The pulmonary vasculature is a high-capacity, low-resis-tance system, and it functions under relatively low pressure. The normal pulmonary arterial systolic and mean pressures are less than 30 mm Hg and less than 20 mm Hg, respectively.
Diseases of this sys-tem impair gas exchange across the pulmonary alveoli and result in an increase in pulmonary arterial pressure with resultant detrimental effects on the function of the right ventricle. As a result of the extensive nature of the pulmonary capillary network and the distensibility of the pulmonary vasculature, the pulmonary system is able to tolerate significant increases in blood flow with little or no rise in pulmonary pressure.

Thus, pulmonary hypertension usually results from an increased pulmonary vascular resistance. is uniquely arranged The pulmonary capillary system to maximize oxygen and carbon dioxide exchange across the alveolar-capillary membrane. Diseases associated with pulmonary hypertension are frequently complicated by impaired alveolar gas exchange with resultant hypoxemia, not as a direct result of the pulmonary vascular disease but, rather, as a result of the primary underlying lung disease. Pulmonary vascular occlusion and obliteration, which may be a secondary result of the pulmonary hypertension, may further decrease the diffusing capacity of the lungs and cause ventilation-perfusion mismatches, thus contributing to the hypoxemia. Because of the low pressure of the normal pulmonary mal contractile work. Elevated pulmonary pressure is important mainly in regard to the effect it has on right faced with an acute increase ventricular function in pulmonary pressure, the right ventricle is able to generate systolic pressure of 50 mm Hg failing.

More indolent development of pulmonary hypertension results in right ventricular hypertrophy and the ability of the right ventricle to generate pressures equal to or greater than systemic pressure chronic pulmonary hypertension may right ventricular failure;

because of the interdependence of the right and left ventricles, the elevated pulmonary pressures can also affect left ventricular function. As the right dilates, the septum the left ventricular cavity, thereby decreasing left ventricular stroke volume and compliance and producing left ventricular failure.