Anatomy and Physiology of the Bladder

Related Subjects: |Anatomy of the Glomerulus |Anatomy of the Bladder

🫙 The urinary bladder is a hollow, distensible muscular sac in the pelvis that temporarily stores urine (typically 400–600 mL comfortably, up to 800–1000 mL max) before voluntary voiding. It expands dramatically during filling (compliance ~20–40 mL/cm H₂O) and contracts forcefully during micturition to expel urine via the urethra.

🧬 Anatomy of the Bladder

• Location & Relations 📍
  • Lies in the true pelvis, behind the pubic symphysis, anterior to the rectum (males) or uterus/vagina (females).
  • Superior surface covered by peritoneum → forms anterior peritoneal reflection (vesicouterine/vesicorectal pouch).
  • Males: prostate inferiorly, seminal vesicles posteriorly.
  • Females: uterus/vagina posteriorly, pubic symphysis anteriorly.
  • When full: rises above pubic symphysis → palpable/percussible in lower abdomen.
• External Features
  • Apex (Fundus anterior): Superior/anterior point; attached to median umbilical ligament (urachus remnant).
  • Base (Fundus posterior): Triangular posterior wall; ureters enter superolaterally.
  • Body: Main expandable portion between apex and base.
  • Neck: Inferior constriction; continuous with prostatic urethra (males) or urethra (females).
• Internal Features – Trigone 🔺
  • Smooth, fixed triangular area on posterior base.
  • Boundaries: two ureteric orifices (superolateral) + internal urethral orifice (inferior apex).
  • Lined by tightly adherent mucosa (no rugae) → less distensible; embryologically mesodermal (vs endodermal body).
  • Interureteric ridge (between ureteric orifices) clinically visible on cystoscopy.
• Histological Layers 🧪
  • Mucosa: Transitional (urothelium) epithelium (6–8 layers when empty → 2–3 when stretched) + lamina propria (connective tissue with blood vessels, nerves, lymphoid follicles).
  • Submucosa (lamina propria continuation): loose connective tissue; allows folding (rugae) when empty.
  • Muscularis (Detrusor muscle): Thick smooth muscle in 3 poorly defined layers (inner/outer longitudinal, middle circular) → powerful contraction during voiding.
  • Adventitia/Serosa: Outer connective tissue; superior surface covered by visceral peritoneum (serosa).

🩸 Vascular Supply

• Arterial ❤️
  • Superior vesical arteries (from internal iliac) → anterosuperior bladder.
  • Inferior vesical arteries (males) / vaginal artery (females) → posteroinferior bladder.
  • Additional: obturator, uterine, middle rectal arteries.
• Venous 💉
  • Vesical venous plexus → internal iliac veins (main drainage).
  • Connections to prostatic/uterine/vaginal plexuses → risk of spread in malignancy.
• Lymphatic: Follow arteries → internal iliac → common iliac → para-aortic nodes.

🧠 Neural Supply & Micturition Reflex

• Storage phase (sympathetic dominance): Low pressure filling (compliance), closed sphincters.
• Micturition reflex:
  • Stretch receptors (Aδ/C fibres) in detrusor → pelvic nerves → sacral cord (S2–S4) → pontine micturition centre → cerebral cortex (voluntary control).
  • Threshold ~200–400 mL → urge; voluntary relaxation of external sphincter + parasympathetic activation → detrusor contraction + sphincter relaxation.
• Neurogenic bladder: Suprapontine (uninhibited), spinal (detrusor-sphincter dyssynergia), infrasacral (atonic).

🫁 Physiology of the Bladder

• Filling/Storage phase:
  • Ureters deliver urine intermittently → bladder fills slowly.
  • Detrusor relaxes (sympathetic β₃), sphincters contracted → pressure remains low (<10–15 cm H₂O).
  • Capacity: functional 400–600 mL; max ~800–1000 mL before strong urge.
• Voiding phase (micturition):
  • Threshold stretch → afferent signals → reflex + voluntary inhibition release.
  • Parasympathetic → detrusor contraction (intravesical pressure 40–100 cm H₂O).
  • Sphincter relaxation → urine expulsion (~15–25 mL/s flow rate).
  • Completion: detrusor relaxes, sphincters close.

🩺 Clinical Relevance

• Urinary incontinence 💦: Stress (pelvic floor weakness), urge (overactive bladder/detrusor instability), overflow (retention), functional (mobility/cognition).
• Urinary retention 🚫: Acute (BPH, drugs, post-op) vs chronic (neurogenic, obstruction) → risk of hydronephrosis, UTI, overflow incontinence.
• Cystitis / UTI 🔥: Bacterial (E. coli), interstitial (painful bladder syndrome), radiation/chemotherapy-induced.
• Bladder cancer 🧬: Transitional cell carcinoma (90%); risk factors: smoking, aromatic amines, schistosomiasis (squamous); painless haematuria most common presentation.
• Neurogenic bladder 🧠: MS, spinal cord injury, diabetic neuropathy → detrusor hyperreflexia/areflexia, dyssynergia → high-pressure retention, reflux nephropathy.

Teaching Point 🩺 Bladder = distensible storage organ (400–600 mL) with transitional epithelium + powerful detrusor. Storage: sympathetic (relax detrusor, contract sphincters). Voiding: parasympathetic (contract detrusor, relax internal sphincter) + voluntary external sphincter relaxation. Trigone = fixed, smooth area (ureteric + urethral orifices). Clinical keys: haematuria → cancer/UTI; retention → overflow incontinence; low compliance → high-pressure damage to kidneys.

📚 References (Feb 2026)

• Gray’s Anatomy 42nd ed. (2020; 2025 online updates).
• Wein AJ et al. Campbell-Walsh Urology (13th ed., 2025).
• EAU Guidelines: Neuro-urology & Incontinence (2025–2026).
• Recent: Bladder mechanosensation & TRP channels (Nature Rev Urol 2025).