Alveolar ventilation (V) and pulmonary perfusion (Q) are both flow rates measured in L/min. The whole-lung V/Q ratio is approximately 0.8 at rest, but this value is an average over a heterogeneous population of alveoli.
Why the ratio, not V or Q alone, determines gas exchange
Oxygen moves from alveolar gas into capillary blood, and carbon dioxide moves from blood into alveolar gas. For this exchange to be efficient, the flow of fresh air and the flow of mixed venous blood must arrive at the alveolus in the right proportion. If V is high and Q is low, a large volume of air contacts a small volume of blood, so the blood is well oxygenated but much of the ventilation is wasted. If Q is high and V is low, a large volume of blood contacts a small volume of air, so the blood leaves poorly oxygenated. The ratio V/Q captures this balance directly.
The ideal lung as a reference model
The ideal single-alveolar lung assumes every alveolus has the same V/Q ratio, equal to the whole-lung average. This model is useful for predicting how changes in V or Q shift alveolar and arterial partial pressures, but it does not describe the real lung, where V/Q varies from region to region. The real lung is a spectrum of ratios, and that heterogeneity has its own consequences for arterial oxygenation.