The stomach is an elongated pouch-like structure lying just below the diaphragm, with most of it to the left of the midline.

It has three divisions:

• the fundus, the enlarged portion to the left and above the entrance of the esophagus;
• the body, the central portion; and
• the pylorus, the lower portions.

Circular sphincter muscles that act as valves guard the opening of the stomach. (The cardiac sphincter is at the esophageal opening, and the pyloric sphincter is at the junction of the stomach and the duodenum, the first portion of the small intestine.)

The cardiac sphincter prevents stomach contents from reentering the esophagus except when vomiting occurs. In the digestive process, two important functions of the stomach are:

a. It acts as a storehouse for food, receiving fairly large amounts, churning it, and breaking it down further for mixing with digestive juices. Semiliquid food is released in small amounts by the pyloric valve into the duodenum, the first part of the small intestine.

b. The glands in the stomach lining produce gastric juices (which contain enzymes) and hydrochloric acid. The enzymes in the gastric juice start the digestion of protein foods, milk, and fats. Hydrochloric acid aids enzyme action. The mucous membrane lining the stomach protects the stomach itself from being digested by the strong acid and powerful enzymes.