Describe the forces that result in fluid exchange across capillary membranes.

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College Answer

The expected answer included a clear explanation of Starling’s forces, including an understanding of the importance of the relative difference along the length of the capillary, with approximate values and examples of factors that influence them. Some explanation of what contributed to the hydrostatic or osmotic pressure gained more marks than merely stating there was a pressure. Several candidates digressed to Fick’s law of diffusion or intracellular flow of ions which was not directly relevant to capillary flow.

Discussion

To be clear, these forces don't belong to Ernest Starling, and he never referred to them as "my forces". When you say "Starling forces", you are usually referring to the equation which decribes how the relationship between hydrstatic pressure and oncotic pressure affects the net magnitude and direction of fluid movement in the microciculation.

Thus:

  • Fluid exchange across capillary membranes depends on a balance between hydrostatic and oncotic pressure gradients in the capillary lumen and the interstitial fluid.
  • This balance can be expressed as the Starling equation:

     J
    v = Lp S [ (Pc - Pi) - σ (Πc - Πi) ]; where
     
  • Pc - Pi is the capillary-interstitial hydrostatic pressure gradient
    • Pc, capillary hydrostatic pressure is usually:
      • 32 mmHg at the arteriolar end of the cpaillary
      • 15 mm Hg at the venular end
      • Affected by gravity (eg. posture) and blood pressure
    • Pi, interstitial hydrostatic pressure is usually:
      • negative (-5-0 mmHg) in most tissues (except for encapsulated organs)
      • Affected by anything that modifies lymphatic drainage, eg. tourniquet or immobility
  • Πc - Πis the capillary-interstitial oncotic pressure gradient
    • Πc, capillary oncotic pressure =  25mmHg
    • Πi, interstitial oncotic pressure = 5 mmHg
  • Lp S is the permeability coefficient of the capillary surface, and is affected by shear stress and endothelial dysfunction.
    • It is a product of the hydrolic permability coefficient (Lp) and surface area of the capillaries (S)
  • σ is the reflection coefficient for protein permeability and is a dimensionless number which is specific for each membrane and protein
    • σ = 0 means the membrane is maximally permeable
    • σ = 1 means the membrane is totally impermeable
    • In the muscles, σ for total body protein is high (0.9)
    • In the intestine and lung, σ is low (0.5-0.7)

References

Starling, Ernest Henry. "On the absorption of fluids from the connective tissue spaces." Classic Papers in Critical Care 19 (1896): 303.

Woodcock, T. E., and Thomas M. Woodcock. "Revised Starling equation and the glycocalyx model of transvascular fluid exchange: an improved paradigm for prescribing intravenous fluid therapy." British journal of anaesthesia 108.3 (2012): 384-394.

Erstad, Brian L. "The Revised Starling Equation: The Debate of Albumin Versus Crystalloids Continues." Annals of Pharmacotherapy (2020): 1060028020907084.

Krogh, August, E. M. Landis, and A. H. Turner. "The movement of fluid through the human capillary wall in relation to venous pressure and to the colloid osmotic pressure of the blood." The Journal of clinical investigation 11.1 (1932): 63-95.