Pulmonary vasoconstriction is a physiological phenomenon and mechanism in response to alveolar hypoxia or low oxygen partial pressures in the pulmonary arterioles and, to some extent, the pulmonary venules. Pulmonary vasoconstriction redirects blood flow within the vasculature away from poorly ventilated parts of the lungs towards better-ventilated portions. Ventilation and perfusion (V/Q) matching is a physiological process that influences gas exchange in the lung, as the lung attempts to efficiently pair oxygenated (ventilated) regions with areas of sufficient blood supply (perfusion). In low-oxygen states, pulmonary vessels constrict in an attempt to shunt blood to better-ventilated regions of the lung. Poor oxygen availability has profound and overarching systemic ramifications manifesting in a plethora of pathologies starting within the lungs itself. Maintaining correct and appropriate oxygen homeostasis is a critical component for systemic stability and functioning, and the process begins within the pulmonary vasculature. While many details pertaining to pulmonary vasoconstriction are not fully understood, the mechanism involves the activity of ion channels as well as several molecular and chemical agents.