How does the urinary system work?
Urination, or voiding, is a complex activity. The bladder is a balloon-like organ that lies in the lowest part of the abdomen. The bladder stores urine, then releases it through the urethra, the canal that carries urine to the outside of the body. Controlling this activity involves nerves, muscles, the spinal cord, and the brain. The bladder is composed of two types of muscles: the detrusor, a muscular sac that stores urine and squeezes to empty; and the sphincter, a circular group of muscles at the bottom or neck of the bladder that automatically stay contracted to hold the urine in and automatically relax when the detrusor contracts to let the urine into the urethra. A third group of muscles below the bladder (pelvic floor muscles) can contract to keep urine back. A baby’s bladder fills to a set point, then automatically contracts and empties. As the child gets older, the nervous system matures. The child’s brain begins to get messages from the filling bladder and begins to send mess
