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The gastrointestinal (GI) tract is the soft tissue tube in the human body that begins at the mouth and ends with the anus, through which all nutrition passes, is processed, and eliminated. Digestion is the primary function of the tract, and it is accomplished through muscle movement, and enzyme and hormone release. An adult human's entire gastrointestinal tract averages 20 to 25 feet (about 6.1 to 7.6 m) long and is comprised of the upper and lower gastrointestinal tracts. Another term that is used for the gastrointestinal tract is the alimentary canal.
Every part of the tract above the duodenum is considered to be part of the upper GI tract. The upper GI tract includes the mouth, pharynx, esophagus, and stomach; the lower GI tract includes the small intestine, large intestine or colon, rectum, and anus. While the liver, gallbladder, and pancreas assist the body in its digestive processing functions, they are not considered part of the actual gastrointestinal tract.
Small intestines typically are about 20 feet (about 6 m) long and are comprised of the duodenum, the jejunum and the ileum, which leads into the large intestine. It is in the duodenum where the principal chemical digestion of food takes place. At the ligament of Trietz, the duodenum ends and the jejunum begins. The purpose of the jejunum is to extract and absorb nutrients via both active and passive transport mechanisms. By the time nutrition reaches the ileum, it has far fewer nutrients to extract, and passes through to the lower GI tract at a faster speed.
Large intestines function by receiving undigested food and removing water from it so that the food will form solid waste that can be excreted as fecal matter. A large intestine is usually about 5 feet (about 1.5 m) long, and includes the cecum, colon, rectum, and anus. More than 700 species of bacteria exist in the large intestine. The large intestine's primary purposes are to produce vitamins for blood absorption, neutralize acidity caused by the formation of fatty acids, produce antibodies, strengthen the autoimmune system, and eliminate waste matter from the rectum through the anus.
Some common problems — such as simple indigestion, gastritis, constipation, or diarrhea — can frequently be controlled and cured through a healthy diet that is high in fiber and low in sugars. Other longer-term gastrointestinal problems — such as Crohn's disease, appendicitis, gastroesophageal reflux disease (GERD), duodenal ulcers, viral infections, and irritable bowel syndrome (IBS) — should not be ignored as they typically require medical attention and care. Gastroenterologists and GI specialists are physicians who usually should be consulted regarding disorders of the gastrointestinal tract.