When a blood vessel is severed or punctured, or when the wall of a blood vessel is damaged, vascular spasm occurs. In vascular spasm, the smooth muscle in the walls of the blood vessel contracts dramatically. This smooth muscle has both circular layers; larger blood vessels also have longitudinal layers. The circular layers tend to constrict the flow of blood, whereas the longitudinal layers, when present, draw the blood vessel back into the surrounding tissue, often making it more difficult for a surgeon to locate, clamp, and tie off a severed blood vessel. The vascular spasm response is believed to be triggered by several chemicals called endothelins that are released by vessel-lining cells and by pain receptors in response to blood vessel injury. This phenomenon typically lasts for up to 30 minutes, although it can last for hours.