Anatomy

The bladder and urethra form the lower urinary tract. This tab covers the bladder's embryology, relations, histology, trigone, and neurovascular supply, followed by the segmental anatomy of the male and female urethra.

Bladder: Embryology and Relations

The cloaca is divided by the urorectal septum into a dorsal portion (rectum and anal canal) and a ventral portion (urogenital sinus). The urogenital sinus forms the bladder in both sexes; in males also the transitional and peripheral prostate zones, the prostatic and penile urethra, and the bulbourethral glands; in females the distal third of the vagina and urethra.

• The urachus anchors the bladder to the anterior abdominal wall; its epithelium-lined lumen persists through life and can rarely give rise to urachal adenocarcinoma.
• Only the superior surface is covered by peritoneum. As the bladder distends it strips the peritoneum off the anterior wall, allowing a suprapubic cystostomy without entering the peritoneal cavity. Posteriorly the peritoneum forms the rectovesical pouch, and anteriorly the space of Retzius is entered by dividing the transversalis fascia.

Bladder: Histology and Trigone

The wall has three layers (superficial to deep):

• Urothelium — lacks blood vessels and lymphatics (low metastatic potential); normally <7 cells thick on a thin basement membrane.
• Lamina propria — rich in vessels and lymphatics (high metastatic potential); contains a poorly defined muscularis mucosae (absent in 70% of specimens; not to be confused with the muscularis propria — invasion of the lamina propria is pT1).
• Detrusor (muscularis propria) — inner longitudinal, middle circular, and outer longitudinal bundles; absent in diverticula.

The filled bladder holds ~500 mL. The ureter pierces the wall obliquely and runs 1.5–2 cm submucosally; vesicoureteral reflux results from insufficient submucosal length and poor detrusor backing (the submucosal tunnel normally acts as a flap valve). The trigone is the triangle between the two ureteric orifices and the internal meatus; the interureteric ridge (Mercier's bar) connects the orifices. Trigonal muscle has a superficial layer (from the ureter's longitudinal muscle) and a deep layer (from Waldeyer's sheath, inserting at the bladder neck) over the detrusor.

Bladder: Vasculature, Lymphatics, and Innervation

• Arterial — superior and inferior vesical arteries (anterior internal iliac), described surgically as the lateral pedicle (lateral to the ureter) and posterior pedicle (posteromedial to the ureter). Venous drainage is via the vesical plexus into the internal iliac vein.
• Lymphatics — mostly to the external iliac nodes, with some drainage to the obturator, internal iliac, and (from the base/trigone) common iliac groups. Cystectomy node-dissection limits: superior — common iliac bifurcation; inferior — Cloquet's node at the femoral canal; lateral — genitofemoral nerve; medial — bladder/internal iliac artery; posterior — obturator fossa.
• Innervation — the wall is richly parasympathetic (cholinergic) for contraction, with sparse functional sympathetic input. In males the internal sphincter is richly α1-adrenergic, providing continence at the bladder neck (perfect continence can persist even if the striated sphincter is destroyed); damage to these sympathetic nerves (diabetes or RPLND for testis cancer) causes retrograde ejaculation. The female bladder neck has little adrenergic innervation. Afferents travel with both the sympathetic (hypogastric) and parasympathetic nerves — in women the parasympathetics run in the cardinal ligament, so presacral neurectomy does not relieve bladder pain.

Urethra

Normal urethral diameter is 8–9 mm (3 Fr = 1 mm; normal 24–27 Fr). The perineal membrane separates the posterior from the anterior urethra.

Male Urethra

• Posterior urethra — bladder neck, prostatic urethra, and membranous urethra. The prostatic urethra turns ~35° anteriorly at its midpoint, separating a pre-prostatic segment (periurethral glands drain here and can contribute to BPH) from a prostatic segment (the ejaculatory ducts open beside the utricular orifice on the verumontanum). The membranous urethra (~2–2.5 cm) is partly surrounded by the external sphincter — intrinsic smooth muscle plus a striated rhabdosphincter, innervated by the pudendal nerve and shaped like a signet ring.
• Anterior urethra — bulbar urethra (the bulbourethral/Cowper glands open here at 3 and 9 o'clock; surrounded by bulbocavernosus and the thickest corpus spongiosum), penile urethra (the mucus glands of Littre open on its posterior wall), and the fossa navicularis (within the glans). The whole anterior urethra is ~15 cm.

Female Urethra

The posterior urethra comprises the bladder neck and membranous urethra only.

Microanatomy and Supply

The posterior urethra is lined by transitional epithelium (which can extend into the prostatic ducts — sample these in persistent positive cytology); the anterior urethra is pseudostratified columnar, becoming stratified squamous in the fossa navicularis. The bulbourethral artery (anterior internal iliac → internal pudendal → common penile) supplies the urethra, corpus spongiosum, and glans; lymphatics drain to the internal and common iliac nodes.

Self-Test

1. What is the arterial supply to the bladder? The superior and inferior vesical arteries, from the anterior branch of the internal iliac artery.

2. Describe the anatomical subdivisions of the urethra, and what separates the anterior from the posterior urethra. Posterior: bladder neck, prostatic urethra, membranous urethra. Anterior: bulbar urethra, penile urethra, fossa navicularis. The perineal membrane separates the two.

3. Where do each of the following drain: periurethral glands, ejaculatory ducts, Cowper's glands, glands of Littre? Periurethral glands → proximal (pre-prostatic) prostatic urethra; ejaculatory ducts → distal prostatic urethra; Cowper's glands → bulbar urethra; glands of Littre → penile urethra.

4. Which part of the urethra is surrounded by the corpus spongiosum? The anterior urethra.