Upper gastrointestinal bleeding is characterized by the sudden onset of bleeding from the GI tract at a site (or sites) proximal to the ligament of Treitz. Acid, pepsin, and helicobacter infection play an important role in the development of gastric ulcers. The gastric mucosal barrier overlies the epithelium. The secretion of mucus and bicarbonate provides a first line defense in maintaining a near-normal pH on the gastric epithelium and protects the mucosal barrier against acid. Gastromucosal prostaglandins increase the barrier’s resistance to ulceration. The integrity of the barrier is enhanced by the rich blood supply of the mucosa of the stomach and duodenum.