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Renal acid–base regulation combines bicarbonate reabsorption with hydrogen-ion secretion and excretion. The proximal tubule reabsorbs most filtered bicarbonate; in this process, secreted H⁺ helps convert filtered bicarbonate into carbon dioxide and water, which can enter tubular cells and be converted back to bicarbonate. Farther along the nephron, Type A intercalated cells secrete H⁺. The kidneys excrete acid as titratable acid, including H₂PO₄⁻, and as ammonium (NH₄⁺). Titratable acid excretion is constrained by phosphate availability, while ammonium excretion can increase during chronic acidosis. For each H⁺ excreted, a new bicarbonate is generated and returned to plasma. Renal compensation takes hours to days, but is important in chronic acid–base disorders.