Ventilation-perfusion matching

For the lung to do its job, air alone is not enough, and neither is blood: both have to arrive at the same place at the same time. Air entering the alveolus, the little sac where exchange happens, is called ventilation, and blood arriving at its capillaries is called perfusion. The ratio between the two, the V/Q, is how we measure whether that meeting actually takes place.

And it does not take place equally throughout the lung. At the base both arrive in greater amounts, but blood increases more than air, so the V/Q ratio there is lower than at the apex, the top of the lung. In other words, mismatch exists even in a healthy lung.

What stands out is that the lung has a way of correcting itself. When an alveolus is poorly ventilated, the arteriole carrying blood to it constricts, and that blood moves toward the areas that are receiving air. Instead of wasting it in a corner with no oxygen, the lung redirects it.

That constriction is thought to be driven by potassium and calcium channels in the arterial wall that are sensitive to oxygen. Even so, many details of how it happens are still unclear.