The stomach is a strong muscular bag that serves to break down and digest food that has been eaten. The process of breaking down the food partly occurs mechanically, by contractions of the stomach muscles.

At the same time, the food is broken down chemically. This happens partly because of a very strong acid, hydrochloric acid, produced by the stomach. The stomach is able to protect itself from this strong acid by producing mucus and bicarbonate that neutralizes the acid. Sometimes this defense within the stomach can break down, in which case the result can be a gastric ulcer.