The release of epinephrine in the blood or norepinephrine released from the sympathetic nerve endings constitutes adrenergic stimulation which can have both the effect of enhancement or inhibition on the body. Stimulation which requires contraction, such as the heart, dilation of the muscles which affect the iris, and smooth muscles which are found surrounding most blood vessels is an example of both inhibitory reaction and enhancement reaction. The smooth muscles which are inhibited from constricting are then dilated through the release of adrenergic chemicals such as those of the bronchioles as well as the remainder of the smooth muscles surrounding the blood vessels.