Which buffer system provides the FASTEST response to an acute acid load in the blood, and why?
- A Bicarbonate-carbonic acid system; because it is the most abundant and its reaction is catalyzed by carbonic anhydrase ✓
- B Phosphate buffer system; because its pKa of 6.8 is closest to blood pH of 7.4
- C Protein buffers (hemoglobin and plasma proteins); because they act within seconds due to their intracellular location
- D Bone mineral buffer; because carbonate salts neutralize acid most efficiently
Correct answer: A. Bicarbonate-carbonic acid system; because it is the most abundant and its reaction is catalyzed by carbonic anhydrase
Explanation
The bicarbonate-carbonic acid system responds within seconds to minutes because carbonic anhydrase in red blood cells catalyzes the rapid conversion of CO2 + H2O to H2CO3, which dissociates to HCO3− + H+. Its effectiveness as a buffer is enhanced by the fact that CO2 is continuously removed by ventilation (an open buffer system), making it far more effective in vivo than its pKa of 6.1 would suggest in isolation. Phosphate buffers are quantitatively minor in blood and mainly important in urine.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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