Final answer:
Blood vessel diameter changes are known as vasodilation (increase in diameter) and vasoconstriction (decrease in diameter), and these changes can dramatically affect blood flow and resistance due to the inverse relationship between vessel diameter and resistance.
Step-by-step explanation:
Changes in the diameter of a blood vessel are collectively known as vasodilation and vasoconstriction. The primary determinant of blood vessel diameter, and thus of resistance and flow, is the vascular tone of the vessel. The relationship is such that the effect of vessel diameter on resistance is inversely proportional, with an increased diameter reducing resistance and increasing flow, while a decreased diameter does the opposite. This is particularly significant because resistance is inversely proportional to the radius of the blood vessel raised to the fourth power, indicating that small changes in diameter can have large effects on blood flow and pressure.