Arteriole. Illustration of peripheral arterioles (small sub-divisions of arteries) carrying red blood cells from the heart. The arterioles divide into capillaries, which supply the tissues. The arteriole wall is encircled by smooth muscle cells which are innervated by sympathetic nerves (yellow). These nerves stimulate the smooth muscle to contract, thereby maintaining blood pressure. Increase in sympathetic stimulation causes constriction of the vessel & an increase in blood pressure. Decrease causes dilation of the vessel & a drop in blood pressure. Presence of adrenaline and local factors also determine the diameter of arterioles, and therefore the blood flow.