One of the roles of the kidneys is to help buffer body fluids so that they are not too acidic or too basic. The cells of the renal tubule secrete H⁺ into the tubule lumen, and absorb bicarbonate (HCO₃^-), passing it into the tissue fluid. Consider the reaction catalyzed by the enzyme carbonic anhydrase, shown in the accompanying figure and below. During heavy exercise, the reaction shifts to the right in the nephron (lumen of the tubule).

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During heavy exercise, there is an increase in the production of carbon dioxide, which combines with water to form carbonic acid, leading to an increase in acidity. To compensate for this, the kidneys increase the secretion of acid (H⁺) into the lumen of the tubule, and absorb bicarbonate (HCO₃^-) from the tubule fluid. This process helps to buffer body fluids and maintain the pH within a normal range. The reaction catalyzed by carbonic anhydrase is important in this process because it allows for the rapid conversion of carbon dioxide and water to form carbonic acid, which can then dissociate into hydrogen ions and bicarbonate ions. During heavy exercise, the reaction shifts to the right in the nephron (lumen of the tubule), leading to the increased secretion of acid and absorption of bicarbonate.

One of the Roles of the Kidneys Is to Help