Pulmonary Hypertension (PH) refers to increased pressure in the pulmonary circulation.  This is the part of the circulatory system that delivers deoxygenated blood that has returned from the body from the right side of the heart to the lungs and then back to the left side of the heart.

The right side of the heart is normally the low-pressure side of the heart, and the left side of the heart is usually the high-pressure side of the heart. However, when the pressure in the pulmonary arteries, capillaries, and veins increases, the right side of the heart has to work harder to move the blood through the lungs, reducing the supply oxygen to the body.