A blood vessel is a tube that carries blood. Blood vessels that take blood away from the heart are arteries. Blood vessels that take blood back to the heart are veins. Capillaries are between veins and arteries and they supply tissue with blood.
The expansion of blood vessels is called vasodilation, it helps the body to get rid of heat energy. The constriction of blood vessels is called vasoconstriction, it prevents the body from losing warmth.