Low oxygen levels in the lungs trigger local vasoconstriction in pulmonary vessels, while high oxygen promotes vasodilation. This directs blood flow toward well-ventilated alveoli and away from collapsed or mucus-blocked air sacs, so blood only reaches functional gas exchange areas.

Pulmonary circulation reverses the usual autoregulatory pattern: low oxygen causes vasoconstriction and high oxygen causes vasodilation. This fits the lungs' role in gas exchange because pulmonary vessels should deliver blood only to air sacs that can actually exchange gases. When alveoli are filled with oxygen-rich air, local capillaries dilate and receive blood, allowing oxygen to be picked up. When alveoli are collapsed or blocked by mucus, the oxygen content in those regions is low, so vasoconstriction reduces blood flow to those nonfunctional areas, shunting perfusion toward healthier, oxygenated regions.

Key points

Low pulmonary oxygen causes local vasoconstriction; high pulmonary oxygen promotes vasodilation.
This is opposite to most tissues, where low oxygen usually causes vasodilation.
The response is consistent with the pulmonary circulation's function of gas exchange.
Oxygen-rich air sacs receive more blood flow, while collapsed or mucus-blocked sacs are largely bypassed.

Why does the pulmonary circulation exhibit vasoconstriction in response to low oxygen levels, and how does this relate to its gas exchange function?

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