When the heart and lungs are functioning normally, a "tree" of blood vessels called the pulmonary artery carries oxygen-poor blood from the heart to the lungs, where it is replenished with fresh oxygen.

Pulmonary hypertension starts when the walls of the blood vessels that supply blood to the lungs constrict, or tighten up. This makes it more difficult for blood to get through to the lungs, and the heart must pump harder to overcome the resistance.

Eventually, the extra stress causes the heart to enlarge and become less efficient. Therefore, it is more difficult for blood to flow out of the heart, through the lungs and into the body.

Pulmonary hypertension is a chronic condition and can be life-threatening, however, new treatments are available that have significantly improved the prognosis.