Smooth muscle is made up of long, spindle-shaped cells. In most hollow organs, these cells are arranged in bundles organized in an outer longitudinal layer and an inner circular layer. The contractions of these muscles work with the same sliding action as in the skeletal muscles. Smooth muscles are not under conscious control; therefore, they are also called involuntary muscles. Many parts of the body contain smooth muscles; within the cardiovascular system, the primary smooth muscle is within the heart. Other smooth muscles include the bronchi of the lungs, the bladder, and the walls of the blood vessels. Smooth muscle tissue causes the movements of internal organs, such as the contractions of the uterus during childbirth.