The adult lung has about 300 million alveoli. Each alveolus is surrounded by a capillary mesh. Gas exchange is optimal in alveoli where ventilation is closely matched to blood flow or perfusion. More specifically, gas exchange is optimal in alveoli where the fraction of alveolar ventilation (A) is matched to the fraction of cardiac output () perfusing that alveolus. For the whole lung, an ideal ventilation to perfusion ratio (A/) is between 0.8 to 1.0.
However, with 300 million alveoli, it is unlikely that ventilation will be precisely matched to perfusion in each alveolus. Some alveoli may be overventilated relative to their blood flow and exhibit a A/ > 1.0. Other alveoli may be overperfused relative to their ventilation and have a A/ of less than 0.8. At the extremes, some alveoli may be ventilated put receive no perfusion (infinite A/), whereas other alveoli may be perfused but not ventilated (very low A/). Normally in the upright lung, the A/ varies slightly from the apex to the base of the lung because of disparities in the distribution of ventilation and blood flow to these regions.