Gastric juice is a colorless, watery, acidic, digestive fluid secreted by the stomach glands, which contains hydrochloric acid, mucin, pepsin, and rennin. Small amounts of gastric juice are present in the resting state. The anticipation of food, or its entry into the mouth, excites a reflex or conditioned secretion. More gastric juice is formed when food actually enters the stomach; a pint of highly acidic juice may be secreted after a heavy meal, the process continuing up to four hours. The stimulus to secretion is not mere contact with food but a hormone, gastrin, which is released into the circulation from the pyloric region of the stomach, and stimulates the body of the organ. Protein, alcohol, and coffee are potent excitants of this phase of secretion. The secretion is strongly acidic, with up 0.5% of hydrochloric acid; some of which is neutralized by mucus, so only some 0.3% is present in the free state.
The enzymes present are pepsin, which converts proteins into polypeptides, and rennin, which curdles milk into casein clot, to be digested by pepsin. Gastric juice acts by virtue of its acidity and through its enzyme action. The acid alone is responsible for minor breakdown of certain foods, but most of the work is done by the pepsin, which also helps liberate the fat of the food by dissolving the fibrous framework round its globules, preparing it for digestion in the duodenum.