MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A patient is diagnosed with a condition where their blood has an abnormally low pH due to an excess of carbon dioxide. The respiratory system attempts to compensate by increasing the rate and depth of breathing. This compensatory mechanism aims to:
- ADecrease the partial pressure of CO2 in the blood.
- BDecrease the production of carbonic acid in the blood.
- CIncrease bicarbonate reabsorption in the kidneys.
- DIncrease the excretion of H+ ions by the kidneys.
Show answer & explanationAnswer & explanation
Correct answer: A. Decrease the partial pressure of CO2 in the blood.
Increasing the rate and depth of breathing (hyperventilation) exhales more CO2. By the equilibrium CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-, reducing CO2 shifts the reaction to the left, decreasing H+ concentration and raising pH.
Why the other options are wrong
- B. Increasing breathing rate decreases CO2, which in turn decreases carbonic acid (H2CO3) formation, but the direct aim of breathing is to remove CO2.
- C. Bicarbonate reabsorption is a renal (kidney) compensatory mechanism, not a direct result of increased breathing.
- D. H+ excretion by kidneys is a renal compensatory mechanism, not directly related to respiratory compensation.
Respiratory Compensation for Acidosis
The physiological response of the respiratory system to restore blood pH balance by altering the rate and depth of breathing, primarily to adjust CO2 levels.
- Hyperventilation reduces CO2, increasing pH.
- Hypoventilation increases CO2, decreasing pH.
- Acts rapidly but may not fully correct severe imbalances.
Memory trick: Respiratory blows off CO2, Renal pees out H+ or keeps HCO3-.