Pulmonary hypertension occurs when the pulmonary arteries and capillaries in the lungs become narrow so that the blood flow through them is obstructed. As a result, the heart muscles need to work harder to pump the sufficient amount of blood into the lungs. This causes weakening of the heart muscles, and high blood pressure in the arteries.
This means the primary cause of pulmonary hypertension is narrow blood vessels or arteries.

Arteries can become narrow in three ways:

1. Tightening of the artery walls.

2. Stiffening of the artery walls caused by the growth of an excess number of cells in the artery walls. Some people have stiff arteries when they are born.

3. Blocking of arteries from blood clots.

All these result in narrower arteries, making it harder for blood to pass through them.
There are different types of pulmonary hypertension. Each is caused by a different factor. For instance, Group 1 pulmonary arterial hypertension is usually hereditary. However, sometimes it may not have any clear cause. It can also be caused by HIV, congenial heart disease, thyroid disease, diet pills and street drugs like cocaine.

On the other hand, groups 2, 3, 4 and 5 pulmonary hypertension can be caused by sarcoidosis, sleep apnea, sickle cell anemia, chronic obstructive pulmonary disease and mitral valve disease.