Fig. 2.

Proximal tubule. Intracellular carbonic acid (H₂CO₃⁻) dissociates into H⁺ and HCO₃⁻ under the action of carbonic anhydrase II (CAII). H⁺ secretion is facilitated by Na⁺/H⁺ exchanger, and Na⁺/HCO₃⁻ cotransporter is responsible for HCO₃⁻ transport. In the lumen, H⁺ reacts with HCO₃⁻ to form H₂CO₃, which dissociates into H₂O and CO₂ through the action of carbonic anhydrase V (CA V). Glucose, amino acids, phosphate and other substances are also reabsorbed through active and passive processes in the proximal tubule (mechanism not shown in schematic diagram). Damage in this region leads to Fanconi’s syndrome