Smooth muscle is a particular kind of muscle that occurs in the digestive system, in blood vessels, in the airways and in some glands of the body. It is not under voluntary control, like the muscles that a person uses to move about. Smooth muscle is generally used by the body to open or close tubes - for example, in arteries they can constrict (tighten) or dilate (open) the vessel, making it easier or harder for blood to get through.