Light microscopy of the stomach lining showing bleeding from a stomach ulcer. Blood vessels and gastric pits are seen. Gastric pits secrete the digestive enzyme pepsin, along with hydrochloric acid and protective mucus. When the mucus is removed or diminished, the acid can attack the lining itself, leading to bleeding. Most ulcers are caused either by infection of the stomach lining by the bacterium Helicobacter pylori, or by drugs that reduce the production of protective mucus, usually common NSAIDs such as aspirin and ibuprofen. For a similar clip of a healthy stomach lining, see K007/4738.