The H+/K+-ATPase in the luminal membrane of parietal cells drives H+ ions into the glandular lumen in exchange for K+ (primary active transport), thereby raising the H+ conc. in the lumen. K+ taken up in the process circulates back to the lumen via luminal K+ channels. For every H+ ion secreted, one HCO3– ion leaves the blood side of the cell and is exchanged for a Cl– ion via an anion antiporter. (The HCO3 – ions are obtained from CO2+ OH–, a reaction catalyzed by carbonic anhydrase, CA). This results in the intracellular accumulation of Cl– ions, which diffuse out of the cell to the lumen via Cl– channels. Thus, one Cl– ion reaches the lumen for each H+ ion secreted.