Pulmonary hypertension (PH) is high blood pressure in the pulmonary arteries, which carry blood from the heart to the lungs to pick up oxygen. It affects the arteries in the lungs and the right side of your heart. What causes pulmonary hypertension? Pulmonary hypertension is usually caused by changes in the cells that line the pulmonary arteries or capillaries which causes them to become narrowed, blocked or damaged. There are five kinds of pulmonary hypertension categorized by cause. Inflammation, tightening, stiffening, and blood clots in the arteries are the most common causes. When this happens, the right ventricle must work harder to move blood through the lungs. As a result, the pressure in your pulmonary arteries increases, causing right heart muscle to weaken and possibly fail. Sometimes pulmonary hypertension can be caused by another medical problem (secondary pulmonary hypertension) i.e. Heart or lung disease or by blood clots. In other cases, the cause of pulmonary hypertension is unknown (idiopathic pulmonary hypertension).