunum and ileum measured from the duodenojejunal flexure to the ileocaecal junction in vivo varies considerably but is approximately 5 m in adults. The duodenum extends from the stomach to the duodenojejunal flexure; the proximal two-fifths of the remaining small intestine is the jejunum and the distal three-fifths is the ileum. The duodenum is mostly retroperitoneal while the jejunum and ileum are intraperitoneal and suspended by a fat-laden mesentery containing blood vessels, lymphatics, and nerves. There is no clear anatomical transition between the jejunum and the ileum but the morphology and histology of the proximal and distal regions of the small bowel differ significantly. The wall of the small intestine consists of a mucosa, submucosa, muscularis externa, and serosa. The mucosal surface is characterized by numerous intestinal villi, which are covered on their luminal surface by enterocytes bearing luminal microvilli. The small intestine is adapted for fluid, electrolyte, and nutrient absorption. Intestinal stem cells located in the intestinal crypts are the source of most cell types of the intestinal epithelium. The functions of the small intestine are regulated by the enteric nervous system, which is modulated by extrinsic autonomic nerves. The development of the human small intestine is largely complete by the end of the 12th week after fertilization.