Glomerulofiltration Lecture Notes

Introduction

• Topic: Glomerulofiltration
• Key Structure: Renal Corpuscle

Renal Corpuscle

• Components:
  • Glomerulus: Tuft of capillaries
  • Bowman's Capsule: Also known as the glomerular capsule

Glomerulus

Structure

• Afferent Arteriole: Feeds the glomerulus
• Efferent Arteriole: Drains the glomerulus (capillary bed fed and drained by arterioles)
• Capillaries Type: Fenestrated capillaries
  • Fenestration Pores: 50-100 nm in diameter
  • Allows Passage of: Water, small proteins, electrolytes, nutrients, waste products
  • Blocks: Formed elements like RBCs, WBCs, platelets

Basement Membrane

• Structure: Glomerular Basement Membrane
  • Layers:
    • Lamina Densa: Type IV collagen and laminins
    • Lamina Rara Interna/Externa: Contain heparan sulfate (negatively charged)
• Function:
  • Repels negatively charged plasma proteins due to negative charge
  • Allows passage of positively charged molecules

Bowman's Capsule

Structure

• Layers:
  • Visceral Layer: Podocytes
    • Filtration Slits: 25-30 nm in diameter
    • Nephrin Protein: Forms slit diaphragm, allowing molecules <7-9 nm
  • Parietal Layer: Continuous with visceral layer

Mesangial Cells

• Functions:
  • Phagocytosis of macromolecules
  • Contractile properties to regulate blood flow
  • Connection with JG cells (juxtaglomerular cells), involved in renin release

Filtration Process

Key Components Filtered

• Freely Filtered Molecules: Glucose, amino acids, lipids, urea, electrolytes, water
• Barriers:
  • Fenestration Pores
  • Negative Charge on Basement Membrane
  • Filtration Slits and Slit Diaphragm

Pressures Involved in Filtration

Net Filtration Pressure

• Equation: Pressures forcing out - Pressures pulling in
• Components:
  • Glomerular Hydrostatic Pressure: 55 mmHg (pushes plasma out)
  • Colloid Osmotic Pressure: 30 mmHg (pulls fluid back in)
  • Capsular Hydrostatic Pressure: 15 mmHg (back pressure from Bowman's capsule)

Glomerular Filtration Rate (GFR)

• Defined: Volume of plasma filtered per minute (avg 125 ml/min)
• Factors Influencing GFR:
  • Surface Area and Permeability of Glomerulus (Filtration Coefficient, KF)
  • Blood Pressure (affects hydrostatic pressure)
  • Protein Levels (affects osmotic pressure)
  • Obstructions (e.g., kidney stones affect hydrostatic pressure)

Clinical Correlations

• Diabetic Nephropathy: Thickens basement membrane, affecting GFR
• Glomerulonephritis: Affects membrane permeability, increases GFR
• Conditions Affecting Blood Pressure and Protein Levels
  • High BP increases glomerular hydrostatic pressure
  • Multiple myeloma increases colloid osmotic pressure
  • Hypoproteinemia decreases colloid osmotic pressure

Conclusion

• Reviewed structure and function of glomerulus and Bowman's capsule
• Detailed the filtration process and pressures involved
• Discussed clinical implications affecting glomerular filtration