Define ventilation–perfusion (V/Q) ratio and its clinical significance.

• A. Ratio of air reaching the alveoli to blood perfusing the alveoli
• B. Ratio of alveolar ventilation to cardiac output
• C. The rate of diffusion of oxygen
• D. The amount of carbon dioxide diffused

Answer: (A)

Ventilation–perfusion ratio measures how well air reaching the gas-exchanging units of the lung (the alveoli) matches the blood flow through the surrounding capillaries. It’s defined as alveolar ventilation divided by pulmonary perfusion. This ratio is crucial because it directly affects how effectively oxygen enters blood and how CO2 is removed.

If ventilation is good but perfusion is poor, the ratio is high. That means air isn’t fully used by blood, creating dead space where gas exchange is inefficient. A common clinical example is a pulmonary embolism, which reduces capillary blood flow and raises the V/Q ratio in affected regions.

If ventilation is poor but perfusion remains (or is relatively high), the ratio is low. Gas exchange is limited because air isn’t reaching the alveoli to contact the blood, producing shunt-like effects and hypoxemia, as can occur with pneumonia, edema, or collapse of alveoli.

Normal lungs maintain a balanced V/Q to maximize gas exchange efficiency; when this balance is off, it helps explain why low oxygen levels occur and guides diagnostic and treatment decisions. The rate of diffusion of oxygen and the amount of CO2 diffused describe diffusion processes, and linking ventilation to total cardiac output mixes systemic flow with local gas exchange, so those aren’t the right way to define the V/Q ratio.