The aerioles are the major site of the resistance to blood flow, and small changes in their caliber cause large changes in the total peripheral resistance. The walls of the aerioles contain less elastic tissue but much more smooth muscle. The muscle is innervated by noradrenergic nerve fibers, which function as constrictors, and in some instances by cholinergic fibers, which dilate the vessels. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 31. Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.