urethra

The female urethra is approximately 3–4 cm long, runs from the bladder neck to the external urethral meatus (in the vestibule of the vulva), and carries only urine. The male urethra is approximately 20 cm long and is divided into four sections: prostatic (passing through the prostate), membranous (through the external sphincter), bulbous, and penile (spongy). The male urethra serves dual functions — conveying urine and, during ejaculation, semen.

Two sphincters regulate urinary flow through the urethra. The internal urethral sphincter is formed by smooth muscle at the bladder neck and is under involuntary (autonomic) control — it automatically closes to maintain continence. The external urethral sphincter is composed of striated skeletal muscle surrounding the urethra and is under voluntary control, allowing conscious delay of urination.

Urethritis — inflammation of the urethra — is most commonly caused by sexually transmitted infections such as gonorrhea (Neisseria gonorrhoeae) and chlamydia (Chlamydia trachomatis). Urethral strictures (scarring that narrows the lumen) cause obstructed flow. In older men, benign prostatic hyperplasia compresses the prostatic urethra, causing lower urinary tract symptoms. Urethral injuries can occur with pelvic fractures or catheterization trauma.