Final answer:
Vasoconstriction is the reduction of the diameter of a blood vessel, while vasodilation is the enlargement of the vessel diameter.
Step-by-step explanation:
Vasoconstriction is the reduction of the diameter of a blood vessel, while vasodilation is the enlargement of the vessel diameter. Vasoconstriction occurs when the smooth muscle in the wall of the blood vessel contracts, narrowing the lumen. This increases resistance and decreases blood flow. Vasodilation, on the other hand, occurs when the smooth muscle relaxes, causing the vessel to widen. This reduces resistance and increases blood flow.