In pulmonary hypertension due to lung diseases and/or hypoxia (WHO Group 3), low levels of oxygen in the alveoli (due to respiratory disease or living at high altitude) cause constriction of the pulmonary arteries. This phenomenon is called hypoxic pulmonary vasoconstriction and it is initially a protective response designed to stop too much blood flowing to areas of the lung that are damaged and do not contain oxygen. When the alveolar hypoxia is widespread and prolonged, this hypoxia-mediated vasoconstriction occurs across a large portion of the pulmonary vascular bed and leads to an increase in pulmonary arterial pressure, with thickening of the pulmonary vessel walls contributing to the development of sustained pulmonary hypertension. Prolonged hypoxia also induces the transcription factor HIF1A, which directly activates downstream growth factor signaling that causes irreversible proliferation and remodeling of pulmonary arterial endothelial cells, leading to chronic pulmonary arterial hypertension.