What is the role of carbonic anhydrase in the kidneys?

a. It drives the recovery of HCO3– and ex-cretion of H+.
b. It is the catalyst for the hamburger phe-nomenon.
c. It promotes the excretion of CO2 in the urine.
d. It promotes the loss of fluids in congestive heart failure.

ANS: A
The HCO3– ions in the filtrate react with the H+ ions secreted by the tubular cells. The resulting carbonic acid breaks down into CO2 and water. Because CO2 is extremely diffusible through bi-ological membranes, it diffuses instantly into the tubule cell. There, CO2 reacts rapidly with water in the presence of carbonic anhydrase, rapidly forming HCO3– and H+. The HCO3– ion diffuses back into the blood. Thus, the reabsorbed HCO3– ion is not the same HCO3– ion that existed in the tubular fluid. If the tubule cells secrete sufficient H+, all HCO3– in the tubular fluid is reab-sorbed in this manner.