The H 2 O within the distal tubular cell dissociates into H + and hydroxide (OH −); in the presence of carbonic anhydrase, the OH − combines with CO 2 to form HCO 3 −, which is transported back into the peritubular capillary, while the H + is secreted into the tubular lumen and joins with freely filtered HCO 3 − to form CO 2 and H 2 O, which are also reabsorbed.