Hypertensive pulmonary vascular disease secondary to chronic alveolar hypoxia is the result of vasoconstriction and then muscularization of the terminal portions of the pulmonary arterial tree. Ultrastructurally these two phases are characterized respectively by muscular evaginations and the hyperplasia of smooth muscle cells. Plexogenic pulmonary arteriopathy secondary to congenital cardiac shunts, primary pulmonary hypertension or rare cases of cirrhosis of the liver are characterized by concentric-laminar proliferation of myofibroblasts. Later there is a development of plexiform lesions which consist of vascular channels separated by a matrix of proteoglycan containing fibrillary cells and myofibroblasts. Fibrinoid necrosis results from the passage of fibrinogen from the vascular lumen into the arterial wall.