Resistive vessel distensibility integrated over a wide range of blood flows and hematocrit values, designated as α, has been developed using a simple model of the pulmonary circulation.,, This coefficient α is a mechanical property of the lung vasculature and represents the percent change in vessel diameter per millimeter of mercury increase in transmural pressure. It is independent of vessel size and can be recalculated from given values of pulmonary artery pressure (PAP), wedge pressure, and cardiac output (CO). In vitro measurements on isolated vessel segments have indicated that normal pulmonary arteries distend approximately 2% of their diameter for each increase in millimeter of mercury, and these results have been replicated in healthy, normoxic volunteers during heart catheterization at rest and at exercise. Decreased α can be expected with chronic hypoxia and with aging, when vascular stiffening and arterial remodeling occur. In a small study of asymptomatic carriers of the bone morphogenetic protein receptor-2 (BMPR-2) mutation, multipoint mean PAP and CO plots from echocardiography were used to derive the α coefficient, which was found to be decreased in this high-risk population. Hence, α may carry high sensitivity for detecting pathological change in the pulmonary vasculature, using both invasive and noninvasive means.