The stomach is the site of food storage and initial protein digestion. The sphincter muscles regulate the movement of food to and from the stomach, these circular muscles act like the drawstrings of a purse. The stomach an store about 1.5 litres of food. Millions of secretory cells line yhe inner wall of the stomach.

The storage of a highly corrosive acid and a protein - digesting enzyme in a stomach that is composed of cells made of protein poses a major problem. The pH of the inside of the stomach is usually between 2.0 and 3.0, sometimes may also reach 1.0.