HOW IS URINARY CONTINENCE MAINTAINED?

(Fig. 3.1 Lateral view of the components of the urethral
support system. Note how the levator ani muscles support
the rectum, vagina, and urethrovesical neck. Also note how
the endopelvic fascia beside the urethra attaches to the
levator ani muscle; contraction of the levator muscle leads
to elevation of the urethrovesical neck. Puborectalis muscle
is removed for clarity. (Redrawn from DeLancey 1994, with
permission of C V Mosby Company, St Louis. © DeLancey
2005.)


Urethral closure pressure must be greater than bladder
pressure, both at rest and during increases in abdominal
pressure to retain urine in the bladder. The resting tone
of the urethral muscles maintains a favourable pressure
relative to the bladder when urethral pressure exceeds
bladder pressure.
During activities such as coughing, when bladder
pressure increases several times higher than urethral
pressure, a dynamic process increases urethral closure
pressure to enhance urethral closure and maintain continence
(Enhörning 1961). Both the magnitude of the
resting pressure in the urethra and the increase in pressure
generated during a cough determine the pressure
at which leakage of urine occurs (Kim et al 1997).
Although analysis of the degree of resting closure
pressure and pressure transmission provides useful
theoretical insights, it does not show how specific
injuries to individual component structures affect the
passive or active aspects of urethral closure. A detailed
examination of the sphincteric closure and the urethral
support subsystems (Fig. 3.1) is required to understand
these relationships.
The dominant element in the urethral sphincter is the
striated urogenital sphincter muscle, which contains a
striated muscle in a circular configuration in the middle
of the urethra and strap-like muscles distally. In its
sphincteric portion, the urogenital sphincter muscle
surrounds two orthogonally-arranged smooth muscle
layers and a vascular plexus that helps to maintain
closure of the urethral lumen.