Based on their size and function, arteries can be grouped into three types. The elastic arteries are the largest arteries located close to the heart. They have wide lumens but proportionately thinner walls. These arteries are abundant in elastic fibers, giving them the elasticity to expand and contract as blood is pumped from the heart. Acting as pressure reservoirs, they enable continuous blood flow during the pulsating rhythm of the heartbeat. The muscular arteries, such as the radial artery, are mid-sized arteries with thicker walls and narrower lumens. Their tunica media contains a high proportion of smooth muscle cells that contribute roughly three-quarters of the total mass of the artery. Due to a low proportion of elastic tissue, the muscular artery cannot recoil and propel blood. Arterioles are the smallest arteries with lumen diameters between 0.3 mm and 10 μm. Their tunica media has 1 to 2 layers of smooth muscle cells and only a few elastic fibers. Arterioles control the blood flow into capillaries through vasoconstriction and vasodilation.