Control of Interstitial Fluid Homeostasis: Roles of Growth Factors and Integrins

Abstract

Fluid and solutes are constantly filtered from the blood stream to the surrounding loose interstitial connective tissue. This flow of fluid has importance for, e.g., cellular metabolism and immunosurveillance. The filtered fluid is moved through the tissues into the lymphatic vessels, which eventually return fluid and solutes back into the blood circulation. The driving force for the filtration results from differences between fluid pressures in the blood vessel and loose connective tissue. The Starling equation, JV = ΔPK, describes fluid filtration (JV) across a capillary wall (for a review, see Ref.1). K is a constant expressing capillary area and permeability. ΔP is the differences in the colloid osmotic pressures in plasma (COPc) and interstitial fluid (COPif), and between capillary hydrostatic pressure (Pc) and interstitial fluid pressure (Pif) according to: ΔPP = (Pc − Pif ) − σ (COPc − COPif), where σ is the plasma protein reflection coefficient. Normally, σ is close to 1, reflecting the low leakage of plasma proteins from normal blood vessels. The higher concentration of diffusible proteins in plasma compared to interstitial fluid together with the properties of the capillary wall causes a higher COPc than COPif, generating a pressure difference that tends to keep fluid within the vessels. Inflammatory processes result in an increased vascular permeability for plasma proteins with a lowering of σ that may cause edema formation since σ (COPc − COPif) is lowered.