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Function

The main action of the urethral sphincter complex is to compress the urethra. This is important as it provides control over urinary continence. The internal sphincter’s smooth muscle resting state is one of contraction or ‘closure’, in which urine is prevented from passing through the internal urethral orifice into the urethra. The external sphincter is also thought to contribute to resting closure, although it’s most important feature is augmenting closure to prevent urination or voluntary opening to allow voiding.

During the voiding phase of micturition, both the internal and external sphincters relax to allow urine to pass through the urethra. When the bladder is full, stretch receptors in its muscular wall stimulate a parasympathetic nervous system response. This results in contraction of the detrusor muscle of the bladder and relaxation of the smooth muscle of the internal urethral sphincter. Thus allowing outflow of urine into the urethra. This process is involuntary, however the flow of urine can be voluntarily stopped by contraction of the external urethral sphincter. At an appropriate time and place, the external urethral sphincter can then be voluntarily relaxed, allowing the passing of urine.

A secondary function of the urethral sphincter complex is to prevent retrograde ejaculation of semen into the bladder. In males, semen and urine share the urethra as a passageway. Sympathetic stimulation during orgasm in males simultaneously causes contraction of the internal urethral sphincter at the neck of the urinary bladder. This serves to prevent semen from traveling proximally along the urethra, through the internal urethral orifice and into the bladder.

References: 

•  Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins. 

•  Standring, S. (2016). Gray's Anatomy (41tst ed.). Edinburgh: Elsevier Churchill Livingstone 

•  Jung, J., Ahn, H. K., & Huh, Y. (2012). Clinical and functional anatomy of the urethral sphincter. International neurourology journal, 16(3), 102–106. https://doi.org/10.5213/inj.2012.16.3.102