Amylase begins the breakdown of com-plex carbohydrates, or starches, into the simple sugars that are used by the body for fuel. Swallowing is an almost unconscious effort that begins with a carefully coordinated closing of the larynx to prevent food from going into the windpipe as it moves into the pharynx, a funnel-shaped structure leading to the esophagus. The esophagus, a muscular tube about a foot long between the mouth and the stomach, connects to the stomach through an opening in the diaphragm, the muscle wall separating the chest and abdominal cavities. The esophageal sphincter, a valve standing guard at the juncture of the esophagus and stomach, relaxes as chewed food approaches, allowing the food to pass into the stomach, and then contracts to prevent a backflow or regurgitation of stomach contents into the esophagus. The antrum contains the cells that make the hormone gastrin, which stimulates the parietal cells to produce hydrochloric acid.
Stomach acid kills most bacteria and other microorganisms present in food.
A special mucous coating protects the stomach wall and other intestinal organs from being burned by this gastric acid. The stomach's 3 layers of muscle, running up and down, horizontally, and crosswise, contract to produce a churning action that mixes food with digestive enzymes and hydrochloric acid. The gastric chemicals help break down complex nutrients carbohydrates, proteins, and fats into their components. Digestion in the stomach generates signals to the brain via the vagus nerve. An intricate combination of neural, chemical, and hormonal factors controls the rate at which stomach contents flow out through the pyloric valve and the pyloric canal.