Lower GI Tract

Your lower digestive tract, also known as your bowel, is approximately 25 feet long and consists of the small intestine and large intestine. Food from the stomach passes through the pyloric valve into the small intestine, a 20-foot tube with three sections: the duodenum, the jejunum, and the ileum. The walls of the small intestine are lined with muscles that contract and relax to carry food along its path. The walls are also covered in microvilli, hairlike projections that help to absorb nutrients into the bloodstream.

When food reaches the duodenum, it begins to get broken down by digestive enzymes and bile. This process turns proteins into amino acids, fats into fatty acids, and carbohydrates into simple sugars. The digested food then moves into the jejunum, where most of its nutrients are absorbed. Vitamin B12 is absorbed in the ileum.

The material left behind—mostly water, electrolytes (such as sodium and potassium), and waste (such as fiber and dead cells)—moves into the cecum, the first part of the colon (also called the large intestine). It then passes through the ascending, transverse, and descending colons; the sigmoid colon; and the rectum.

No nutrients are absorbed by the colon. Its job is to remove excess water from the intestinal waste and return it to the bloodstream. Thus, as the material moves along the colon, it slowly dries out and forms a more solid substance called stool.

Waste usually spends a day or two in the colon before it is expelled from the body. When stool moves into the rectum, it stretches the walls of the rectum, which signals the need for a bowel movement. The stool then moves into the anal canal. At the end of the anal canal is the anal sphincter, which is a muscle that usually remains closed; however, it opens to allow stool to pass out of the body.

Publication Review By: H. Franklin Herlong, M.D.

Published: 25 Mar 2011

Last Modified: 02 Dec 2014