Digestion is the process by which the body converts food into basic substances that can either be absorbed in the bloodstream as nutrients or passed out of the body as waste. This process of breakdown and assimilation occurs within the digestive tract, a convoluted tube more than 30 feet long that is lined with a mucous membrane.

The tract includes several hollow organs -- the mouth, esophagus, stomach, small intestine, and large intestine -- each of which has a specific function in digestion. The muscles of these organs move the food through the system, while mucus lubricates the tract and prevents irritation. The liver and pancreas are also critical organs in digestion. While not an essential organ, the gallbladder is involved in digestion as well.

Food first enters the digestive tract through the mouth. Movement of the jaws allows the teeth to cut and grind the food into smaller pieces, which are mixed with saliva (a secretion of the glands in the mouth). Saliva moistens food for easier swallowing and contains an enzyme (a special type of protein) that begins the chemical breakdown of starches.

From the mouth, food passes down the throat and into the esophagus, the muscular tube through which the food is conducted to the stomach. The stomach is a large pouch in the abdominal cavity, where food is combined with acid-and enzyme-containing digestive juices secreted by glands located within the stomach walls.

The food becomes semifluid, which allows it to pass easily into the small intestine. The small intestine consists of three portions -- the upper section, or duodenum; the middle section, or jejunum; and the lower section, or the ileum.

In the duodenum, digestive juices from the liver and pancreas continue the process of breaking down the food into its constituent nutrients, which can then be absorbed into the bloodstream throughout the remainder of the small intestine. The liver aids digestion by producing bile, which is necessary for absorption of fat in the small intestine.

The gallbladder, which can be found on the underside of the liver, is another organ that performs an indirect digestive function. The gallbladder stores the bile that has been manufactured by the liver. As bile is needed, the gallbladder contracts and releases the fluid into the duodenum. If it becomes necessary for the gallbladder to be removed for medical reasons (such as in severe gallbladder disease), however, the liver can compensate for its role, and digestion is not hindered.

Other digestive juices required by the small intestine to digest and absorb food, particularly fats and starches, come from the pancreas, an organ located just behind the stomach. The pancreas also secretes insulin and other hormones into the blood. Insulin is the hormone responsible for aiding absorption and use of glucose.

Whatever substances are not assimilated into the bloodstream through the small intestine move into the large intestine. Within the large intestine, waste material is processed into stool (feces), and water and certain chemicals are absorbed into the bloodstream to preserve the body's fluid balance.

The fecal matter continues to move through the colon to the rectum. Once in the rectum, waste is ready to be passed out of the body through the anus (the opening at the end of the digestive tract), thus completing the process of digestion.



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