The main purpose of this study is to investigate whether bosentan (Tracleer®) affects the
wall thickness of the pulmonary arteries in patients with idiopathic pulmonary arterial
hypertension (iPAH) and PAH related to systemic sclerosis (PAH-SSc).
The second purpose is to investigate if bosentan affects the enlargement of small vessels in
the lungs in response to natural chemicals in patients with iPAH and PAH-SSc.