A blood vessel is a tube that carries blood in the circulatory system. 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 (vas- in Latin means "container" or "vessel"). The constriction of blood vessels is called vasoconstriction, it prevents the body from losing warmth.
There are 100,000 km (60,000 miles) of blood vessels in an adult human body.