Digestive System

Chemical digestion begins in the stomach. The stomach, a large, hollow, pouched-shaped muscular organ, is shaped like a lima bean. When empty, the stomach becomes elongated; when filled, it balloons out.

Food in the stomach is broken down by the action of the gastric juice containing hydrochloric acid and a protein-digesting enzyme called pepsin. Gastric juice is secreted from the linings of the stomach walls, along with mucus, which helps to protect the stomach lining from the action of the acid. The three layers of powerful stomach muscles churn the food into a fine semiliquid paste called chyme. From time to time, the chyme is passed through an opening (the pyloric sphincter), which controls the passage of chyme between the stomach and the beginning of the small intestine.