As food reaches the stomach, an expandable muscular sack capable of holding two to three liters of food and liquids, a number of things happen.Food is mechanically broken apart by the contracting, churning movements of the stomach's muscular walls.The chemical breakdown of food is initiated by the hormone gastrin, which stimulates the secretion of hydrochloric acid and pepsinogen from glands lining the walls of the stomach. Pepsinogen is an inactive form of the protein-digesting enzyme, pepsin. Pepsin must be secreted in the inactive pepsinogen form in order to prevent it from digesting the cells that produce it. It is the highly acidic conditions in the stomach created by the release of hydrochloric acid that converts pepsinogen into pepsin. Once pepsinogen is converted into pepsin, it breaks proteins down into shorter amino acid chains called peptides. So the stomach itself isn't digested by hydrocholoric acid and pepsin, cells lining the stomach produce a very thick mucous that coats the stomach. However, the protection is not perfect and cells lining the stomach must nonetheless be replaced ever few days due to damage. The result of the mechanical and chemical processes in the stomach is a thick, acidic liquid called chyme, consisting of partially digested food and digestive secretions. Peristaltic waves occurring at about twenty second intervals propel the chyme towards the small intestine. A circular ring of muscle, called the pyloric-sphincter, at the base of the stomach allows only about a teaspoon of chyme to be released into the small intestine with each wave. Depending on the size of the meal, it takes two to six hours to empty the stomach into the small intestine.