The gastric glands have three different types of cells which secrete three different substances. The parietal cells secrete hydrochloric acid, the chief cells secrete pepsinogen, and the mucous neck cells secrete mucin. Hydrochloric acid causes the acidity of gastric juice to be in the range of pH 1 to pH 3. The hydrochloric acid of the stomach practically guarantees that any bacteria which enters with the food is destroyed. The few that are not destroyed can in some cases cause food poisoning.
Hydrochloric acid combines with pepsinogen to form the enzyme called pepsin.
Pepsin digests only protein. Mucin is a secretion which protects the stomach from being dissolved by the hydrochloric acid. The presence of the acid producing bacteria H. pylori, increases the acidity of the stomach so that the stomach lining is dissolved. This condition s called a peptic ulcer. The quantity of gastric juices secreted to digest and average meal is a little over a half quart.