Sphincter urethrae

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, at the bladder neck
 * the external, 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".

Function
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.

Sphincter urethrae membranaceae
The Sphincter urethrae membranaceae surrounds the whole length of the membranous portion of the urethra, and is enclosed in the fasciæ of the urogenital diaphragm.

Its external fibers arise from the junction of the inferior rami of the pubis and ischium to the extent of 1.25 to 2 cm., and from the neighboring fasciæ.

They arch across the front of the urethra and bulbourethral glands, pass around the urethra, and behind it unite with the muscle of the opposite side, by means of a tendinous raphé.

Its innermost fibers form a continuous circular investment for the membranous urethra.