The large intestine or colon is about 4-5 feet long and consists of the ascending colon, transverse colon, descending colon, and the sigmoid colon. The main function of the colon appears to be extraction of water and salts from chyme (digested food). The colon from cecum to the mid-transverse colon is also known as the right colon. The remainder is known as the left colon.
The blood supply to the colon comes from branches of the superior and inferior mesenteric arteries (SMA and IMA) which arise from the aorta. Flow between these two systems communicates via a "marginal artery" that runs parallel to the colon for its entire length and becomes important during surgical procedures when part of the colon is removed. Venous drainage usually mirrors colonic arterial supply, with the inferior mesenteric vein draining into the splenic vein, the superior mesenteric vein joining the splenic vein which form the portal vein that then enters the liver. It is this venous drainage to the liver that is thought to be the source of liver metastasis from colon cancer.
Ascending colon
The ascending colon, on the right side of the abdomen, is about 30 inches long. It is the part of the colon from the cecum to the hepatic flexure (the turn of the colon by the liver). It is retroperitonal (covered by a thin peritoneal lining) within the abdominal cavity. Arterial supply of the ascending colon comes from the ileocolic artery and right colic artery, both branches of the SMA. While the ileocolic artery is almost always present, the right colic artery can be absent in 5-15% of individuals.
Transverse colon
The transverse colon is the part of the colon from the hepatic flexure (the turn of the colon by the liver) to the splenic flexure (the turn of the colon by the spleen). The transverse colon hangs off the stomach, attached to it by a wide band of fatty tissue called the greater omentum.
The transverse colon is encased in peritoneum, and is therefore mobile (unlike the parts of the colon immediately before and after it). As digested food moves along the colon the contents become more solid (water is removed) in order to form feces. The proximal two-thirds of the transverse colon is perfused by the middle colic artery, a branch of SMA, while the latter third is supplied by branches of the IMA.
Descending colon
The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. It is retroperitoneal like the ascending colon. Arterial supply comes via the left colic artery.
Sigmoid colon
