Functional Anatomy of Urinary System

Kidneys - Bean-Shaped Filters

• Paired, bean-shaped, retroperitoneal organs (T12-L3). Right kidney slightly lower.
• Dimensions: 11x6x3 cm. Weight: ~150g (M), ~135g (F).
• Coverings (deep to superficial): Fibrous capsule, Perirenal fat, Renal fascia (Gerota's), Pararenal fat.
• Hilum (medial indentation):

  ⭐ Structures anterior to posterior: Renal Vein → Renal Artery → Renal Pelvis (Mnemonic: VAP).

• Internal Gross Structure:
  • Cortex: Outer, granular. Contains glomeruli, PCT, DCT. Renal columns (of Bertin).
  • Medulla: Inner, striated. Composed of 8-18 renal pyramids; apex = renal papilla.
  • Drainage: Papilla → Minor calyces (7-13) → Major calyces (2-3) → Renal pelvis → Ureter.

Nephron & JGA - Tiny Titans of Filtration

• Nephron: Kidney's functional unit (~1 million/kidney).
  • Parts: 📌 Glory Boy, Please Let Down Collecting! (Glomerulus, Bowman's capsule, PCT, Loop of Henle [thin/thick limbs], DCT, Collecting Duct).
  • Types: Cortical (85%), Juxtamedullary (15%, long loops, urine concentration).
• Juxtaglomerular Apparatus (JGA): Regulates GFR & BP.
  • Components: Macula Densa (DCT, senses NaCl), JG Cells (afferent arteriole, secrete renin), Extraglomerular Mesangial cells.

⭐ JGA: JG cells secrete renin (↑BP via RAAS). Macula densa senses filtrate NaCl for tubuloglomerular feedback (TGF).

Ureters & Bladder - Conduits & Container

• Ureters: Muscular tubes (25-30 cm) conveying urine from kidneys to bladder.
  • Constrictions (📌 PUJ, Pelvic Brim, VUJ):
    • Pelviureteric Junction
    • Crossing Pelvic Brim/Iliac vessels
    • Vesicoureteric Junction (narrowest)
• Urinary Bladder: Hollow muscular organ; stores urine. Capacity ~500ml.
  • Detrusor muscle: Smooth muscle; contraction aids micturition.
  • Trigone: Triangular area; ureteric orifices & internal urethral meatus.
  • Internal urethral sphincter: Smooth muscle; involuntary control.

⭐ Ureter Histology: Transitional epithelium; muscularis (inner longitudinal, outer circular smooth muscle); additional outer longitudinal in lower third.

Urethra & Vasculature - Exit Route & Lifeline

• Male Urethra (approx. 20cm):
  • Prostatic: 3-4cm, widest, most dilatable. Receives ejaculatory ducts.
  • Membranous: 1-2cm, passes urogenital diaphragm. External sphincter.
  • Spongy (Penile): ~15cm, in corpus spongiosum. Narrowest at external meatus.
• Female Urethra: ~4cm, anterior to vagina. Prone to UTIs due to short length.
• Renal Arterial Supply:

• Renal Venous Drainage: Follows arterial path in reverse (Interlobular v. → Arcuate v. → Interlobar v. → Renal v.). Renal veins lack valves.

⭐ Membranous urethra is the shortest, narrowest, and least dilatable part of the male urethra, making it most vulnerable to injury from pelvic fractures.

and female urethra; Schematic of renal arterial supply and venous drainage)

Micturition & Innervation - The Emptying Act

• Bladder capacity: First urge 150-250ml; Fullness 350-450ml.
• Innervation & Reflex:
  • Storage (Sympathetic T11-L2): Detrusor relax, Internal Urethral Sphincter (IUS) contracts.
  • Voiding (Parasympathetic S2-S4): Detrusor contracts, IUS relaxes. 📌 P for Pee.
  • Somatic (Pudendal n. S2-S4): External Urethral Sphincter (EUS) voluntary control.

⭐ Pontine Micturition Center (Barrington's nucleus) in pons: key micturition reflex coordinator.

High‑Yield Points - ⚡ Biggest Takeaways

• Kidneys: Retroperitoneal, T12-L3; right kidney slightly lower.
• Nephron: Functional unit; juxtamedullary type key for urine concentration.
• JGA: Macula densa, JG cells regulate renin secretion and GFR.
• Glomerular filtration barrier: Endothelium, GBM, podocyte slits prevent proteinuria.
• Ureteric constrictions: PUJ, pelvic brim, VUJ - common sites for calculi.
• Bladder: Detrusor muscle (wall); trigone (smooth, stretch-sensitive).
• Micturition: Parasympathetic (emptying), sympathetic (storage), somatic (sphincter control).