Hypoxic pulmonary vasoconstriction (HPV) is the reflex constriction of pulmonary arterioles in response to low a PO₂ (below 80 - 100 mmHg; 11 - 13 kPa) in adjacent alveoli.

It is independent of the innervation to the vessel walls and can occur when blood with a high PO₂ is perfused through lung which has a low alveolar PO₂.

Thus a low PO₂ in the alveoli has been shown to influence hypoxic pulmonary vasoconstriction (HPV) more than a low PO₂ in the blood.

HPV results in the blood flow being directed away from poorly ventilated areas of the lung and helps to reduce the ventilation/perfusion mismatch.

Volatile anaesthetic agents reduce HPV in animals, but in adults the evidence is less convincing, although it certainly does not increase the effect.

20 parts per million (ppm) of nitric oxide will inhibit the HPV response.