Pulmonary hypertension is caused by changes to the pulmonary arteries, the blood vessels that carry blood from the heart to the lungs.
There are five main types of pulmonary hypertension, depending on the underlying cause. These are described below.
Pulmonary arterial hypertension (PAH) is caused by changes in the smaller branches of the pulmonary arteries.
The walls of the arteries become thick and stiff, narrowing the space for blood to pass through and increasing blood pressure.
PAH can be associated with other conditions, including:
A small number of people develop PAH without having any other medical condition. This is called idiopathic PAH. In very rare cases, PAH can be inherited.
In rare cases, newborn babies can have high pressure inside their blood vessels, which means their heart can't pump enough oxygenated blood around their body. This is known as persistent pulmonary hypertension of the newborn (PPHN).
Treatment in an intensive care unit may be needed if simple measures such as keeping the baby warm and giving oxygen don't increase oxygen levels to normal.
The Great Ormond Street Hospital for Children website has more information about persistent pulmonary hypertension of the newborn.
If there are problems with the left side of the heart, the right side has to work harder to pump blood through the lungs. This increases blood pressure in the pulmonary arteries.
Problems with the left side of the heart are thought to be the most common cause of pulmonary hypertension. These include mitral valve problems, left ventricle problems and aortic valve conditions.
Pulmonary hypertension is also sometimes linked with lung diseases or lack of oxygen (hypoxia), including:
Low levels of oxygen in the blood make the pulmonary arteries narrow. This squeezes the blood into a smaller space, which increases blood pressure, causing pulmonary hypertension.
Pulmonary hypertension can sometimes be caused by scars from previous blood clots that narrow or block the pulmonary arteries. This is called chronic thromboembolic pulmonary hypertension.
A blood clot that blocks one of the blood vessels that supply your lungs is called a pulmonary embolism.
Other, less common, causes of pulmonary hypertension include:
Read about how pulmonary hypertension is diagnosed.