Lecture Notes on Glomerular Filtration

Introduction

• Focus on glomerular filtration.
• Key structure: Renal Corpuscle.

Renal Corpuscle

• Comprised of:
  • Glomerulus: Tuft of capillaries.
  • Bowman's Capsule (also known as the glomerular capsule).

Glomerulus

• Structure:
  • A tuft of capillaries fed by afferent arteriole and drained by efferent arteriole.
  • Contains fenestrated capillaries with pores (fenestrae) ~50-100 nm in diameter.
• Function:
  • Allows passage of small proteins, water, electrolytes, nutrients, and waste products.
  • Prevents passage of formed elements (red/white blood cells, platelets).

Glomerular Basement Membrane (GBM)

• Layers:
  1. Lamina Densa: Contains type IV collagen and laminins.
  2. Lamina Rara Interna/Externa: Contains heparan sulfate (negatively charged).
• Function:
  • Repels negatively charged plasma proteins.
  • Permits positively charged particles (e.g., sodium, calcium) and neutral particles to pass.

Bowman's Capsule

• Layers:
  • Visceral Layer: Contains podocytes with foot processes.
  • Parietal Layer: Continuous with visceral layer, encases the glomerulus.
• Podocytes:
  • Create filtration slits (~25-30 nm wide).
  • Nephrin protein forms the slit diaphragm, allowing passage of particles <7-9 nm.

Filtration Process

• Substances Filtered:
  • Electrolytes (Na⁺, K⁺, Cl⁻, Ca²⁺, Mg²⁺), water, glucose, amino acids, lipids, urea, creatinine, vitamins.
• Barriers:
  • Fenestrated endothelium, GBM (negatively charged), slit diaphragm.

Mesangial Cells

• Functions:
  • Phagocytose trapped macromolecules.
  • Contract to regulate blood flow.
  • Communicate with juxtaglomerular (JG) cells (renin release for blood pressure regulation).

Net Filtration Pressure (NFP)

• Components:
  • Glomerular Hydrostatic Pressure: Pushes plasma out of capillaries (~55 mmHg).
  • Colloid Osmotic Pressure: Retains water in blood (~30 mmHg).
  • Capsular Hydrostatic Pressure: Opposes filtration (~15 mmHg).
• Net Result: NFP = ~10 mmHg.

Glomerular Filtration Rate (GFR)

• Definition: Volume of plasma filtered per minute (~125 mL/min).
• Factors Affecting GFR:
  • NFP directly proportional to GFR.
  • Surface area and permeability of the glomerulus (filtration coefficient, Kf).

Clinical Correlations

• Glomerular Hydrostatic Pressure:
  • Increases with high systemic blood pressure.
• Colloid Osmotic Pressure:
  • Increases with high plasma protein levels (e.g., multiple myeloma).
  • Decreases with protein loss (e.g., hypoproteinemia).
• Capsular Hydrostatic Pressure:
  • Increases with kidney obstructions (e.g., stones, hydronephrosis).

Conclusion

• Detailed overview of glomerular filtration mechanisms.
• Importance of pressure dynamics and structural integrity in renal function.