The bladder is continually filled with urine and then emptied periodically. When full, the bladder stretches to increase its
capacity. When the muscle of the internal sphincter is relaxed, the muscles of the wall contract, and the urine exits
through the urethra. In adults this occurs voluntarily in response to an order issued by the nervous system. In infants, on the other hand, this evacuation occurs spontaneously, as soon as the bladder is filled.