Urethrae

Urethrae are the plural form of urethra, the duct that conducts urine from the bladder to the exterior in humans and many other animals. The term is used in anatomical and comparative contexts to discuss multiple instances of the channel within a single organism or across species.

In humans, urethrae differ by sex. The male urethra is roughly 18 to 20 cm long and traverses the prostate, the pelvic floor, and the penis (proximal prostatic, membranous, and distal spongy or penile segments). The female urethra measures about 4 cm and runs from the bladder neck to the external urethral meatus anterior to the vagina.

Function and lining: The urethra carries urine from the bladder to the outside, and in males also serves as a conduit for semen during ejaculation. It is guarded by internal and external sphincters and is lined by mucosa whose epithelium changes along its length, from transitional or pseudostratified proximally to stratified squamous near the meatus.

Clinical relevance: Common issues include urethral stricture (narrowing due to injury or inflammation), urethritis (inflammation usually from infection), and congenital malformations such as hypospadias in males, where the urethral opening is on the underside of the penis. In females, the shorter urethra is more susceptible to infections. Catheterization and instrumentation can cause trauma or strictures; diagnosis may involve urethrography or cystoscopy.

Development and variation: Urethral development involves the urogenital sinus; the male urethra has contributions from endoderm and ectoderm, while the female urethra arises largely from the urogenital sinus. Across species, urethrae vary in length, path, and timing of development depending on reproductive anatomy.