Abstract
A principal function of the vasculature is to transport and deliver O2. Thus, it is not surprising that a variety of mechanisms have evolved to optimize gas exchange in the lungs and O2 delivery to tissues. In both the systemic and pulmonary circulations changes in O2, induce a change in vessel caliber. Decreasing arterial pO2 causes vasodilation in coronary, skeletal muscle, cerebral and gastrointestinal circulations. In contrast, in the pulmonary circulation both airway hypoxia and pulmonary arterial hypoxemia cause vasoconstriction in a process known as hypoxic pulmonary vasoconstriction. Systemic hypoxic vasodilation provides a feedback mechanism of increasing blood flow to tissues with increased metabolic demands. In the pulmonary circulation hypoxic pulmonary vasoconstriction provides a mechanism for optimizing tissue oxygenation, matching perfusion to ventilation, therefore improving gas exchange.