ventilation–perfusion ratio

Imagine a busy restaurant where waiters bring fresh food (air) and customers arrive to eat it (blood). Things run smoothly only when the flow of food roughly matches the flow of customers. The ventilation–perfusion ratio, written V/Q, is exactly this match-up in each region of lung: how much fresh air reaches a patch of air sacs compared with how much blood flows past them.

In an ideal patch, ventilation (V) and perfusion (Q) are balanced, giving a ratio near one, and the blood leaving is fully loaded with oxygen and stripped of carbon dioxide. Across the whole lung the average ratio for both quantities is roughly 0.8. Gravity, however, makes it uneven even in health: in an upright person the lung bases receive proportionally more blood and the apices proportionally more air, so the ratio is lower at the bottom and higher at the top.

Matching V to Q is the central job of efficient gas exchange, and the lung actively defends it. When a region is poorly ventilated, its small arteries constrict to divert blood toward better-aired regions — a reflex called hypoxic pulmonary vasoconstriction. A caveat: this matching is imperfect and easily disrupted, and most clinically important hypoxemia traces back to regions where the ratio has drifted too high or too low.