Diseases of the GI tract are common, accounting for one out of seven complaints, and disorders of the stomach and duodenum make up a large part of these.

It has been known for many centuries that the gastric juice is acid in nature, but it was not until 1824 that William Prout established that the acid in the stomach is hydrochloric acid. Since then physicians have been fascinated by the ability of the healthy stomach and duodenum to withstand hydrochloric acid and pepsin. In particular they have studied extensively the mechanisms controlling gastric secretion in the hope of finding a satisfactory way of explaining and treating peptic ulcer disease. Recent attention has turned to the role of mucus, bicarbonate and prostaglandins in maintenance and defense of the gastric mucosa against acid injury. This chapter will review the anatomy, physiology and related common disorders of the stomach and duodenum.