The distribution of ventilation and perfusion in the normal lung, studied with Xe133.

Anatomically the lung consists of two conducting systems to bring the inspired air and venous blood into contact. The effectiveness of gas exchange is largely dependent on the efficiency of these distribution systems. If parts of the lung are ventilated but not perfused, or perfused but not ventilated, no gas exchange will occur in these areas. Furthermore, ventilation in excess of perfusion, or perfusion in excess of ventilation are relatively inefficient. The optimum distribution is clearly when the quantities of ventilation and perfusion are matched, that is, the Ventilation/Perfusion ratio is equal in all lung units. To predict gas exchange it is therefore necessary to know not only the total quantities of alveolar ventilation and pulmonary capillary blood flow, but also their distribution.