7.1a Nephrology - Glomerular Filtration - video 2

Main Topic: Urine Production

Urine production involves four main processes:

1. Filtration
2. Reabsorption
3. Secretion
4. Excretion

Filtration Process

Glomerular Filtration

• Definition: First step in urine production.
• Plasma Volume:
  • 100% enters the afferent arteriole.
  • 20% filtered into the nephron; 80% remains in circulation.
  • More than 99% of filtered plasma is reabsorbed back into circulation.
  • Less than 1% excreted.

Filtration Fraction

• Definition: Amount of plasma filtered from the glomerulus into the nephron (20%).

Filtration Mechanism

Anatomy Involved

• Afferent and Efferent Arterioles: Blood entry and exit routes.
• Bowman’s Capsule: Site of filtration.
• Glomerular Capillaries: Site of initial filtration.
• Mesangial Cells: Have actin; alter blood flow through contraction.
• Granular Cells: Secrete renin hormone.
• Macula Densa Cells: Chemoreceptors monitoring sodium and chloride.

Filtration Membrane

• Components:
  • Endothelial cells (fenestrated)
  • Basement membrane
  • Podocytes (foot-like projections)
• Function: Controls type of substances passing into the nephron; blocks large and negatively charged molecules.

Charge Influence on Filtration

• Negatively Charged Proteoglycans: Present on podocytes and basement membrane.
• Substance Movement:
  • Positively charged molecules pass easily (e.g., Sodium).
  • Large negatively charged proteins like Albumin are repelled.

Filtration Pressure

Understanding Filtration Dynamics

• Pressures Favoring Filtration:
  • Glomerular Hydrostatic Pressure (P_GH): 60 mm Hg
• Pressures Opposing Filtration:
  • Bowman’s Capsule Pressure (P_BC): 16 mm Hg
  • Glomerular Colloid Osmotic Pressure (P_GCO): 34 mm Hg

Net Filtration Pressure (NFP)

• Formula: P_GH - P_BC - P_GCO = NFP (10 mm Hg)

Glomerular Filtration Rate (GFR)

Definition and Importance

• Definition: Volume of fluid filtered into Bowman’s capsule per unit time (125 mL/min or 180 L/day).
• GFR Calculation: GFR = KF x NFP
• KF: Filtration coefficient based on capillary surface area and permeability.
• Significance: Indicator of kidney function; alterations in GFR suggest kidney issues.

Altering GFR

• Influence Factors:
  • Alterations in Glomerular Hydrostatic Pressure affect NFR and GFR.
  • Resistance changes in afferent/efferent arterioles impact hydrostatic pressure.
• Examples:
  • Increased afferent resistance or decreased efferent resistance can lower GFR.
  • Decreased afferent resistance or increased efferent resistance can increase GFR.

Conclusion

• Summary: Detailed exploration of the filtration process in urine production.
• Next Steps: Discussion on reabsorption in the following video.