Hypoxic pulmonary vasoconstriction

Overview
Hypoxic pulmonary vasoconstriction is a physiological phenomenon in which pulmonary arteries constrict in the presence of hypoxia (low oxygen levels) without hypercapnia (high carbon dioxide levels), redirecting blood flow to alveoli with higher oxygen tension.

The process might at first seem illogical, as low oxygen levels should theoretically lead to increased blood flow to the lungs to receive increased gaseous exchange. However, it is explained by the fact that constriction leads to increased bloodflow to better-aerated areas of the lung, which increases the total area involved in gaseous exchange.