*HCO<sub>3</sub><sup>-</sup> is reabsorbed here in a different way to other substances. The presence of hydrogen in the lumen causes some amount of it to dissociate to carbon dioxide and water via the enzyme carbonic anhydrase. These enter the epitehlial cells where they are reformed using the same enzyme. The HCO<sub>3</sub><sup>-</sup> then leaves the cell using the basolateral 3HCO<sub>3</sub><sup>-</sup>/Na<sup>+</sup> co-transporter. | *HCO<sub>3</sub><sup>-</sup> is reabsorbed here in a different way to other substances. The presence of hydrogen in the lumen causes some amount of it to dissociate to carbon dioxide and water via the enzyme carbonic anhydrase. These enter the epitehlial cells where they are reformed using the same enzyme. The HCO<sub>3</sub><sup>-</sup> then leaves the cell using the basolateral 3HCO<sub>3</sub><sup>-</sup>/Na<sup>+</sup> co-transporter. |