text: In hypoxic pulmonary hypertension ( WHO Group III ) , the low levels of oxygen are thought to cause vasoconstriction or tightening of pulmonary arteries . This leads to a similar pathophysiology as pulmonary arterial hypertension .
simplified: When a person has pulmonary hypertension without any other cause , this is called idiopathic pulmonary arterial hypertension or IPAH .