Apelin is an endogenous peptide with physiological actions in the cardiovascular system and is abundantly expressed in the pulmonary vasculature. Pre-clinical models and preliminary clinical data indicate that Apelin deficiency may mediate or contribute to the pathogenesis of pulmonary hypertension and heart failure. Apelin causes peripheral vasodilatation and increased cardiac contractility. The investigators will determine the effects of Apelin on the pulmonary circulation in 3 groups; healthy control, people with pulmonary arterial hypertension and people with pulmonary hypertension due to heart failure. Each subject will receive both Apelin infusion and saline placebo infusion in a crossover design. The infusions will be given in a random order which the subject and the investigator will be blinded to. The investigators hypothesise that Apelin will have more marked pulmonary haemodynamic effects than that observed in the systemic circulation. Moreover, the investigators propose that Apelin will have a marked vasodilatory effect on the human pulmonary vasculature and reduce pulmonary vascular resistance in patients with pulmonary arterial hypertension or pulmonary hypertension due to left heart disease.