Need-to-know anatomy

The stomach is an arc-shaped organ located in the upper part of the abdomen just beneath the diaphragm. One end of the stomach joins the esophagus where a ring of muscle called a sphincter allows food into the stomach as it's swallowed. The other end narrows to a channel called the pylorus that connects the stomach and the duodenum, or the upper part of the small intestine.

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The duodenum, or the first portion of the small intestine, is about 12 inches long and extends from the narrow opening of the pylorus, a thick ring of muscle that helps regulate the flow of food from the stomach, to about the middle of the small intestine. It is shaped in a sharp curve that almost completes a circle.

Glands in the stomach produce acid and an enzyme called pepsin that help the body digest food. Normally, the stomach and the duodenum also produce mucus that protect them from the effects of the acid. In people with peptic ulcers, the defenses break down, often because of an infection with the H. pylori bacterium, and sores form in the lining of the stomach (gastric ulcer) and the duodenum (duodenal ulcer).