Colon

The Human Colon or Large Intestine

The colon, also referred to as the large intestine is the final section of the gastrointestinal (GI) tract. Its primary function is to extract water, salts and other substances from food content passing through the body’s digestive system. Most nutritional absorption occurs in the small intestine, the part of the gastrointestinal tract positioned above the colon. However, in the colon, absorption of fluids, sodium and fat-soluble vitamins forms solid fecal matter that can be expelled from the body. The colon walls are coated in special bacteria that help with this process.

The first part of the colon, also known as the cecum, begins in the lower right-hand side of the abdomen. The remainder of the colon has four distinct sections, which listed from “top” to “bottom” of the large intestine are:

1. The ascending colon—goes up the right-hand side of the abdomen. Its length totals approximately 25 centimeters.
2. The transverse colon—goes across the abdomen, with one side hanging off the stomach by way of a wide segment of tissue and with the other side connected to the abdominal wall. Unlike the ascending and descending colons, the transverse colon is mobile because it is covered with a special membrane called peritoneum.
3. The descending colon—goes down the left-hand side of the abdomen. This is the area of the body where food is stored while it waits to enter the rectum.
4. The sigmoid colon—the curve-shaped, final section of the large intestine, right before the rectum. The muscles lining the walls of the sigmoid colon are particularly strong because they need to contract in order to push stool toward the rectum. Your GI doctor can perform a sigmoidoscopy to check for signs of cancer or other colorectal disease located in the lower-third of the colon.

Colon cancer can occur anywhere within the large intestine; however, is more likely to emerge lower down the gastrointestinal tract (the left colon).  That being said, cancers involving the right side of the colon can only be detected early by means of colonoscopy examination or CT colonography (or virtual colonoscopy) and would be missed by sigmoidoscopy exam.   The rectum and anus have lower rates of cancer than the remainder of the colon.  If you have questions about colon anatomy or health, please contact a colonoscopy specialist.

Reviewed 12/14/2011 by David M. Nolan, M.D.
Diplomate of the American Board of Internal Medicine, 2011
Currently a Fellow of Gastroenterology, at UCI 2011-2014