Male Urethra Anatomy
The male urethra is a fibromuscular
tube approximately 20 cm long. Beginning at the internal urethral meatus of the
bladder, it descends through the prostate and the pelvic floor and enters the
bulb of the penis (Fig. 5.19). It then traverses the corpus spongiosum and
glans of the penis and terminates at the external urethral meatus. In the male
the urethra not only drains urine from the bladder but also receives secretions
from the prostatic ducts, the ejaculatory ducts and the ducts of the
bulbourethral glands.
The male urethra is described in
three parts: prostatic, intermediate (membranous) and spongy. The prostatic and
inter-mediate parts pass downwards, while the spongy part turns forwards in the
bulb of the penis (Fig. 5.19). A catheter or cystoscope must be
manoeuvred carefully at this angulation to avoid urethral damage. Although the
spongy and prostatic parts can be readily dilated, the external meatus and the
intermediate urethra are comparatively narrow.
Passing downwards through the
prostate, the prostatic urethra is approximately 3 cm long. A midline ridge,
the urethral crest, projects from the posterior wall, producing bilateral
grooves, the prostatic sinuses (Fig. 5.20). Opening into each sinus are
numerous prostatic ducts. The urethral crest is most prominent near its
midpoint, where it presents a rounded elevation, the seminal colliculus. A
midline orifice on the colliculus leads into a blind-ending sac, the prostatic
utricle, which is a remnant of the ducts that give rise to the uterus in the
female embryo. On each side of the utricle is the opening of the ejaculatory
duct. Pro-static surgery, which disrupts the bladder neck and weakens the
urethral sphincters, may impair urinary continence.
Emerging from the anterior aspect
of the apex of the prostate, the intermediate urethra descends through the
pelvic floor and pierces the perineal membrane. It is approximately 2 cm long
and its mucosa is folded, giving the lumen a stellate appearance on
crosssection. Encircling the intermediate urethra is the striated muscle of the
external urethral sphincter (Fig. 5.21), the main structure responsible for
urinary continence. Lateral to the sphincter are the medial borders of the
levatores ani.
Posterolateral to the intermediate
urethra are the paired bulbourethral glands, each about 1 cm in diameter (Fig.
5.21). Their ducts pierce the perineal membrane and open into the spongy
urethra. An additional posterior relation of the intermediate urethra is the
ampulla of the rectum, while anteriorly lies the lower border of the pubic
symphysis, to which the urethra is anchored by the pubourethral ligaments.
The spongy urethra is approximately
15 cm in length, commencing in the bulb of the penis and traversing the
erectile tissue of the corpus spongiosum and glans (Fig. 5.19). The mucosa
presents numerous small recesses or lacunae and most of its lumen forms a
transverse slit. Within the bulb, the urethra is wider, forming the intrabulbar
fossa. The lumen is also expanded within the glans to form the navicular fossa,
which opens at the surface as a vertical slit, the external meatus. Urine may
leak (extravasation) into the superficial perineal pouch if the spongy urethra
is torn by perineal trauma or pelvic fracture (p. 244).
Blood supply
The prostatic and intermediate
parts of the urethra receive blood from the inferior vesical arteries. The
spongy part is supplied by the internal pudendal artery via the dorsal arteries
of the penis and the arteries to the bulb. Venous blood passes into the
prostatic venous plexus and the internal pudendal veins.
Nerve supply
The principal sensory innervation
of the mucosa is provided by the pudendal (S2, S3 & S4) nerve, a branch of
the sacral plexus. The same spinal cord segments innervate the external sphincter.
