The purpose of the study is to find out how the blood flow changes in the lungs of people with pulmonary hypertension compared to healthy individuals without pulmonary hypertension. We would like to find out if there are differences in how blood flows when subjects are given a drug to dilate (widen) the arteries in their lungs and when they breathe extra oxygen. We will compare the results to when subjects don't receive any drug or extra oxygen. We hope that knowing about these differences will help us to better understand pulmonary hypertension and how to diagnose it earlier.