HIF1A and pulmonary arterial hypertension: The development of pulmonary hypertension in chronic lung diseases is multifactorial, the main factor is hypoxic vasoconstriction of pulmonary vessels to shift blood to well ventilated areas to optimize ventilation-perfusion matching, sustained hypoxia promotes intracellular mediators and hypoxia inducible factor 1α (HIF-1α) leading to increase vascular remodeling and increase pulmonary vascular resistance with further increase in pulmonary artery pressure [29].