The kidneys constantly produce urine, which flows through two tubes (the ureters) to the bladder, where urine is stored (see figure Viewing the Urinary Tract Viewing the Urinary Tract 
). The lowest part of the bladder (the neck) is encircled by a muscle (the urinary sphincter) that remains contracted to close off the channel that carries urine out of the body (the urethra), so that urine is retained in the bladder until it is full.
When the bladder is full, messages travel along nerves from the bladder to the spinal cord. The messages are then relayed to the brain, and the person becomes aware of the urge to urinate. A person who has control of urination can consciously and voluntarily decide whether to release the urine from the bladder or to hold it for a while. When the decision is made to urinate, the sphincter muscle relaxes, allowing urine to flow out through the urethra, and the bladder wall muscles contract to push the urine out. Muscles in the abdominal wall and floor of the pelvis can be contracted voluntarily to increase the pressure on the bladder.
A variety of disorders can interfere with the control of urination, among them
Urinary incontinence (in adults Urinary Incontinence in Adults Urinary incontinence is involuntary loss of urine. Incontinence can occur in both men and women at any age, but it is more common among women and older people, affecting about 30% of older women... read more
and children Urinary Incontinence in Children Urinary incontinence is defined as the involuntary release of urine occurring two or more times per month after toilet training has been completed. Incontinence may be present During the day... read more )
