There are three causes for this abnormal narrowing:

• The muscles within the walls of the arteries may contract or squeeze, making it harder for blood to flow through them
• The walls of the arteries may thicken from too much cell growth in the lining of the vessels
• Tiny blood clots may form within the smaller arteries, blocking blood flow

Any of these changes in the body will make it difficult for blood to pass through the lungs. When that happens, the heart is forced to work very hard, resulting in the eventual weakening of the heart muscle and the loss of its ability to pump blood efficiently.

There are several subtypes of pulmonary hypertension and they are categorised according to various causes ranging from genetic mutations to unknown causes.