Urethra
Urethra
Also known as: male urethra, female urethra, prostatic urethra, membranous urethra, spongy urethra, penile urethra
The fibromuscular tube that conveys urine from the bladder to the exterior: short (≈4 cm) and purely urinary in females; long (≈18–20 cm), with prostatic, membranous and spongy parts, and shared with the reproductive tract in males.
Overview
Female urethra (≈4 cm, ≈6 mm diameter) runs anteroinferiorly from the internal urethral orifice, embedded in the anterior wall of the vagina, through the pelvic floor and perineal membrane, to the external urethral orifice in the vestibule between the clitoris and the vaginal orifice. Paraurethral (Skene) glands open beside it. Its shortness and proximity to the anus explain the higher incidence of UTI in women.
Male urethra (≈18–20 cm) has four parts: (1) preprostatic (intramural) — ≈0.5–1.5 cm, in the bladder neck, surrounded by the internal sphincter; (2) prostatic — ≈3–4 cm, the widest and most dilatable part; its posterior wall bears the urethral crest with the seminal colliculus, on which open the prostatic utricle and the two ejaculatory ducts, with the prostatic ducts opening into the prostatic sinuses either side; (3) membranous (intermediate) — ≈1–2 cm, the shortest and least dilatable part (apart from the meatus), passing through the deep perineal pouch surrounded by the external urethral sphincter, with the bulbourethral glands alongside; (4) spongy (penile) — ≈15 cm, the longest, within the corpus spongiosum, dilated proximally in the bulb (receiving the bulbourethral ducts) and distally in the glans as the navicular fossa, ending at the external urethral orifice (the narrowest point). Urethral glands (of Littré) open along it.
The male urethra has two curvatures in the flaccid penis (S-shape) — relevant to catheterisation.
- Location
From the bladder neck through the pelvic floor and perineum to the external urethral orifice (vestibule in females; tip of the glans in males).
Parts & subdivisions
- ≈3–4 cm through the prostate; seminal colliculus, ejaculatory ducts, prostatic sinuses; widest part.
- ≈1–2 cm through the deep perineal pouch and external sphincter; least distensible; injured in pelvic fractures.
- ≈15 cm in the corpus spongiosum; bulbar part injured in straddle injuries; navicular fossa distally.
- Female urethra≈4 cm, embedded in the anterior vaginal wall; opens in the vestibule.
Anatomy
- Innervation
Autonomic from the prostatic/vesical plexuses (inferior hypogastric); somatic sensory and motor via the pudendal nerve (S2–S4).
- Arterial supply
Female: internal pudendal and vaginal arteries. Male: inferior vesical and middle rectal (prostatic part), internal pudendal branches — artery of the bulb, urethral artery (membranous and spongy parts).
- Venous drainage
Internal pudendal veins and the vesical/prostatic venous plexuses.
- Lymphatic drainage
Female and male proximal urethra: internal iliac nodes; male spongy urethra: deep inguinal nodes (glans: deep inguinal and external iliac).
- Histology
Urothelium (prostatic urethra and proximal female urethra) → pseudostratified/stratified columnar (membranous and most of the spongy urethra) → non-keratinised stratified squamous (navicular fossa, distal female urethra).
- Development
Urogenital sinus (pelvic and phallic parts); the distal male urethra forms by fusion of the urethral folds — failure produces hypospadias (ventral meatus), the commonest penile anomaly. Posterior urethral valves (males) obstruct the prostatic urethra and can cause oligohydramnios and bilateral hydronephrosis.
Function
- Conduct urine from the bladder to the exterior
- Contribute to continence (sphincters, mucosal seal)
- In males, conduit for semen during ejaculation
Clinical significance
Urethral injury
Pelvic fracture shears the membranous urethra at the prostatomembranous junction (posterior injury: blood at the meatus, high-riding prostate, urine extravasates into the deep pelvis — do not blindly catheterise). Straddle injury ruptures the bulbar (spongy) urethra below the perineal membrane; urine and blood track into the superficial perineal pouch, scrotum, penis and anterior abdominal wall deep to Scarpa (membranous) fascia but not into the thighs.
Urethral catheterisation
In males the catheter must negotiate the external meatus, the navicular fossa, the bulbar curve and the membranous urethra (narrowest and most resistant); elevating the penis straightens the S-curve. Urethral strictures follow infection (gonococcal), trauma or instrumentation.
Urethritis
Dysuria and discharge from Neisseria gonorrhoeae or Chlamydia trachomatis; reactive arthritis may follow chlamydial infection.
Mnemonics
PMS
Prostatic — Membranous — Spongy (proximal → distal).
Test yourself
In a pelvic fracture with urethral injury, a "high-riding" prostate and blood at the meatus, which part of the male urethra is most likely torn?
- Prostatic
- Membranous
- Bulbar (proximal spongy)
- Penile (distal spongy)
Answer: Membranous
The membranous urethra is fixed within the deep perineal pouch and is sheared off at the prostatomembranous junction in pelvic fractures. Straddle injuries typically rupture the bulbar urethra.
Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.