Pulmonary Circulation

The pulmonary vasculature is unique, both in volume and function. The pulmonary circulation, a low-pressure vascular bed that accommodates the entire cardiac output, carries the mixed venous blood to the alveoli, where gas exchange occurs, and then back to the left heart for distribution of oxygenated blood to the rest of the tissues in the body. As compared with the systemic circulation, the pulmonary arteries have thinner walls with much less vascular smooth muscle. Moreover, in order to maintain the low pulmonary arterial pressure, normal pulmonary vascular resistance is approximately one-tenth that of the systemic circulation. Factors that control pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. A unique aspect of the pulmonary circulation is the pressor response to hypoxia, as the systemic circulation dilates in response to decreased oxygen concentrations. In addition to gas exchange, the pulmonary circulation also serves to filter the blood, removing microemboli, and participates in the metabolic regulation of a variety of vasoactive hormones. Several diseases can affect the function of the pulmonary circulation, including primary and secondary pulmonary hypertension, arteriovenous malformation, embolism, and fibrotic lung disease.