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: The ventilation-perfusion ratio is \(\dfrac{V_A}{Q}\), where \(V_A\) is alveolar ventilation and \(Q\) is the alveolar blood flow (perfusion). The ratio describes how well ventilation is matched to perfusion in an alveolus.
Step 2: When \(\dfrac{V_A}{Q}\) tends to infinity, the denominator (perfusion) approaches zero while ventilation continues. This is alveolar dead space: air enters the alveolus but no blood flows past it, so there is no exchange of oxygen and carbon dioxide across the respiratory membrane. Hence the answer is 'no exchange of O2 and CO2'.
Step 3: Why the others are wrong. With no perfusion the alveolar gas simply equilibrates with humidified inspired air, so the alveolar oxygen stays high (about 149 mmHg) rather than falling to zero, and the carbon dioxide falls to about 0 mmHg only as a consequence of the absent exchange. The defining feature being tested is the loss of gas exchange itself, not any single normal pressure value.