FIGURE 11-31 Acidification of the stomach lumen by parietal cells in the gastric lining. The apical membrane of parietal cells contains a H⁺/K⁺ ATPase (a P-class pump) as well as Cl⁻ and K⁺ channels. Note the cyclic K⁺ transport across the apical membrane: K⁺ ions are pumped inward by the H⁺/K⁺ ATPase and exit via a K⁺ channel. The basolateral membrane contains an anion antiporter that exchanges HCO₃⁻ and Cl⁻ ions. The combined operation of these four different transport proteins and carbonic anhydrase acidifies the stomach lumen while maintaining the neutral pH of the cytosol.