membranous urethra

**Anatomy of the male urethra (4 parts, proximal to distal):** (1) Preprostatic (intramural), (2) Prostatic, (3) Membranous, (4) Spongy (penile), which includes the bulbar and pendulous portions and the navicular fossa.

**Membranous urethra:** - Length about 1–2 cm. It is the shortest and narrowest part after the external urethral meatus, and the least distensible part. - It lies in the deep perineal pouch (deep perineal space), where it passes through the urogenital diaphragm/perineal membrane region. - The external urethral sphincter (sphincter urethrae) surrounds it. This is skeletal muscle under voluntary control, innervated by the pudendal nerve (S2–S4, deep perineal branch). - The bulbourethral (Cowper) glands lie lateral to it in the deep pouch. Their ducts open into the proximal spongy (bulbar) urethra, not the membranous part. - Epithelium is stratified columnar or pseudostratified columnar. The prostatic urethra is transitional (urothelium), and the distal navicular fossa is stratified squamous.

**Clinical significance:** - **Pelvic fracture urethral injury:** Shearing forces on the puboprostatic ligaments tear the membranous urethra. Presentation: blood at the meatus, inability to void, perineal/scrotal hematoma, and a high-riding or "floating" prostate on DRE. Obtain a retrograde urethrogram **before** placing a Foley catheter. Extravasation of urine/blood goes above the perineal membrane, into the retropubic/pelvic space. - **Contrast with bulbar (spongy) urethral injury:** A straddle injury ruptures the bulbar urethra below the perineal membrane. If Buck fascia is intact, extravasation is limited to the penis. If Buck fascia is breached, urine is contained by Colles fascia, producing a "butterfly" perineal hematoma. It can spread to the scrotum and up the abdominal wall under Scarpa fascia but not to the thigh, because Colles fascia attaches to the fascia lata. - **Iatrogenic injury:** Radical prostatectomy can damage the external sphincter and cause stress incontinence. Instrumentation can also injure the membranous urethra. - **Sphincter function:** It allows voluntary continence. The internal urethral sphincter (smooth muscle at the bladder neck, sympathetic control) prevents retrograde ejaculation. - **Female correlate:** The female urethra has a sphincter urethrae but no distinct "membranous" segment.

**High-yield mnemonic:** "Membranous = Most often injured in pelvic fracture; Bulbar = Bicycle (straddle) injury." Another version: a "Rupture above the diaphragm" (membranous) causes a high-riding prostate, and a "rupture below the diaphragm" (bulbar) causes a butterfly hematoma.