PH is a disease of abnormally high blood pressure in the arteries of the lungs (also known as pulmonary arteries). The pulmonary arteries become narrowed, and can be scarred to the point of being closed.

The pulmonary arteries normally carry blood from the right ventricle (RV; lower chamber on the right side) of the heart to the lungs. Since the pulmonary arteries are narrowed in PH, blood flow to the lungs is reduced. Reduced blood flow to the lungs weakens the lung’s ability to transfer oxygen into the blood. As a result, patients with PH may have low blood oxygen levels.

Other blood vessels of the lungs can also be affected by PH. This includes the smallest capillaries, and sometimes the pulmonary veins which carry blood from the lungs back to the left-side of the heart.