An arteriole is a blood vessel that branches off an artery. Arterioles themselves branch to form capillaries. Capillaries interlink to form networks; they join small veins (venules), which gradually merge into larger veins.

Arterioles are intermediate in size between arteries and capillaries. They have a high proportion of smooth muscle in their walls, and their nerve supply enables them to be narrowed or widened to meet decreases or increases in the blood-flow needs of the tissues they supply.