Basic Anatomy

The term colorectal cancer refers to cancer that arises in the lower gastrointestinal tract, which is divided into the cecum, the ascending (right) colon, the transverse colon, the descending (left) colon, the sigmoid colon, and the rectum. The large intestine (colorectum) begins at the cecum, which is a pouch of approximately two to three inches in length. Ileal contents empty into the cecum through the ileocecal valve. The appendix extends from the base of the cecum.

The ascending colon rises from the cecum along the right posterior wall of the abdomen, to the right upper quadrant and to the undersurface of the liver, where it turns toward the midline (hepatic flexure), becoming the transverse colon. The transverse colon crosses the abdominal cavity toward the spleen in the left upper quadrant; where it turns downward at the splenic flexure. Continuing along the left side of the abdomen, the descending colon turns medially and inferiorly to form the S-shaped sigmoid colon. The rectum makes up the last five to ten inches of the large intestine, beginning from the end of the sigmoid colon down to the anal canal.

The rectum is located within the pelvis and is not a true intra-abdominal structure. The diameter of the rectum is larger than that of the colon, and serves primarily as a storage reservoir.

The pelvic musculature, or levator ani muscles and the internal and external anal sphincter muscles, bind the most distal rectum and anus. The sphincter muscles maintain bowel continence.

 

Normal colorectal anatomy. A: anatomy of the colon; B: anatomy of the rectum.

The entire large intestine is approximately five to six feet in length, with a diameter that varies from one to two inches.

It is the site of salt and water absorption. Glands secrete large quantities of alkaline mucus that lubricates the intestinal contents and neutralizes acids formed by bacteria in the intestine. These bacteria aid in the decomposition of undigested food residue, unabsorbed carbohydrates, amino acids, cell debris, and dead bacteria through the process of segmentation and putrefaction. Short-chain fatty acids formed by bacteria from unabsorbed complex carbohydrates provide an energy source for the cells of the left colon. Maintenance of potassium balance is also performed by the colon, wherein the epithelium absorbs and secretes potassium and bicarbonate.


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