The urethra is the vessel responsible for transporting urine from the bladder to an external opening in the perineum. It is lined by stratified columnar epithelium, which is protected from the corrosive urine by mucus secreting glands.
The male urethra is approximately 15-20cm long. In addition to urine, the male urethra transports semen – a fluid containing spermatozoa and sex gland secretions.
According to the latest classification, the male urethra can be divided anatomically into three parts (proximal to distal):
- Prostatic urethra:
- Begins as a continuation of the bladder neck and passes through the prostate gland.
- Receives the ejaculatory ducts (containing spermatozoa from the testes and seminal fluid from the seminal vesicle glands) and the prostatic ducts (containing alkaline fluid).
- It is the widest and most dilatable portion of the urethra.
- Membranous urethra:
- Passes through the pelvic floor and the deep perineal pouch.
- Surrounded by the external urethral sphincter – which provides voluntary control of micturition.
- It is the narrowest and least dilatable portion of the urethra.
- Penile (bulbous) urethra:
- Passes through the bulb and corpus spongiosum of the penis, ending at the external urethral orifice (the meatus).
- Receives the bulbourethral glands proximally.
- In the glans (head) of the penis, the urethra dilates to form the navicular fossa.
In females, the urethra is relatively short (approximately 4cm). It begins at the neck of the bladder, and passes inferiorly through the perineal membrane and muscular pelvic floor. The urethra opens directly onto the perineum, in an area between the labia minora, known as the vestibule.
Within the vestibule, the urethral orifice is located anteriorly to the vaginal opening, and 2-3cm posteriorly to the clitoris. The distal end of the urethra is marked by the presence of two mucous glands that lie either side of the urethra – Skene’s glands. They are homologous to the male prostate.