Lung gas tensions and active regulation of ventilation/perfusion ratios in health and disease.

THE lung is a large organ (over 30 cm in height, in man) perfused with blood at a relatively low pressure. Thus the influence of gravity on the distribution of blood flow and ventilation is considerable. In normal lungs significant differences in perfusion, ventilation and ventilation/perfusion ratio exist between apex and base (West 1g62), and in disease mismatching of ventilation and perfusion in different lung units may be very marked. Nevertheless, the distribution of perfusion and ventilation is not determined for the lung wholly by external factors such as gravity or disease processes. For example, the pulmonary blood vessels constrict in the presence of alveolar hypoxia. This review examines the contribution of active mechanisms such as local vasoconstriction and bronchoconstriction to the efficiency of the lung as a gas exchanger.