Question

Where in the body would you find low oxygen levels causing vasoconstriction and high levels causing vasodilation?

Answer 1

The correct answer is the lungs. Usually, in the systemic organs such as the liver, kidneys, and the heart; hypoxia or low oxygen levels cause the blood vessels supplying them to vasodilate to increase flow of blood and prevent ischemia. Adequate oxygenation of systemic organs cause the blood supply to vasoconstrict to prevent pooling of oxygenated blood and shunt it to other organs that need the oxygen more.

However, in the lungs, the opposite is true because in the setting of hypoxia, the blood vessels of the lungs vasoconstrict as to shunt the blood to other parts of the lungs with adequate oxygenation for the blood to be adequately oxygenated. If there is adequate oxygen in the lungs, then blood vessels vasodilate to efficiently get the oxygen from the lungs. 

Answer 2

The answer for the question is the lungs.
Vasodilation means the smooth muscles in the blood vessel walls relax causing them to widen. The widening results in less vsacular resistance, thus the blood flow through the dilated vessel increases. Vasocontsriction on the other hand means the smooth muscles in the blood vessel walls contract causing them to narrow. The action results in blood flow through the vessels to be restricted.