In mammals the flow of urine from the urinary bladder is controlled by a group of muscles collectively called the urethral sphincter, named for their proximity to the urethra. Both sexes have at least two areas of muscle: the internal sphincter, or bladder neck; and the external, or distal, sphincter.
In males the internal and external urethral sphincters are more powerful, able to retain urine for twice as long as females, and are furthermore backed up by the tertiary rings of urethral sphincters along the length of the penis.
Both sexes can use the levator ani, the voluntary muscle of the pelvic floor, to control urination. In females this muscle is more important because of the weakness of the urethral sphincters. Kegel exercises are a form of exercise intended to strengthen the pelvic floor muscles.
Urinary incontinence is the inability to control urination, and is more common in women than men.
There are anecdotal reports that many men, although able to control the onset of urination, are unable to stop urinating once they have started. It is uncertain whether this is a common phenomenon in real life.
When the external urinary sphincter is relaxed urine will flow from the urinary bladder when the pressure there is great enough to force urine to flow through the urethra.
Farther along the urethra is a sphincter of skeletal muscle, the sphincter of the membranous urethra (externalurethralsphincter).
It is through the learned ability to maintain the externalsphincter in a contracted state that adults are able to delay urination until the opportunity to void presents itself.
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