 The NHE3 antiporter imports one sodium ion into the cytosol of a tubule cell as it ejects one hydrogen ion from the cell into the lumen of the proximal tubule.The sodium within the tubule cell may then be retained by the body rather than excreted in the urine.The NHE3 antiporter indirectly contributes to blood buffering capacity because hydrogen ions that are ejected are the products of the carbonic anhydrase enzyme, which also makes bicarbonate.