Question

When is the ventilation-perfusion ratio (Va/Q) equal to 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 pressures of both CO2 and O2 remain normal

Hint:

Infinite Va/Q means dead space - ventilation without perfusion, so no gas exchange.

Answer 1

The Correct Option is B

Step 1: Treat \(V_A/Q\) as a fraction. A ratio of zero means ventilation has stopped while blood still flows (a shunt). A ratio of infinity means the perfusion term \(Q\) has dropped to zero while ventilation persists (dead space). The question concerns the infinity end.
Step 2: If no blood perfuses the alveolus, there is no medium to take up oxygen or to deliver carbon dioxide, so gas exchange ceases across that respiratory membrane. The single defining consequence of \(V_A/Q = \infty\) is therefore no exchange of O2 and CO2.
Step 3: Reject the others. Because fresh air keeps entering an unperfused alveolus, its gas equilibrates with humidified inspired air, giving an alveolar \(P_{O_2}\) near 149 mmHg (not zero) and a \(P_{CO_2}\) near 0 mmHg. These pressure values are downstream results, while the essence of the infinite ratio is the abolished gas exchange.
\[\boxed{\text{No exchange of O2 and CO2}}\]