In adults with Pulmonary hypertension, smooth cells proliferate in the intima, thickening and clogging the arteriole walls. This altered tissue inside the arterioles forms a plexiform lesion. The lesion clogs the inside of the arterioles, causing scar formation of the intima and media arteriole layers. This narrows the vessel, with a resultant increase in MPAP. This scar tissue is fibrous and prevents the arterioles from expanding and contracting, thereby decreasing the normal compensatory mechanism of the pulmonary circulatory system. This thickening or remodeling of the intima arteriole layer is irreversible and leads to permanent vasoconstriction of the arterioles, with a resultant increase in MPAP.