Urinary system
Urination, or voiding, is a complex
activity. The bladder is a balloonlike muscle 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 made 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 develops. 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 the simple to the
complex.