Bulk of reabsorption occurs in the proximal tubule

Secreted H⁺ combines with filtered HCO₃^-, generating carbonic acid. This in turn dissociates into carbon dioxide and water by the action of Carbonic anhydrase. Carbon dioxide and water diffuses readily into the cells recombine into carbonic acid and then into hydrogen and bicarbonate ions. The bicarbonate ions diffuse into the intercellular space.

A similar mechanism is involved in the reabsorption of the small amount of bicarbonate ions that leave the proximal tubule and are absorbed by the distal tubule. The difference is that there is no carbonic anhydrase in the brush border of the distal tubule. This compensated by the presence of a larger amount of hydrogen ion due to the presence of a H⁺-ATPase pump. The relatively high concentration of hydrogen ions shifts the reaction towards the right (i.e. more carbonic acid and thus more carbon dioxide and water)