How Glomerular Filtration Rate is Determined by Hydrostatic and Oncotic Pressures

Introduction

  • The kidneys filter large volumes of blood plasma through ultrafiltration.

  • Pressure gradients across the glomerular filtration barrier facilitate the movement of small molecules while excluding larger molecules like proteins and blood cells.

  • Purpose of this video: To discuss main determinants of this filtration process and their influence on the glomerular filtration rate (GFR).

Glomerular Filtration Rate (GFR)

  • Definition: The rate at which blood plasma is filtered from the glomerular capillaries into Bowman's space.

  • Main Determinants of GFR:

    • Net Filtration Pressure (P-net):

      • expressed in mmHg.

    • Capillary Filtration Coefficient:

      • measured in mL/min/mmHg.

      • Proportional to the surface area of the filtering membrane (glomerulus).

      • Indicates fluid conductivity.

Key Characteristics of the Glomerulus

  • Highly conductive capillary bed.

  • The filtration coefficient is ~400 times greater than systemic capillaries, indicating specialization for ultrafiltrate formation.

  • Changes can occur in filtration coefficient under normal or pathological conditions, but net filtration pressure is the primary regulatable parameter affecting GFR.

Net Filtration Pressure (NFP)

  • Definition: The pressure driving the movement of fluid from glomerular capillaries into Bowman's space, calculated by the balance of Starling's forces.

Components of Net Filtration Pressure

  1. Hydrostatic Pressure in Glomerular Capillaries:

    • Blood pressure in glomerular capillaries (

  2. Hydrostatic Pressure in Bowman's Space:

    • Back pressure due to tubular fluid in the nephron.

  3. Oncotic Pressure in Glomerular Capillaries:

    • Osmotic pressure from dissolved plasma proteins (e.g., albumin).

  4. Oncotic Pressure in Bowman's Space:

    • Usually negligible, low concentration of dissolved proteins, but can be increased in conditions like glomerular nephritis.

Favoring and Opposing Filtration

  • Pressures Favoring Filtration:

    • Glomerular hydrostatic pressure.

    • Usually negligible oncotic pressure in Bowman's space.

  • Pressures Opposing Filtration:

    • Glomerular oncotic pressure: pulls fluid back into capillaries.

    • Hydrostatic pressure in Bowman's space: pushes fluid back into capillaries.

  • Calculation of NFP:

    • Sum of pressures favoring filtration - Sum of pressures opposing filtration.

Example Calculation of Net Filtration Pressure

  • Forces Favoring Filtration:

    • Glomerular Hydrostatic Pressure: ~60 mmHg.

    • Oncotic Pressure in Bowman's Space: ~0 mmHg (negligible).

  • Forces Opposing Filtration:

    • Hydrostatic Pressure in Bowman's Space: ~18 mmHg.

    • Oncotic Pressure in Glomerulus: ~32 mmHg.

  • NFP Calculation:

    • NFP = (60 mmHg + 0 mmHg) - (18 mmHg + 32 mmHg) = 60 mmHg - 50 mmHg = 10 mmHg.

Significance of Net Filtration Pressure

  • Importance of NFP: Determines driving force for fluid movement from glomerular capillaries to Bowman's space.

  • Although 10 mmHg seems low, considering the high filtration coefficient, it leads to significant GFR.

Calculating Glomerular Filtration Rate (GFR)

  • Formula:

    • GFR = NFP × Capillary Filtration Coefficient.

  • Example Calculation:

    • For a healthy 70 kg animal:

      • NFP = 10 mmHg.

      • Filtration Coefficient = 12.6 mL/min/mmHg.

  • GFR = 10 mmHg × 12.6 mL/min/mmHg = 126 mL/min.

  • Indicates a filtration rate of over 1 liter of blood plasma every 8 minutes.

Regulation of Glomerular Filtration Rate (GFR)

  • Regulatable Parameter: GFR can be increased or decreased through various factors.

  • Responsible for kidneys' responses to body fluid challenges by adjusting NFP.

Main Regulatable Components

  • Glomerular Hydrostatic Pressure:

    • Increasing it favors filtration and raises GFR.

  • Glomerular Oncotic Pressure:

    • Both increases and decreases can impact GFR.

Conclusion

  • Changes in glomerular hydrostatic pressure and oncotic pressure modulate GFR.

  • Future videos will explore how alterations in afferent and efferent arterial resistance impact these pressures.