Despite popular belief, the gastric acid of the stomach does not perform any digesting itself. However, it does play a key role in the digestion process. When the brain senses food, the hormone gastrin triggers two events: cells in the stomach lining secrete hydrochloric acid into the gastric acid, and chief cells in the stomach release pepsinogen. When the pepsinogen and the hydrochloric acid meet, they undergo a chemical reaction and generate pepsin. Studies show this reaction can occur outside the stomach as well, but it is much slower.

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