How Urinary System
Urination, or voiding, is a complex activity. The bladder is a
balloon like 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.