Gastrointestinal Tract and Liver

Abstract

The functions of the gastrointestinal tract (GI tract) are digestion and absorption of food, and elimination of waste as feces. After ingestion, food passes down the esophagus and is temporarily stored in the stomach, where it is mixed with digestive juices, and broken down into small particles. It then passes through the duodenum, jejunum, ileum, large intestine, and finally the rectum and anal canal (Figure 14.1). During its passage through the tract it is mixed with saliva, gastric juice, bile and pancreatic juice, and other enzymes which are secreted by the small bowel. The digested food particles are thus absorbed and further metabolized in the liver, which is involved in storage and breakdown of many metabolic products. In the majority of the gastrointestinal tract the thin outer layer of the bowel is the peritoneum, beneath which is a longitudinal and a circular layer of muscle. In some parts of the bowel, the circular muscle is condensed to form sphincters which regulate the passage of bowel contents through the tract. Between the inner mucosal layer of the bowel and the muscle is a submucosal layer which contains a rich lymphatic, autonomic nerve, and blood supply.Mucus-secreting cells in themucosa lubricate the bowel contents, and in the submucosal layer are the glands that secrete the digestive enzymes. The liver, pancreas, and GI tract all are intimately involved in digestion, absorption, and metabolism of food and interact with each other via the nervous system and hormonal excretion. 14.1.1 Esophagus