HCl

Meaning: Parietal cells in the gastric glands secrete HCl into the stomach lumen. Inside the cell, carbonic anhydrase converts CO₂ and H₂O to H₂CO₃, which dissociates to H⁺ and HCO₃⁻. An H⁺/K⁺ ATPase pumps H⁺ into the lumen, Cl⁻ follows through channels, and HCO₃⁻ exits to the blood. The acid denatures proteins, kills many microbes, and converts pepsinogen (from chief cells) to active pepsin. Gastrin, histamine and acetylcholine stimulate secretion.

Equation: pH = −log[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L and pH is unitless. HCl is a strong acid, so it dissociates essentially completely and [H⁺] ≈ [HCl].

On the MCAT: Digestion passages ask what happens if acid secretion is blocked or the stomach pH rises: pepsin activity falls and protein digestion in the stomach drops. Other questions follow acidic chyme into the duodenum, where secretin prompts the pancreas to release bicarbonate that neutralizes the acid so pancreatic enzymes can work. Activity-versus-pH graphs show pepsin peaking near pH 2 and trypsin near pH 8.

Common confusion: HCl mainly denatures proteins; pepsin cleaves the peptide bonds. Parietal cells make HCl, while chief cells make pepsinogen. The stomach wall survives because mucus and bicarbonate from mucous cells coat it, not because its cells resist acid.