Question 18

College Answer

The question required an accurate statement of Starling’s Equation, including the
filtration and reflection co-efficients, and definitions of terms. Marks were awarded
for numerical values pertaining to hydrostatic and oncotic pressure gradients and net
filtration in a 24 hour period.
A satisfactory answer explained the factors which cause imbalance in Starling’s
relationship including; precapillary vasodilation, increased venous pressures, gravity
/ posture, fall in plasma protein concentration, changes to capillary permeability and
lymphatic obstruction.
Syllabus: E1
Recommended sources: Review of Medical Physiology, Ganong, Chp 23 and other
sections

Discussion

• Fluid movement into tissue across capillary membranes, which is the origin of oedema, 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:
  
   Jv = L_p S [ (P_c - P_i) - σ (Π_c - Π_i) ]; where
   
• P_c - P_i is the capillary-interstitial hydrostatic pressure gradient
  • P_c, 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)
      • Blood pressure, eg. venous drainage
      • Precapillary vasodilatation, eg. in localised inflammation
  • P_i, interstitial hydrostatic pressure is usually:
    • negative (-5-0 mmHg) in most tissues (except for encapsulated organs, where it is slightly positive, +3 to +6 mmHg)
    • Affected by anything that modifies lymphatic drainage, eg.:
      • Tourniquet
      • Immobility, decreased muscle pump activity
      • Inflammation, eg burns (where it becomes extremely negative, eg. -20 to -30 mmHg)
• Π_c - Π_i is the capillary-interstitial oncotic pressure gradient
  • Π_c, capillary oncotic pressure =  25mmHg
    • Affected by the protein content of blood, eg.:
      • hypoalbuminaemia (eg. liver disease)
      • Hypoproteinaemia (eg. malnutrition, nephrotic syndrome)
  • Π_i, interstitial oncotic pressure = 5 mmHg
    • Affected by the protein content of interstitial fluid, which is usually low but which can increase in local inflammation
• L_p 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 (L_p) 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)

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.