The male urethra measures, on average, 18-20 cm in length. It commences at the internal urethral orifice in the trigone of the bladder and opens in the navicular fossa of the glans penis at the external urethral meatus, which is the narrowest part of the urethra.
The male urethra can be divided into anterior and posterior portions. The anterior urethra is composed of the penile and bulbar urethra to the level of the urogenital diaphragm. The posterior urethra is composed of the membranous and prostatic urethra.
- penile (spongy, pendulous) urethra (~16 cm long): encased by corpus spongiosum of the penis
- the longest portion
- the fossa navicularis is a small normal dilatation of the distal penile urethra
- bulbar (bulbous) urethra: traverses the root of the penis
- it receives the ducts from the bulbourethral glands and the glands of Littré
The anterior urethra is lined by pseudostratified columnar epithelium. The very distal portion (at approximately the level of the fossa navicularis) is lined by squamous epithelium.
- membranous urethra (1 cm long): passes through the urogenital diaphragm, surrounded by sphincter urethrae
- the shortest and narrowest portion
- prostatic urethra (3 cm long): surrounded by the prostate gland; on its posterior wall runs the urethral crest and the prominent smooth muscle verumontanum
The posterior urethra is lined by transitional columnar epithelium.
- prostatic urethra – inferior vesical artery
- membranous urethra – bulbourethral artery
- penile urethra – branches of the internal pudendal artery
- dorsal veins of the penis and internal pudendal vein that drain to the prostatic and vesical venous plexuses
- prostatic and vesical venous plexuses drain to internal iliac veins
- posterior urethra:
- anterior urethra:
- prostatic plexus
- posterior urethral valves
- urethral duplication
- congenital rectourethral fistulae
The division into anterior and posterior urethras is important in terms of pathology and in imaging the urethra: the anterior urethra being visualized by performing a retrograde (ascending) urethrogram and the posterior urethra with an antegrade (descending or micturating) urethrogram.
- axial: membranous urethra is a low-intensity outer ring surrounding a high-intensity inner epithelium
- sagittal and coronal images may show the course of the anterior and posterior urethra but often the proximal prostatic and penile urethra are not well seen unless there is an indwelling Foley catheter