In which region of the body would low oxygen levels cause vasoconstriction and high oxygen levels cause vasodilation?

• A. Lungs
• B. Brain
• C. Kidneys
• D. Skin

Answer: (A)

In the lungs, blood flow is matched to air flow through a local response to oxygen levels. When a region of the lung has low oxygen, the small pulmonary arteries constrict (hypoxic pulmonary vasoconstriction), reducing blood flow to that poorly ventilated area. When oxygen levels are high, these vessels relax and dilate, increasing blood flow to well-ventilated regions. This pattern helps optimize gas exchange by directing blood toward areas with the most oxygen. Other tissues don’t use this same O2-driven constriction-dilation pattern, so the lungs are the region where low oxygen causes vasoconstriction and high oxygen causes vasodilation.