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).
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.
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.