Original: 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.