Any condition which causes blood to slow its forward progress can cause this condition. If there is a problem that exists that possibly can cause pulmonary hypertension, it is called Secondary Pulmonary Hypertension. When a cause cannot be found, it is called Primary or Idiopathic Pulmonary Hypertension. Any condition that existed prior to the pulmonary hypertension, is blamed for the cause, despite the fact that the doctor may not agree. Whatever the disease process, when the blood can't move forward, it begins to back up.
Blood vessels are similar to garden hoses, some are larger and some are as small as one cell.
If you can imagine kinking the hose, behind the kink, the hose would be large and pressure would be high, with the water at the other end trickling out. Physiologically, that is just what happens in the body. Since Primary Pulmonary Hypertension and most secondary forms starts with the vessels in the Pulmonary Arteries, the back up begins in the right heart. The right heart begins to enlarge and swell with blood. The blood then backs up in the liver where ascites can occur and in the legs where the serum is pushed into the cells and your feet, ankles and legs distend with water.