How your digestive system works

A colostomy creates a major physical change for a patient, but it doesn’t really change the digestion of food or body chemistry. To understand how a colostomy works, it helps to know how the digestive tract normally works (see Figure 1).

After food is chewed and swallowed, it passes through the esophagus (swallowing tube) into the stomach. From there it goes into the small intestine. Hours can go by before it moves into the large intestine or colon. After hours or even days, it leaves the storage area called the rectum by way of the anus. For most of its passage, the food is liquid and loose. Water is absorbed in the colon, causing the stool to become a firm mass as it nears the rectum.

Figure 1

Illustration showing the normal digestive system including locations of the stomach, transverse colon, small intestine, descending/sigmoid colon, rectum, cecum and ascending colon

The small intestine

The small intestine is the longest section of the digestive tract. Food nutrients are digested and absorbed here as food is moved through by peristalsis. (Peristalsis is the wave-like muscle contractions that move food through the digestive tract.)

The small intestine is about 20 feet long. It’s made up of 3 sections:

  • Duodenum (first part) – 10 to 12 inches beginning at the outlet of the stomach
  • Jejunum (second part) – about 8 to 9 feet long
  • Ileum (third part) – about 12 feet long; it connects to the colon at the cecum

The small intestine lies loosely curled in the belly (abdominal cavity).

The large intestine

The large intestine (also called large bowel) joins the small intestine where the ileum and cecum meet on the body’s right side.

The colon

The colon is by far the biggest part of the large intestine. It’s about 5 to 6 feet long, and is made up of these sections:

  • Cecum – the entry point for food that has been through the small intestine and is now a highly acidic liquid. It contains a valve that keeps food from going back into the small intestine.
  • Ascending colon – the contents are acidic liquid. This section goes up the right side of the body.
  • Transverse colon – the contents are less acidic liquid. This section goes across the belly.
  • Descending colon – the contents become more formed. This section goes down the left side of the body into the pelvis.
  • Sigmoid colon – the contents are usually solid. The sigmoid is an S-shaped curve at the end of the descending colon, just above the rectum.

The main jobs of the colon are absorbing water and electrolytes (salts and minerals the body needs, like sodium, calcium, and potassium), moving stool, and storing waste until it’s passed out of the body.

There are 2 major types of activities in the colon, peristalsis and mass reflex. During peristalsis, the muscles of the colon are constantly contracting (squeezing) and relaxing. These movements happen in all the different parts of the colon, but can’t be felt. The purpose of peristalsis is to mix and knead the liquid from the small intestine and remove water. This makes the end product, formed or solid stool. When stool collects in a part of the colon, muscles in that part relax and stretch to hold it. Pressure builds as the stretch limit is reached. At this point, a mass reflex, stronger than peristalsis, pushes the stool into the next part of the colon. Over time, the stool moves into the rectum. This reflex happens several times a day, usually after you eat or drink.

The rectum and anus

The rectum is the last part of the large intestine that connects the sigmoid colon to the anus. Normally at this point the bowel contents are solid. Special nerve pathways to the brain make us aware when the stool reaches the rectum. As stool enters the rectum, we feel the need to have a bowel movement. The anal sphincter muscle is like a valve that allows us to control this. Unlike the rest of the digestive tract, it closes (contracts) or opens (relaxes) at our will to allow stool to pass out of the body.

In its original form this document was written by the United Ostomy Association, Inc. (1962-2005) and reviewed by Jan Clark, RNET, CWOCN and Peg Grover, RNET. It has since been modified and updated by:

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: December 2, 2014 Last Revised: December 2, 2014

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