How the Body Works : Release of Urine

The release of urine from the bladder is controlled by the autonomic nervous system, which acts on the muscular wall of the bladder and an internal sphincter. Voluntary nerve impulses control the actions of an external sphincter, so that urine can be voided at convenient times. As the bladder fills, sympathetic nerve impulses, shown in red, increase, keeping the internal sphincter firmly closed. When the parasympathetic nerve output and the internal output, shown in blue, cause the bladder wall to contract, the internal sphincter is opened. As this occurs, controlled voluntary impulses may also cease and urine can then be released.