Answer: If the radius of a blood vessel decreases, blood pressure will increase.
Explanation: A radius is the distance from the center of a circle to any part of the circumference. Blood vessels are like tubes, if the radius decreases, it means that the diameter decreases too and the vessel becomes narrower. The narrowing of the blood vessels results in increased blood pressure as the heart is being made to increase its pumping action in order to meet the body's demand for blood circulation.
