The arteries of the lungs (also known as pulmonary arteries) normally carry blood from the right ventricle (RV; lower chamber on the right-side of the heart) to the lungs.

In normal humans without PH, the pulmonary arteries are relaxed, wide open, and are not stiff, so that blood flows very easily through the lungs. For this reason, blood pressure in the pulmonary arteries is normally very low.

In normal humans, the blood pressure in the pulmonary arteries (also known as pulmonary artery pressure = PAP) normally fluctuates continuously:

• PAP fluctuates between a high value (also known as systolic) of about 25 millimeters of mercury (= mm Hg) and a low value (also known as diastolic) of 10 mmHg. Thus, the PAP is said to be 25/10 mm Hg
• The average PAP (also known as mean PAP) is usually between 15-20 mmHg, but is always less than 25 mmHg in normal humans at rest
• PAP may rise slightly during exercise, but mean PAP is always less than 30 mm Hg in normal humans

PH is often due to disease in the smallest pulmonary arteries. These pulmonary arteries are narrowed, and some may even be closed, because of several abnormalities:

• The walls of the pulmonary arteries are thicker and stiffer
• There is scar tissue (also known as fibrosis) in the walls of the pulmonary arteries
• There are more cells in the walls of the pulmonary arteries. This is because cells have multiplied (also known as divided or proliferated) abnormally to create more cells. These cells include:
  • The inner lining cells (also known as endothelial cells) of the pulmonary arteries
  • The smooth muscle cells that contract to cause spasm of pulmonary arteries
  • The cells that make scar tissue (also known as fibroblasts)

In PH, because of the narrowed and closed pulmonary arteries, the lungs may grow new blood vessels. These new blood vessels are often very small, and twisted, and form little balls of blood vessels called “plexiform lesions”.

Because of the high blood pressure in the lungs in PH, the large pulmonary arteries can enlarge (also known as dilate). This can be seen on a chest x-ray or CT scan of the lungs.