General Information about the Digestive Tract
The two main roles of the intestine are to digest and absorb nutrients and to maintain a barrier against the external environment. The length of the small intestine is quite variable, ranging from 8 ft. to 26 ft. (1) Special circular folding’s within the lumen of the small intestine increase its absorptive capacity three-fold. Finger-like projections called villi further increase the absorptive capacity. Each villous contains a dense capillary network as well as lacteals which drain the lymphatic fluid. In between each villous are crypts which contain epithelial stem cells that work to repair the intestinal mucosa. The large intestine is much shorter than the small intestine, usually only ranging from 3 ft to 5 ft. The intestine is a highly vascularized organ and is supplied by numerous blood vessels. The majority of the intestine, from the second half of the duodenum to the distal one-third of the colon, is supplied by branches of the superior mesenteric artery (SMA). This is an important anatomical note because thrombosis of this artery can result in devastating loss of intestinal mass.
The motor activity of the small intestine is largely governed by the enteric nervous system which receives sympathetic and parasympathetic input as well as paracrine and endocrine signals. Specialized cells called Interstitial Cells of Cajal (ICC) are the pacemaker cells of the gut and generate and propagate slow electrical waves. Motility of the gut is governed based on if the gut is in the fasting or fed state.
Of particular importance for intestinal transplant are the Peyer’s patches scattered throughout the intestine. These are aggregates of lymphoid tissue, or immune tissue, that contain numerous antigen sampling and presenting cells. These dense areas of lymphatic tissue play a role in the immune response the body may mount toward foreign pathogens or invaders.
Langnas AN, Goulet O, Quigley EMM, Tappenden KA. Intestinal Failure Diagnosis, Management, and Transplantation. Malden, MA: Blackwell; 2008.