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, which is 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 has 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 closed 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 messages to the bladder to keep it from automatically emptying, until the child decides it is the time and place to void.
Failures in this control mechanism result in incontinence. Reasons for this failure range from simple to complex.
Experts don't know the exact cause of bedwetting. Young people who experience bedwetting are usually physically and emotionally normal. These experts do think, however, that a combination of factors probably results in most cases of bedwetting. These factors include:
- Slower physical development
- Overproduction of urine at night
- Medical conditions or structural problems.
Slower Physical Development
Between the ages of 5 and 10, bedwetting may be the result of a small bladder capacity, long sleeping periods, and underdevelopment of the body's alarms that signal a full or emptying bladder. This form of urinary incontinence usually fades away as the bladder grows, and the natural alarms become operational.