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The digestive tract -- also called the gastrointestinal tract or alimentary canal -- provides the pathway through which foods move through the body. During this process, foods are broken down into their component nutrients to be available for absorption.
Digestion actually begins in the mouth, as the enzymes in saliva begin to break down carbohydrate (starch). As food is chewed, it becomes lubricated, warmer, and easier to swallow and digest. The teeth and mouth work together to convert each bite of food into a bolus that can readily move into the esophagus ("the food pipe"). In the meantime, taste buds located in the mouth help you to enjoy each mouthful -- or to find the food distasteful, as is sometimes the case. After the bolus is swallowed, it enters the esophagus where it continues to be warmed and lubricated as it moves toward the stomach.
The acidic environment of the stomach and the action of gastric enzymes convert the bolus into chyme, a liquefied mass that is squirted from the stomach into the small intestine. Carbohydrates tend to leave the stomach rapidly and enter the small intestine; proteins leave the stomach less rapidly; and fats linger there the longest.
The small intestine is the principal site of digestion and absorption. There, enzymes and secretions from the pancreas, liver, gallbladder, and the small intestine itself combine to break down nutrients so that they can be absorbed. The pancreas is a veritable enzyme factory, supplying enzymes to digest proteins, fats, and carbohydrates. Intestinal cells also supply some enzymes. The liver produces the bile required for the emulsification of fat, and the gallbladder stores the bile until it is needed. The absorption of nutrients in the small intestine is facilitated by tiny projections called villi, which provide more surface area for absorption. The nutrients pass through the intestinal membranes into the circulatory system, which transports them to body tissues. Nutrients are then absorbed into the cells, where they are used for growth, repair, and the release or storage of energy. The overall process -- called metabolism -- is highly complex.
Undigested chyme proceeds from the small intestine into the large intestine (colon), where it becomes concentrated, as liquid is absorbed in preparation for excretion. Bacteria cause fermentation, which facilitates further breakdown, but absorption of nutrients from the large intestine is minimal.
The key points to remember about digestion are:
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This article was revised on May 23, 2000