An acidic mixture of inorganic salts, hydrochloric acid, mucus, and pepsinogens secreted by gastric glands in the stomach lining and important for digestion of food. Oxyntic (parietal) cells in the gastric glands secrete hydrogen and chloride ions, while chief cells secrete the inactive proenzyme pepsinogen. In the lumen of the stomach, the hydrochloric acid converts pepsinogen into the active enzyme pepsin, which catalyses the initial digestion of food proteins.