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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 narrowing of the pulmonary arteries means the right ventricle (RV = lower chamber on the right side) of the heart has to work harder to pump blood through the lungs. Because the RV of the heart has to work harder in PH, it is the RV that eventually gets weak, leading to RV failure, a type of heart failure. Heart failure means the heart cannot pump enough blood for the body’s needs. There is reduced blood flow to the organs of the body and patients with PH may have low blood oxygen levels. Reduced blood flow causes the symptoms of difficulty breathing, fatigue, chest pain, and faintness.
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).

If PH is not treated, the RV of the heart will eventually fail in many PH patients, which may lead to death.