Renal System Lecture Review

Holistic Importance of the Renal (Urinary) System

• No single body system is truly “most important,” yet the renal system is indispensable; without functional kidneys, every other system’s stability collapses.
  • Dialysis frequency (2–5× wk) in end-stage renal disease illustrates dependency.
• Interdependence with other systems
  • Cardiovascular delivers blood to be filtered.
  • Respiratory supplies O₂; brain regulates renal blood flow/hormonal outputs.

Gross Anatomy & Basic Layout

• Organs
  • 2 kidneys (retroperitoneal)
  • 2 ureters (kidney → bladder)
  • 1 urinary bladder (pelvic girdle)
  • 1 urethra (bladder → exterior)
• Terminology
  • “Renal” = kidney-related; entire setup may also be called the urinary system.

Core Functions (Simplified)

• Filter blood plasma (except most plasma proteins & blood cells).
• Regulate blood osmolarity → directly modulates blood volume → influences blood pressure.
  • Osmolarity depends on water + solutes (ions, nutrients, hormones, wastes like urea/uric acid, etc.).
  • High solute → water retention → ↑ BP; low solute/water elimination → ↓ BP.

Kidney Micro-Architecture

• ~10^6 nephrons per kidney (≈2\times10^6 total).
• Coronal section terms
  • Renal cortex (outer shell) – houses most nephron components.
  • Renal medulla (inner pyramids) – contains loops/collecting ducts.

Nephron Components

• Bowman's capsule → proximal convoluted tubule (PCT) → loop of Henle → distal convoluted tubule (DCT) → collecting duct.
  • Bowman's capsule + glomerulus = renal corpuscle.
• Associated vessels
  • Afferent arteriole → glomerulus (capillary tuft) → efferent arteriole → peritubular capillaries (and vasa recta for juxtamedullary nephrons).
  • Arrangement constitutes a portal system (arteriole–capillary–arteriole–capillary).

Quantitative Highlight: Glomerular Filtration Rate (GFR)

• Normal target \approx 125\;\text{mL}\,\text{min}^{-1}(all nephrons combined).
• Daily filtrate production
  125\;\text{mL}\,\text{min}^{-1}\times60\,\text{min}\,\text{h}^{-1}\times24\,\text{h}\,\text{day}^{-1}\times\frac{1\,\text{L}}{1000\,\text{mL}}=180\;\text{L}\,\text{day}^{-1}
• Yet total blood plasma ≈3 L → plasma filtered ≈60×/day.
• Urine volume only \approx1{-}1.5\;\text{L}\,\text{day}^{-1} ( <1 % of filtrate).
• Urine osmolarity range: 50 mOsm L⁻¹ (very dilute) to 1200 mOsm L⁻¹ (very concentrated) vs blood 280{-}300 mOsm L⁻¹.
  • Concentrated urine ⇒ water reabsorbed (dehydration, low BP).
  • Dilute urine ⇒ excess water excreted (high BP, diuretic use).

Three Fundamental Nephron Processes

1. Filtration (renal corpuscle)
   • Plasma → nephron lumen (excludes proteins & cells).
2. Reabsorption (mainly PCT; continues along nephron)
   • “Wanted” solutes & water returned to blood (peritubular caps).
3. Secretion (primarily DCT for fine-tuning)
   • Blood → nephron lumen (additional waste/ion balance).

Overall urine formation equation
{\rm Excretion}=\underbrace{\rm Filtration}{F}-\underbrace{\rm Reabsorption}{R}+\underbrace{\rm Secretion}_{S}

Regulation of GFR (Hemodynamics)

• If GFR ↓ (too low)
  • Dilate afferent arteriole → ↑ flow into glomerulus.
  • Constrict efferent arteriole → “dam” outflow, ↑ intraglomerular pressure.
• If GFR ↑ (too high) → opposite adjustments.
• Pressures at play in renal corpuscle
  • Glomerular hydrostatic pressure (BP driven) – drives filtration outward.
  • Capsular hydrostatic pressure – back-pressure from filtrate volume trying to push fluid back in.
  • Colloid osmotic pressure – plasma proteins in glomerulus pull water back in.
  • Net filtration occurs as long as P{\text{glomerular}} > P{\text{capsule}}+\pi_{\text{oncotic}}.

Urine Pathway Post-Collecting Duct

Collecting duct → renal papilla → minor & major calyces → renal pelvis → ureter → bladder → urethra → exterior.

Detailed Look: Bulk Reabsorption in the PCT

• Lining: simple cuboidal epithelium with extensive microvilli → ↑ surface area.
• Surface nomenclature
  • Apical (faces lumen/filtrate).
  • Basal (faces interstitium/peritubular capillary).

Ionic & Nutrient Transport

• Initial filtrate iso-osmotic (≈300 mOsm) to plasma.
• Sodium
  • High in filtrate, low intracellular.
  • Apical entry via passive channels/co-transporters (down gradient).
  • Basal exit via \text{Na}^+/\text{K}^+-ATPase (against gradient).
• Glucose
  • Apical uptake against gradient by SGLT (sodium-glucose co-transport). Uses Na⁺ kinetic energy.
  • Basal release via GLUT (facilitated diffusion) → interstitium → blood.
• Potassium
  • High intracellular; low initial filtrate.
  • After massive water reabsorption, filtrate [K⁺] rises → passive paracellular reabsorption.
• Water
  • Follows solute (esp. Na⁺, glucose) osmotically.
  • Primarily paracellular; some aquaporin-mediated transcellular flow.

Summary Chain

1. Solutes (Na⁺, glucose, etc.) move apical → cell → basal → interstitium → blood.
2. Osmosis drags H₂O.
3. Concentration changes enable additional solute (e.g., K⁺) reuptake.

Comparative/Real-World & Clinical Notes

• Portal arrangement (afferent–capillary–efferent–capillary) unique → enables massive filtration yet precise reclamation.
• Imperfect design: bulk “over-filter then reclaim” strategy wastes energy but offers fine control.
• Diuretics, dehydration, heart failure, hypertension all modulate urine concentration via nephron transporters/pressures.
• Pronunciation challenges: glomerulus, peritubular, Bowman's—the lecturer acknowledges difficulty, stresses practice for mastery.

Key Numerical & Conceptual Takeaways

• \text{GFR}_{\text{normal}}\approx125\,\text{mL}\,\text{min}^{-1} → 180 L filtrate/day.
• Only 1{-}1.5 L urine/day ( <1 %).
• Plasma filtered ~60× daily; indicates continuous blood cleansing.
• Urine osmolarity spans 50{-}1200 mOsm L⁻¹.
• Equation E=F-R+S underpins all renal clearance calculations.
• Hemodynamic tweaks (afferent/efferent tone) correct GFR deviations.
• Bulk reabsorption in PCT reclaims ≈75\% of filtrate before Loop of Henle.