The pulmonary vasculature generates chemical messengers that produce, repair, and destroy cells in the vessel wall.2 The blood vessels of the pulmonary circulatory system are controlled by a system of molecular messengers that facilitate the metabolic compensation through dilatation and constriction of these blood vessels. Prostacyclin, nitric oxide (NO), and endothelin (ET-1) are the molecular messengers that control the flow of blood in the pulmonary circulation. Prostacyclin is a prostaglandin that promotes blood vessel vasodilatation and prevents the overgrowth of cells in the blood vessels. Prostacyclin prevents platelets from clumping together, which lowers the incidence of clogged blood vessels. Nitric oxide is a powerful vasodilator that prevents smooth muscle growth and inhibits platelets from sticking together. Endothelin is a messenger that causes vasoconstriction of the blood vessels. In healthy adults, these molecular messengers ensure appropriate compensatory blood flow based on metabolic demands.