PHYSIOLOGY OF THE PULMONARY CIRCULATION



The pulmonary circulation has a rich capillary network similar to that of the systemic circulation. The pulmonary alveoli are adjacent to the capillaries, permitting oxygen to diffuse into and carbon dioxide out of the capillary blood. Oxygen is the major mediator of pulmonary autoregula-tion. In regions where the partial pressure of oxygen is high, pulmonary vasodilation occurs and blood flow is directed preferentially toward well-oxygenated areas of the lung. When the partial pressure of oxygen is low, pulmonary vasoconstriction occurs, preventing the perfusion of areas of the lung that have relatively poor oxygen availability. These vasodilatory effects of oxygen are opposite to those in the systemic circulation. Acidemia potentiates the pulmonary vasoconstrictive effect of hypoxemia, also opposite to its effect on systemic arterioles.

The lungs receive blood through the bronchial arteries as well as the pulmonary arteries (dual blood supply). The bronchial arteries supply arterial blood to the pulmonary tissue and drain into the bronchial veins, some of which drain into the systemic venous bed. Some bronchial veins drain into the pulmonary veins, creating a small physiological right-to-left shunt.

Pulmonary vascular resistance is normally one tenth that of systemic vascular resistance and accounts for the small pressure gradient required to propel blood across the pulmonary vascular bed. Because the pulmonary vasculature is very distensible (compliant), a relatively large left-to-right intracardiac shunt may exist with only a minimal rise in pulmonary arterial pressure.