Anatomy

The colon, or large intestine, is a long, muscular tube about 2 ½ inches in diameter. It extends 5 or 6 feet from the cecum, a pouch that opens to the small intestine, located in the lower right abdomen near the appendix. The colon runs up to just under the rib cage, around and down into the S-shaped region in the lower left abdomen referred to as the sigmoid colon, and then to the end of the gastrointestinal tract, the rectum and anal canal, through which waste passes. The colon is the site of salt and water absorption, and glands in the colon secrete slippery mucus to lubricate the intestines, and aid decomposition of undigested food, cell debris, dead bacteria, and other wastes that must leave the body. Maintaining proper potassium balance is also a task of the colon. The tissue lining the colon absorbs and secretes potassium as needed to maintain proper bodily functions, including heart rate.

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Inflammation from ulcerative colitis damages the mucosa, and sores form in the lining of the colon. Under a microscope, the tissue may appear only red and irritated, or florid ulceration may be present. Bleeding and pain are common.

The typical lesion doctors see in ulcerative colitis is called a "crypt abscess." The lining of a healthy colon is like a honeycomb. In ulcerative colitis, the spaces in the honeycomb, or crypts, which maintain the delicate structure of the lining, fill up with polymorphonuclear cells. These are white blood cells summoned by the body to fight against infection, injury or foreign invasion, and are the first cells on the scene at the site of inflammation. Over time, chronic inflammation destroys the crypts, and the normal architecture of the mucosa is lost, replaced by ulcers and scarring, which can shorten or narrow the colon. In severe disease, obstruction of the colon can occur.

While diarrhea, pain, and blood in the feces are common with UC, the extent of disease varies greatly from person to person. In about one third of patients, the disease affects only the rectum, about the first 6 inches inward from the anus. This condition is also called ulcerative proctitis. Proctitis may result in swelling, redness, pain in the anal canal and rectum, and the urge to pass feces, even if only blood, mucus, and pus are present.

The disease may also involve only the left or distal side of the colon, extending some 15 inches from the rectum to the bend in the colon near the spleen. In patients with distal ulcerative colitis, tissue under a microscope indicates chronic inflammation and the destruction of the cell architecture of the colon lining. Those with distal UC for 10 to 15 years are at higher risk of developing colon cancer.

Pancolitis is ulcerative colitis that extends for the entire length of the colon. Diarrhea and blood in the feces are common and may be accompanied by abdominal pain and cramps, fever, and weight loss. In severe UC, the lining of the colon may develop nodules, discrete ulcer craters, abnormal cells, or carcinoma. Patients who have suffered pancolitis for eight to 10 years are at increased risk of colon cancer.