Anatomy of the Colon
Located behind the abdominal cavity or in the retroperitoneum, the colon is a two and a half inch diameter, five foot long structure at the end of the digestive tract.
Also called the large intestine, or large bowel, the colon is the last stop for digested food waste as it travels through the digestive system on its way to be expelled through the anus. The features of the colon play important roles nutrient and water absorption.
The Parts of The Large Intestine
The connection between the large intestine and small intestine is the ileum. Digested food, undigested cellulose, and water pass from the ileum through the ileocecal valve, which acts as a sphincter at the junction with the cecum.
The appendix, a small piece of lymphatic tissue, is attached to the cecum, but does not transport fecal material. Appendicitis occurs if waste diverts into the small organ.
The bowel circles around the small intestine as the ascending, transverse, and descending colon. The large intestine turns downward as the sigmoid colon and then ends at the anus as the six inch long rectum and one inch anal canal.
The Colonic Wall
Passage of fecal matter through the large intestine is lubricated by secretion of mucus by the colonic mucosa. Mucosa is an epithelial lining on the inner wall of the tissue. The colonic tissue is made up of three longitudinal bands of smooth muscle. They are called taeniae coli.
The large intestine looks puckered because of the gathering of tissue to correlate with these bands. The pockets that are formed are called haustra and provide more surface area for absorption. Approximately 80% of the water that enters the colon is absorbed along with minerals and vitamins. Within the smooth muscle layer of the rectum are stretch receptors. The receptors send sensory impulses to the sacral spinal cord and cause contraction of the rectum and relaxation of the internal anal sphincter.
Normal Intestinal Flora
Trillions of bacteria live in the gastrointestinal tract, including the colon. Necessary vitamins are produced by this naturally occurring flora. The daily requirement for vitamin K, which is important in blood clotting, is almost solely produced by intestinal bacteria. Smaller amounts of other vitamins, requiring dietary intake in addition, are produced as well. Other products include the B vitamins riboflavin and thiamin, biotin, and folic acid.
Everything absorbed by the colon first circulates to the liver so an additional role of the colonic flora is to prevent pathogens from making their way to the other organ.
Blood Supply and Lymphatic Drainage
An important part of any organ is the blood supply and venous drainage. The large intestine is supplied blood by branches of the superior and inferior mesenteric arteries. A marginal artery runs parallel to the length of the colon.
Deoxygenated blood leaves the intestinal capillaries and drains through the inferior mesenteric vein and into the splenic vein. They also drain in the superior mesenteric vein which combines with the splenic vein to form the portal vein that enters the liver. Lymphatic drainage of excess tissue fluid and pathogens involves the paraortic nodes that drain into the cisterna chyli.
