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Gastrointestinal system

The organs and structures that ingest and digest food, and pass digestive waste from the body. The gastrointestinal system supplies the body with the nutrients it needs to fuel its myriad activities, from molecular interactions to integrated networks of function. What enters the body as a meal embarks on a turbulent, convoluted journey through 30 feet of muscular conduits, also called the alimentary canal. The average meal takes 24-30 hours to complete the passage, by the end of which all nutritionally useful substances have been extracted to leave residue that bears no resemblance to its original composition. Seven stations along the way blend, churn, and dissolve the meal: mouth, esophagus, stomach, small intestine, large intestine (colon), rectum, and anus. Three additional organs support these stations: the pancreas, the gallbladder, and the liver.

The Journey’s Start: The Mouth and Esophagus

Food enters the gastrointestinal system through the mouth. The teeth tear and grind the food into small particles. Three pairs of salivary glands in the mouth produce between a half-ounce and an ounce of saliva for each mouthful of food. Saliva contains a few digestive enzymes, though its primary purpose is to form the mouthful of food into a semisolid ball called an alimentary bolus. The tongue pushes the bolus to the back of the throat and into the top of the esophagus, a muscular tube about 10 inches long. A powerful series of wavelike contractions pull it down the esophagus to the stomach. Chewing also releases hormones that promote digestion in the intestines.

Middle Passage: Stomach and Small Intestine

In the stomach, the digestive action starts in earnest, bathing the bolus in a powerful acid solution. The stomach is a hollow structure tucked under the bottom of the rib cage, in the upper left abdomen. The stomach’s outer structure is three layers of muscle, with the fibers of each layer running a different direction — one layer across, one layer lengthwise, and one layer wrapped around. Gastric glands, which produce hydrochloric acid and digestive enzymes, line the inner wall of the stomach. Interspersed among them are cells that secrete a thick mucus to protect the stomach from its digestive juices. Empty, the stomach has a volume of about 16 ounces or one pint. It can stretch to hold more than three times that volume, about 56 ounces (3 ½ pints). The stomach’s muscular wall compresses and churns the food, mixing it with acid and enzymes until, after about six hours, the bolus has become a liquified blend called chyme. A ring of muscle at the bottom of the stomach, the pyloric sphincter, periodically opens to allow small surges of chyme to enter the duodenum, the first segment of the small intestine.

The small intestine is the longest component of the gastrointestinal system; its 18 feet or so of soft, tubelike structure lay in convoluted folds within the central abdomen. There are three segments to the small intestine: the duodenum (about 12 inches long), the jejunum (about 6V2 to seven feet long), and the ileum (about 10 feet long). For the next 12-20 hours, gentle contractions massage the chyme through the small intestine. Various digestive enzymes enter the mix along the way, separating out nutrients and breaking them into their molecular components. Millions of microscopic tendrils, the intestinal villi, line the walls of the small intestine. The villi extend into the capillary beds, where waiting blood picks up the molecules of nutrients that migrate across the membrane coverings of the villi. By the time the chyme reaches the end of the small intestine, little of nutritional value remains.

Adding to the Mix: Pancreas, Liver, and Gallbladder

The liver and pancreas produce numerous digestive enzymes that enter the intestinal tract through channels called ducts. The liver also produces bile and cholesterol, which are necessary to digest and transport lipids and fatty acids. The gallbladder stores bile to make it more rapidly available; in response to a rise in the digestive enzyme cholecystokinin (CCK), the gallbladder releases bile into the common bile duct, which then drains into the duodenum. Some digestive enzymes the pancreas produces are inactive until they mix with other enzymes in the duodenum.

Journey’s End: Colon, Rectum, and Anus

The remnants of digestion pass from the small intestine to the large intestine or colon. The colon’s lining absorbs much of the water still in the sludgelike material and compacts the residue that is left. At the end of the six to eight feet of colon, the digestive waste is in a semisolid form. This material — feces — enters the rectum, where it waits to be expelled through the anus as a bowel movement.

Maintaining Gastrointestinal Health

A healthy, efficient gastrointestinal system requires a diet with adequate fiber and a balance of nutrients. Fiber gives substance to the chyme, aiding in its movement through the small intestine. Fiber is particularly essential at the end of the digestive journey, helping to retain enough fluid in the feces so they pass easily. Fiber also absorbs cholesterol and fatty acids, reducing the amounts of each that enter the bloodstream. Daily physical exercise such as walking stimulates a meal’s movement through the gastrointestinal system, helping to prevent stagnating delays. Some health experts believe that the longer the digestive journey takes, the greater the risk for diseases such as colorectal cancer. Digestive waste that spends an extended time in the colon and rectum exposes those tissues to any environmental residues that are potentially harmful or carcinogenic (cancer-causing). Bowel habits are important indicators of bowel function; changes in the nature or frequency of bowel movements warrant a doctor’s evaluation.

See also dyspepsia; Helicobacter pylori; hemorrhoids.

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