Digest This – How Your Body Does It

Last Updated: June 1, 2003

The digestive system is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. Inside this tube is a lining called the mucosa. In the mouth, stomach and small intestine, the mucosa contains tiny glands that produce juices to help digest food.

Two solid organs, the liver and the pancreas, produce digestive juices that reach the intestine through small tubes. In addition, parts of other organ systems (for instance, nerves and blood) play a major role in the digestive system.

How is food digested?
Digestion involves the mixing of food, its movement through the digestive tract, and chemical breakdown of the large molecules of food into smaller molecules. Digestion begins in the mouth, when we chew and swallow, and is completed in the small intestine. The chemical process varies somewhat for different kinds of food.

Movement of food through the system
The large, hollow organs of the digestive system contain muscle that enables their walls to move. The movement of organ walls can propel food and liquid and also can mix the contents within each organ. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.

The first major muscle movement occurs when food or liquid is swallowed. Although we are able to start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the nerves.

The esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ringlike valve closing the passage between the two organs. However, as the food approaches the closed ring, the surrounding muscles relax and allow the food to pass.

The food then enters the stomach, which has three mechanical tasks to do. First, the stomach must store the swallowed food and liquid. This requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material. The second job is to mix up the food, liquid and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine.

As the food is digested in the small intestine and dissolved into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward to allow further digestion.

Finally, all of the digested nutrients are absorbed through the intestinal walls. The waste products of this process include undigested parts of the food, known as fiber, and older cells that have been shed from the mucosa. These materials are propelled into the colon, where they remain, usually for a day or two, until the feces are expelled by a bowel movement.

Digested molecules of food, as well as water and minerals from the diet, are absorbed from the cavity of the upper small intestine. Most absorbed materials cross the mucosa into the blood and are carried off in the bloodstream to other parts of the body for storage or further chemical change.

What does the colon do?
The colon, which is about five feet long, connects the small intestine with the rectum and anus. The major function of the colon is to absorb water, nutrients and salts from the partially digested food that enters from the small intestine.

Colon motility (the contraction of the colon muscles and the movement of its contents) is controlled by nerves and hormones and by electrical activity in the colon muscle. Contractions move the contents slowly back and forth but mainly toward the rectum. During this passage, water and nutrients are absorbed into the body. What remains is stool. A few times each day, strong muscle contractions move down the colon, pushing the stool ahead of them. Some of these strong contractions result in a bowel movement.

The muscles of the pelvis and anal sphincters have to relax at the right time to allow the stool to be expelled. If the muscles of the colon, sphincters and pelvis do not contract in a coordinated way, the contents do not move smoothly, resulting in abdominal pain, cramps, constipation or diarrhea, and a sense of incomplete stool movement.



Source: National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

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