Final answer:
Vasodilation and vasoconstriction are terms that describe changes in the diameter of blood vessels, regulated by the vascular tone of the smooth muscle, affecting blood flow and pressure.
Step-by-step explanation:
Changes in the diameter of a blood vessel are collectively known as vasodilation when they widen and vasoconstriction when they narrow. The vascular tone, which refers to the contractile state of the smooth muscle in the blood vessel walls, is the primary determinant of the vessel's diameter.
Vasodilation results in a larger diameter, reduced resistance, and increased blood flow, whereas vasoconstriction involves a smaller diameter, increased resistance, and decreased blood flow. These mechanisms allow the body to regulate blood pressure and the distribution of blood to various tissues, such as during a fight-or-flight response.