Pulmonary blood pressure is one eighth of systemic pressure and pulmonary circulation normally has low resistance. In pulmonary hypertension, pulmonary blood pressure reaches one fourth or more of systemic blood pressure. This usually develops secondary to the decreased area of pulmonary vascular bed or increased pulmonary blood flow.

Causes of pulmonary hypertension:

Kronik obstrüktif ya da interstisyel akciğer hastalıkları. Bu durumlarda normal akciğer parankiması ortadan kalkar ve dolayısı ile alveoler kapgillerler azalır. Bu durum pulmoner arter direncinde artışa ve bu da pulmoner arter basıncında artışa neden olur.

Recurrent pulmonary embolisms reduce the functional area of pulmonary vascular bed, which, as a result, lead to an increased vascular resistance.

Previous heart diseases such as mitral stenosis cause an increased pulmonary venous pressure by elevating the left atrial pressure. This eventually increases pulmonary arterial pressure. Congenital left-to-right shunts are also causes of secondary pulmonary hypertension. Pulmonary hypertension observed when all causes that may lead to diffuse pulmonary hypertension are eliminated is called primary or idiopathic pulmonary hypertension. It is rarely seen. Most of these cases are sporadic. Only 6% thereof is congenital and shows autosomal dominant transmission.