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Sphincter urethrae
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The urethral sphincter is a collective name for the muscles used to control the flow of urine from the urinary bladder. These muscles surround the urethra, so that when they contract, the urethra is closed.
- There are two distinct areas of muscle: the internal sphincter muscle of urethra, at the bladder neck
- the external sphincter muscle of urethra, or distal, sphincter
Human males have much stronger sphincter muscles than females, meaning that they can retain a large amount of urine for twice as long, as much as 800mL, i.e. "hold it".
In addition to the internal and external sphincters, in the male only, the longer urethra, and the presence of the prostate gland help to close the urethra, preventing leakage.
Both sexes can use the levator ani, the voluntary muscle of the pelvic floor, to control urination. In females this muscle may be damaged, particularly by pregnancy, leading to weakness of the sphincter mechanism, and stress incontinence. Kegel exercises are a form of exercise intended to strengthen the pelvic floor muscles. They are also known as Pelvic Floor Muscle Training.