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How the colon and rectum work

Once food is digested, the colon and rectum move the waste products out of the body.

Food is a powerful fuel for our bodies. When we eat, the food enters a labyrinth of hollow organs, beginning with the esophagus, stomach and small intestine. Muscles in these organs move the food along in waves—an activity known as peristalsis. Meanwhile, digestive juices break the food into nutrients that can be absorbed and used by the body. But not every morsel that you eat can be put to work. Just as waste is produced in preparing some foods (think melon rinds, for example), it's also produced in the digestive system. Anything that is not processed needs to be eliminated. And that is the work of the colon and the rectum—the lower end of the digestive system.

Getting rid of waste

The small intestine, which is about 20 feet long, processes and absorbs most of the nutrients from food. It then passes the nutrients into the bloodstream, which carries them to various areas of the body. What's left in the intestine at the end of this process are undigested parts of the food, known as fiber, and old cells that have been shed from the organ linings (mucosa). These byproducts are moved by peristalsis from the small intestine into a muscular tube called the colon.

The colon is about 5 to 6 feet long. It is the longest part of the large intestine, or bowel, which also includes the rectum.

The colon is coiled in a triangle-like pattern in the abdomen. It has several sections:

The ascending colon winds upward through the abdomen.

The transverse colon crosses the abdomen close to the stomach.

The descending colon follows a downward direction, then turns toward the middle of the lower abdomen, where it meets the rectum.

The major function of the colon is to absorb water and salts from digestive byproducts that enter from the small intestine. Naturally occurring bacteria in the colon help this process along.

According to the American Gastroenterological Association (AGA), the liquid byproducts that enter the colon from the small intestine can take several days to move through the entire colon. During its passage, the fluids are absorbed into the body, leaving solid waste (feces) behind. The feces, commonly known as stool, collect in the lower part of the descending colon. Contractions of the colon move the waste along until it enters the rectum.

The rectum is only a few inches long. Stool is held in the rectum until contractions from the colon move it through the anus and out of the body.

Normal bowel function varies from person to person, the AGA points out. For example, normal bowel movements range from as many as three stools a day to as few as three a week. A normal movement is firm but not hard, contains no blood, and is passed without cramps or pain.

Reviewed 3/2/2023

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