The digestive tract from the mouth to the anus includes the esophagus, stomach, small intestine, and large intestine. The main physiological functions are to ingest, transport and digest food, absorb nutrients and excrete waste. Various enzymes secreted by the pancreas and gastrointestinal glands, and bile secreted by the liver digest food components into small molecular substances that are absorbed by the intestines. Various diseases (inside and outside the digestive tract) can change the normal function of the digestive tract and cause disease as long as they cause structural abnormalities of the digestive tract (erosion, ulcer, perforation, stenosis, cancer), bleeding, and abnormal nutrition and electrolyte absorption and secretion.