Pathophysiology
Clinical meaning
The pulmonary circulation is a low-pressure, high-compliance system (mean PAP 8-20 mmHg) receiving the entire cardiac output. Unlike systemic arterioles which dilate in response to hypoxia, pulmonary arterioles constrict during hypoxia (hypoxic pulmonary vasoconstriction or HPV) to redirect blood away from poorly ventilated alveoli, optimizing V/Q matching. Chronic hypoxia from COPD, ILD, or OSA causes sustained pulmonary vasoconstriction, smooth muscle hypertrophy, and intimal fibrosis leading to pulmonary hypertension (mPAP >= 20 mmHg at rest). The right ventricle, adapted for volume work against low pressures, eventually fails under chronic pressure overload, producing cor pulmonale. Pulmonary embolism acutely increases pulmonary vascular resistance, causing right ventricular dilation and potential hemodynamic collapse.