Transport mechanisms involved in hydrogen secretion and HCO3– and K+ reabsorption in type A intercalated cells, which are present from the late distal convoluted tubule to the initial portion of the inner medullary collecting duct. Water within the cell dissociates into H+ and OH– ions. The former are secreted into the lumen by H-ATPase pumps in the luminal membrane, where they combine with urinary buffers to generate titratable acid (eg, convert HPO4–2 to H2PO4–) and convert NH3 to NH4+. The OH– ions in the cell combine with CO2 to form HCO3– in a reaction catalyzed by carbonic anhydrase (CA). Driven by their electrochemical concentration gradients, cellular bicarbonate enters the peritubular capillaries in exchange for extracellular chloride via Cl-HCO3 exchangers on the basolateral membrane. H-K-ATPase pumps, which secrete H+ and reabsorb K+, are also present in the luminal membrane of the type A intercalated cells. The number and activity of these pumps are increased by K+ depletion, suggesting that they may be important for K+ conservation.