4.1. The Margin of Safety Against Edema Formation – Edema Safety Factors
While increases in capillary pressure, reductions in plasma oncotic pressure, and/or disruption of endothelial barrier function are all accompanied by an increase in transmicrovascular filtration, the accumulation of fluid is resisted by a number of edema safety factors that work in concert to limit edema formation. This margin of safety against edema formation was first recognized in 1932 by Krogh and coworkers [148] as a means to explain why elevations in venous pressure by 10–15 mmHg failed to cause substantial accumulation of tissue fluid. Only when venous hypertension exceeded these levels did gross edema form, indicating that the margin of safety against edema formation could be overwhelmed. From the Starling equation (Equation (1.4)), one can readily see that increases in interstitial fluid pressure, reductions in tissue colloid osmotic pressure or microvascular surface area for exchange, or increases in lymph flow may all act to limit accumulation of excess fluid, and thus represent important edema safety factors against edema formation