Glomerular Filtration Lecture Notes

Introduction to Glomerular Filtration

• Focus on the renal corpuscle structure.

Renal Corpuscle Definition

• Components:
  • Glomerulus: A tuft of capillaries (fenestrated capillaries).
  • Bowman's Capsule: Also known as the glomerular capsule.

Glomerulus

• Afferent Arteriole: Vessel feeding into the glomerulus.
• Fenestrated Capillaries:
  • Diameter of pores: 50 to 100 nanometers.
  • Allows passage of:
    • Water, small proteins, electrolytes (e.g., sodium, potassium, calcium, chloride), nutrients, and waste products.
  • Restriction: Formed elements like red blood cells, white blood cells, and platelets cannot pass through.
• Efferent Arteriole: Drains the glomerulus (unusual for capillary beds).

Glomerular Basement Membrane (GBM)

• Structure: Composed of three layers:
  • Lamina Densa: Type IV collagen and laminins.
  • Lamina Rara Interna: Thinner layer with heparan sulfate (negatively charged).
  • Lamina Rara Externa: Similar to interna.
• Function:
  • Repels negatively charged particles (e.g., albumin).
  • Allows positively charged particles and neutral ones to pass through more easily.

Bowman's Capsule

• Components:
  • Visceral Layer: Contains podocytes (foot cells).
  • Parietal Layer: Continuous with the visceral layer.
• Filtration Slits:
  • Spaces between podocytes, approximately 25 to 30 nanometers in diameter.
  • Nefrin: Protein that forms the slit diaphragm, allowing molecules less than 7 to 9 nanometers to pass through.

Summary of Filtration Process

• Filtered Molecules:
  • Electrolytes, nutrients, waste products, glucose, amino acids, lipids, water.
• Not Filtered: Negatively charged plasma proteins (e.g., albumin).

Net Filtration Pressure (NFP)

• Definition: Difference between forces pushing out and pulling in.
• Pressures Involved:
  • Glomerular Hydrostatic Pressure (GHP): About 55 mmHg (pushes fluid into Bowman's capsule).
  • Colloid Osmotic Pressure (COP): About 30 mmHg (pulls fluid back into capillaries due to plasma proteins).
  • Capsular Hydrostatic Pressure (CHP): About 15 mmHg (pushes fluid back into capillaries).
• NFP Calculation: NFP = GHP - (COP + CHP)
  • Example: 55 - (30 + 15) = 10 mmHg.

Factors Affecting Glomerular Filtration Rate (GFR)

• GFR Rate: Average is about 125 mL/min.
• Determinants of GFR:
  • Surface Area: More surface area increases GFR. E.g., diabetic nephropathy decreases GFR.
  • Permeability: More permeability increases GFR. E.g., glomerulonephritis increases permeability.
• Formula:
  • GFR = NFP × Filtration Coefficient (Kf).

Clinical Correlations

• Systemic Blood Pressure: Affects GHP. High BP increases GHP and vice versa.
• Colloid Osmotic Pressure:
  • Increased by high protein levels (e.g., multiple myeloma) leading to increased COP.
  • Decreased by low protein levels (e.g., due to malnutrition or disease).
• Capsular Hydrostatic Pressure:
  • Increased by conditions like kidney stones (renal calculi) or hydronephrosis, which back up fluid into the glomerulus.

Conclusion

• Reviewed components, functions, and the importance of glomerular filtration.
• Emphasis on understanding the dynamics of filtration and its clinical relevance.