Upon incurring some type of trauma, blood flow through damaged vessels is immediately limited through a process known as vascular spasm. During a vascular spasm, pain can cause reflexes in the nervous system to stimulate smooth muscle contraction around the blood vessel. As the smooth muscle of the impaired vessels contract, the diameter of the vessels decrease. In some instances, the blood vessel can be entirely occluded by a vascular spasm. However, this occlusion is short lived and the smooth muscle will eventually relax and then bleeding out of the damaged vessel can greatly increase. As effective as a vascular spasm is, it is temporary and cannot establish hemostasis alone.