Question

When is the ventilation-perfusion ratio (Va/Q) infinity?

• A. Partial pressure of O2 becomes zero
• B. No exchange of O2 and CO2
• C. Partial pressure of CO2 alone becomes zero
• D. Partial pressure of both CO2 and O2 remain normal

Hint:

Infinite Va/Q means ventilation without perfusion - alveolar dead space.

Answer 1

The Correct Option is B

Step 1: Think of the two extremes of the ventilation-perfusion spectrum. The ratio $\frac{Va}{Q}$ goes to infinity when its denominator, the perfusion Q, falls to zero while ventilation Va persists. Step 2: Physiologically this is dead-space ventilation: fresh air enters the alveolus but no capillary blood arrives to pick up oxygen or unload carbon dioxide. Step 3: Because no blood contacts the alveolar membrane, the alveolar air ends up identical to humidified inspired air, with PO2 near 149 mmHg and PCO2 near 0 mmHg, and crucially no net transfer of gases occurs. Step 4: The key consequence asked for is that gas exchange ceases, so the correct choice is that there is no exchange of O2 and CO2. \[\boxed{\text{No exchange of O}_2 \text{ and CO}_2}\]