Glomerulofiltration Lecture Notes

Introduction

• Focus: Glomerulofiltration
• Pre-topic: Structure of the Renal Corpuscle
  • Composed of two parts:
    • Glomerulus: Tuft of capillaries
    • Bowman's Capsule: Also known as glomerular capsule

Glomerulus

• Structure: Tuft of capillaries
• Afferent Arteriole: Feeds the glomerulus
• Efferent Arteriole: Drains the glomerulus
  • Unique system: Arteriole-fed and arteriole-drained capillary bed

Capillaries in Glomerulus

• Fenestrated Capillaries
  • Fenestrations: Small pores in endothelial cells
    • Size: 50 to 100 nanometers
    • Allow passage:
      • Small proteins
      • Water
      • Electrolytes (e.g., Na⁺, K⁺, Ca²⁺)
      • Nutrients and waste products
    • Block passage of formed elements (e.g., RBCs, WBCs, platelets)

Glomerular Basement Membrane (GBM)

• Three Layers
  1. Lamina Densa: Contains type IV collagen and laminins
  2. Lamina Rara Interna: Contains heparin sulfate (negatively charged)
  3. Lamina Rara Externa: Contains heparin sulfate (negatively charged)
• Function
  • Repels negatively charged plasma proteins (e.g., albumin)
  • Permits passage of positively charged and neutral particles

Bowman's Capsule

• Two Layers
  • Parietal Layer
  • Visceral Layer: Composed of podocytes
    • Podocytes: Foot processes create filtration slits
    • Filtration Slits
      • Size: 25 to 30 nanometers
      • Nephrin: Protein forming slit diaphragm, allowing particles <7-9 nm

Mesangial Cells

• Located near glomerulus
• Functions:
  • Phagocytose trapped macromolecules
  • Contractile function: Regulate blood flow
  • Communication via gap junctions with juxtaglomerular cells

Filtration Process

• Net Filtration Pressure (NFP)
  • Dependent on pressures pushing out and pulling in
  • Key Pressures
    • Glomerular Hydrostatic Pressure: ~55 mmHg
    • Colloid Osmotic Pressure: Exerts about 30 mmHg
    • Capsular Hydrostatic Pressure: ~15 mmHg

Glomerular Filtration Rate (GFR)

• Amount of plasma filtered per minute: ~125 mL/min
• Influenced by:
  • Net Filtration Pressure
  • Surface Area of glomerulus
  • Permeability of filtration barrier

Clinical Correlations

• Blood Pressure: Directly affects glomerular hydrostatic pressure
• Protein Levels: Influence colloid osmotic pressure
  • High protein: Increases colloid osmotic pressure
  • Low protein: Decreases colloid osmotic pressure
• Obstructions (e.g., kidney stones): Affect capsular hydrostatic pressure

Diseases Affecting Glomerular Filtration

• Diabetic Nephropathy: Changes glomerular surface area
• Glomerulonephritis: Affects permeability of filtration barrier

Summary

• Key Concepts: Understand components of renal corpuscle, pressures affecting filtration, factors influencing GFR.
• Glomerular filtration is crucial in regulating plasma filtration into the Bowman's capsule, heavily influenced by pressures and structural integrity.