<prompt> Hypoxic pulmonary vasoconstriction due toIrreversible pulmonary vasocontriction hypoxiaReversible pulmonary vasoconstriction due to hypoxiaDirect blood to poorly ventilated areasOccurs hours after pulmonary vasoconstriction  <response>: Hypoxic pulmonary vasoconstriction HPV, also known as the Euler-Liljestrand mechanism, is a physiological phenomenon in which small pulmonary aeries constrict in the presence of alveolar hypoxia low oxygen levels. Ref Harrison20th edition pg 2334 