Why vasoconstriction in hypoxia?
Pulmonary arterial vasoconstriction in hypoxia is an adaptive response to the local lack of oxygen – if a particular section of a lung is unable to oxygenate venous blood sufficiently, then vasoactive substances are released locally that act to constrict the arterioles in the area, reducing blood flow to the ‘inefficient’ bit of the lung, and allowing more flow to parts that are working better. Unfortunately, in the case of generalised processes (such as COPD) where the entire lung suffers from poor efficiency, this becomes a maladaptive process whereby there is generalised vasoconstriction. This leads to pulmonary hypertension, which in many diseases portends a poor prognosis.
