FEMALE URETHRA
The urethra, situated at the lowest portion of the bladder and passing downward and forward beneath the symphysis, varies from 3 to 5 cm in length and averages about 6 mm in diameter. The angle formed by the internal urethral orifice and the bladder at the bladder neck and surrounded by the intrinsic sphincter is critical to maintaining normal urinary continence; to with-stand the hydrostatic pressure of the bladder, this area is further supported by the fascia and tensing muscles of the pelvic diaphragm. Its mucosal surface is thrown into longitudinal folds by the constricting action of the external supporting structures. The most prominent of these longitudinal folds, situated on the posterior aspect of the urethra, is sometimes referred to as the urethral crest. The endopelvic fascia that covers the bladder is continuous over the entire urethra just below the mucosal layer, and contiguous to it is a thin layer of erectile tissue formed by the cavernous venous plexus. The muscular coats that surround the bladder also cover the urethra but become thinner as it passes down-ward toward the external meatus. The upper two-thirds of the urethra lie behind the symphysis pubis and are referred to as the intrapelvic urethra. It is this portion that passes through the musculofascial attachments forming the interlevator cleft. The perineal portion extends from the superior fascia of the urogenital diaphragm to the meatus. As it passes through the urogenital diaphragm, the urethra is surrounded by the sphincter urethrae membranaceae, the homologue of the muscle of the same name in the male but a far weaker and less important structure. Near the external meatus, the urethra is adjacent to the upper ends of the vestibular bulbs and the surrounding bulbocavernosus muscles. At its meatus, the urethra lies in the anterior vaginal wall between the folds of the labia minora 2 to 3 cm below the clitoris. Along its entire length, but especially in its perineal portion, the urethra is perforated by the openings of numerous small periurethral glands, the homologues of the prostatic ducts in the male.
The schematic reconstruction of this duct system shows that although the
ducts of the small glands may enter the urethra independently, the majority of
them form an interdependent conducting system terminating in the large
paraurethral (Skene) ducts, which open on either side of the midline, posterior
to the urethral meatus. These are vestigial remnants that serve no specific
purpose but are important in that their position predisposes them to infection,
especially by the gonococcus, and that their relatively poor drainage increases
the risk of a chronic infection.
Cross sections through the lower urethra show the mucosal folds and the
immediate supporting structures:
The submucosal lamina propria is a loose network of fibrous and elastic
tissue containing a prominent venous system, the cavernous plexus or corpus
spongiosum, which accounts for the extreme vascularity in the area. The muscle
coats consist of an inner longitudinal and an outer circular layer, both quite
thin and mutually interdependent. A thin layer of striated muscle referred to
as the external sphincter and supplied by the pudendal nerve also surrounds the
lower urethra, but these distal muscle groups have
little to do with micturition. Under high-power microscopy, it can be seen that
the epithelium of both the urethra and the periurethral ducts are of the
stratified squamous type. The epithelium of the intrapelvic portion of the
urethra, as it approaches the bladder neck, tends to be transitional. The
glandular epithelium, on the other hand, is of the columnar type, not
infrequently stratified. The submucosal connective tissue is relatively poor in
cells.
