Pulmonary Hypertension (PH) disease that is a type of high blood pressure that affects the arteries in the lungs and the right side of your heart. Pulmonary hypertension begins when tiny arteries in your lungs, called pulmonary arteries, and capillaries become narrowed, blocked or destroyed. This makes it harder for blood to flow through your lungs, and raises pressure within your lungs' arteries. As the pressure builds, your heart's lower right chamber (right ventricle) must work harder to pump blood through your lungs, eventually causing your heart muscle to weaken and eventually fail.