Question

When the value of the ventilation-perfusion ratio (V/Q) is infinity, it means?

• A. No O2 goes from alveoli to blood and no CO2 goes from blood to alveoli
• B. Dead space
• C. The PO2 of alveolar air is 159 mmHg and PCO2 is 40 mmHg
• D. Partial pressures of O2 and CO2 are equal

Hint:

If Q (perfusion) drops to zero, what is left is air that never meets blood.

Answer 1

The Correct Option is B

Think of the two extremes of the V/Q spectrum as a memory anchor. At one end, ventilation is zero but perfusion continues: that is a true shunt (V/Q = 0). At the opposite end, perfusion is zero but ventilation continues: V/Q heads toward infinity, and that is dead space, which is option (b).
Dead space alveoli get fresh air but no blood arrives to pick up oxygen or drop off CO2, so no gas exchange occurs in that unit. Normal matching sits at V/Q $\approx 0.8$. A value above 0.8 leans toward dead-space physiology (more air than blood), while a value below 0.8 leans toward shunt physiology. Option (a) describes a complete failure of exchange but is not the named term, and options (c) and (d) quote specific gas tensions that do not define the infinity condition. The single correct label is dead space.
Ref: Arvind Arora Review Book of Physiology.