An arteriole's a small diameter blood vessel that extends and branches out from an artery and leads to capillaries. Arterioles have thin muscular walls (usually only one to two layers of smooth muscle) and are the primary site of vascular resistance. Arterioles receive autonomic nervous system innervation and respond to various circulating hormones in order to regulate their diameter. Blood pressure in the arteries supplying the body's a result of the work needed to pump the cardiac output (the… (More on Arterioles)