Pulmonary hypertension is a disorder where the blood flow that leaves the right side of the heart faces an increased resistance (pressure).

Normally blood flows from the right side of the heart into the pulmonary arteries and smaller blood vessels in the lungs. The blood vessels have muscles in their walls that can relax or contract to allow more or less blood flow to enter.

In pulmonary hypertension, the blood vessels of the lungs have an increased amount of muscle in the walls. This causes a higher resistance in the lungs. The right side of the heart then has to work harder to pump blood out to the lungs. The right side of the heart will enlarge and thicken in response to this extra work. With time, the extra work placed on the right side of the heart can cause it to fail.