Which compound is the principal acid in the stomach?

Hydrochloric acid is the principal acid in the human stomach. It is an aqueous solution of hydrogen chloride, HCl, secreted by parietal cells in the stomach lining.

Stomach acid helps denature proteins and activates pepsin, an enzyme that begins protein digestion. Its strongly acidic environment also provides a barrier against many microorganisms swallowed with food. The stomach lining protects itself with mucus, bicarbonate, tightly joined cells, and continuous repair.

Hydrochloric acid is not the same as pure hydrogen chloride gas. Hydrogen chloride becomes hydrochloric acid when dissolved in water. In water, it dissociates extensively into hydronium ions and chloride ions, making it a strong acid.

The concentration and effects of stomach acid are regulated rather than left constant. Excess acid can contribute to conditions such as reflux or ulcers, while medicines including proton-pump inhibitors reduce acid secretion. Hydrochloric acid is also an important industrial reagent used for metal cleaning and chemical manufacture.