<prompt> Hypoxic pulmonary vasoconstriction due to -Irreversible pulmonary vasocontriction hypoxiaReversible pulmonary vasoconstriction due to hypoxiaDirect blood to poorly ventilated areasOccurs hours after pulmonary vasoconstriction  <response>: Ans. is b i.e., Reversible pulmonary vasoconstriction due to hypoxiaPulmonary hypoxic vasoconstrictiono Hypoxic pulmonary vasoconstriction HPV is an adaptive vasomotor response to alveolar hypoxia which redistributes blood to optimally ventilated lung segments by an active process of vasoconstriction, particularly involving the small muscular resistance pulmonary arteries PA.o HPV is intrinsic to the lung and the mechanism involves redox based O2 sensor.o Alveolar hypoxia triggers the pulmonary vessels to constrict. Alveolar hypoxia triggers the pulmonary vessels to constrict, directing blood away from poorlynonventilated areas to better ventilated segments thereby improving ventilationperfusion matching.o Clinically HPV is proportional to the degree of local hypoxia and is triggered primarily in response to an alveolar oxygen partial pressure PAO2 less than 100 mmHg.o HPV onsets within seconds of moderate hypoxia and reverses quickly on restoration of normoxic ventilation.o HPV shunts bloodfrom poorly oxygenated areas to better ventilated lung segments, thereby optimizing ventilation perfusion matching, reducing shunt fraction and optimizing O2 delivery in conditions such as atelactasis and pneumonia.The onset of HPV occurs in two phase:- Initial rapid onset phase minutes Delayed phase hours 