Fig. 2.

Simplified schematization of H⁺ and HCO₃⁻ handling by the kidneys. Plasma is filtered from the renal blood supply at the glomerulus of each nephron (panel A). Acids are secreted into the filtered fluid by epithelial cells in the proximal tubule (panel B) and the collecting duct (panel C), generating new HCO₃⁻ that is absorbed into circulation to replace that titrated by acids in circulation. In the tubule lumen, if secreted H⁺ is titrated by a non-HCO₃⁻ buffer such as NH₃ or phosphate, it can be excreted. If a secreted H⁺ reacts with a filtered HCO₃⁻ in the proximal tubule lumen (catalyzed by CAIV), the CO₂ enters the proximal tubule cell and is converted back into a H⁺ and HCO₃⁻ (by CAII). In that case no H⁺ is excreted and the HCO₃⁻ is considered to have been reabsorbed. Virtually no filtered HCO₃⁻ is excreted in the urine and the amount of H⁺ excreted varies with the body’s acid load. NHE3: Na⁺/H⁺ exchanger 3; CAIV: carbonic anhydrase IV; CAII: carbonic anhydrase II; NBCe1: electrogenic Na⁺/HCO_3⁻ cotransporter. Note that, because the stoichiometry of NBCe1 in the renal proximal tubule is not yet established [43], we have indicated with "n" the number of HCO₃⁻ ions carried by NBCe1; AE1: Cl^–/HCO₃^– exchanger 1.