In the context of renal physiology, how many hydrogen ions (\(\mathrm{H^+}\)) must be secreted into the filtrate in order to reabsorb one bicarbonate ion (\(\mathrm{HCO_3^-}\))?

• A. Four
• B. Three
• C. One
• D. Two

Answer: (C)

Verified step by step guidance

1. Understand the relationship between hydrogen ions (\(\mathrm{H^+}\)) and bicarbonate ions (\(\mathrm{HCO_3^-}\)) in renal physiology. The kidneys play a crucial role in maintaining acid-base balance by secreting hydrogen ions into the filtrate and reabsorbing bicarbonate ions.
2. Recall that for every bicarbonate ion (\(\mathrm{HCO_3^-}\)) reabsorbed, a hydrogen ion (\(\mathrm{H^+}\)) must be secreted into the filtrate. This process occurs in the proximal tubule of the nephron.
3. The mechanism involves the enzyme carbonic anhydrase, which facilitates the conversion of carbon dioxide (\(\mathrm{CO_2}\)) and water (\(\mathrm{H_2O}\)) into carbonic acid (\(\mathrm{H_2CO_3}\)). Carbonic acid then dissociates into a hydrogen ion (\(\mathrm{H^+}\)) and a bicarbonate ion (\(\mathrm{HCO_3^-}\)).
4. The hydrogen ion (\(\mathrm{H^+}\)) is actively secreted into the filtrate, while the bicarbonate ion (\(\mathrm{HCO_3^-}\)) is reabsorbed into the bloodstream. This exchange helps regulate blood pH and maintain homeostasis.
5. Based on this physiological process, the secretion of one hydrogen ion (\(\mathrm{H^+}\)) is required to reabsorb one bicarbonate ion (\(\mathrm{HCO_3^-}\)).