Pulmonary arterial hypertension (PAH) is a rare, progressive disorder characterized by high blood pressure (hypertension) in the arteries of the lungs (pulmonary artery) for no apparent reason. The pulmonary arteries are the blood vessels that carry blood from the right side of the heart through the lungs.The exact cause of PAH is unknown and although treatable, there is no known cure for the disease. PAH usually affects women between the ages of 30-60.