American Board of Internal Medicine (ABIM) Certification Practice Exam

What mechanism is responsible for the acute fall in serum bicarbonate with decreasing PCO2?

1. Extracellular-to-intracellular shift of hydrogen ions

2. Intracellular-to-extracellular shift of hydrogen ions

3. Increased renal excretion of bicarbonate

4. Increased respiratory rate

The correct answer is: Intracellular-to-extracellular shift of hydrogen ions

The mechanism responsible for the acute fall in serum bicarbonate with decreasing PCO2 involves the intracellular-to-extracellular shift of hydrogen ions. When PCO2 decreases, such as during hyperventilation, the resulting drop in carbonic acid concentration leads to a decrease in hydrogen ion concentration in the extracellular fluid. As hydrogen ions shift from the intracellular space to the extracellular compartment to maintain electrochemical balance, this causes a relative increase in bicarbonate ions in the serum, which is a compensatory response. This intracellular-to-extracellular movement of hydrogen ions is critical because it allows the body to quickly adjust to changes in the acid-base balance. The bicarbonate buffer system plays an essential role in maintaining pH, and changes in PCO2 directly influence bicarbonate levels due to shifts in hydrogen ion distribution. In contrast, while increased renal excretion of bicarbonate could eventually contribute to acid-base balance in chronic conditions, it is not an immediate mechanism associated with acute changes in PCO2. An increased respiratory rate, while it may decrease PCO2, does not directly account for the immediate alteration in bicarbonate levels as described in the question. Moreover, shifts of hydrogen ions are more integral to the abrupt changes in bicarbonate seen during rapid