Physiology and pathophysiology of pulmonary circulation.

The lungs provide not only respiratory function, but also are involved in metabolism and maintaining homeostasis. The lungs receive the entire cardiac output, circulating blood cells and hormones. The metabolically active vascular endothelial cell lining participates in degrading and synthesizing various hormones and vasoactive substances which affect the pulmonary as well as systemic circulation. The normal state is maintained through complex interaction of the circulating hormones, maturation of the system, the physiological state of the lung cells and metabolism. Any noxious stimulus can disrupt this delicate balance and adversely affect the metabolism of various substances, thus producing an abnormal state. In this review, some of these factors including therapeutic implications are discussed.