Pulmonary hypertension is a clinical syndrome where the blood flow that leaves the right side of the heart faces an increased resistance (pressure).
Normally blood flows from the right ventricle into the pulmonary arteries and then flows into smaller and smaller blood vessels until it reaches the capillaries (thin walled blood vessels near the small air chambers of the lungs or alveoli). There gas exchange occurs where oxygen is picked up and carbon dioxide is released. The blood vessels have muscles in their walls that can regulate their diameter and thereby allow more or less blood flow to enter the capillaries.
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 ventricle then has to work harder to pump blood out to the lungs. The right ventricle 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.