An analysis of the interaction between interstitial plasma protein, interstitial flow, and fenestral filtration and its application to synovium.

An analysis of the interstitial flow of a protein solution is presented, based on physical interactions with extracellular matrix biopolymers. Previous models were extended by incorporation of three features--(a) interstitial osmotic reflection coefficients and attendant partial molecular sieving, (b) the effect of solute exclusion on apparent intramatrix fluid viscosity, and (c) the effect of discrete capillary porosity on near-pore oncotic and hydraulic gradients. The principles were applied to a distributed, finite difference model of the synovial lining and its fenestrated capillaries. Numerical solutions were used to assess the significance of some of the factors involved and predictions were compared with experimental measurements of the transsynovial flow of albumin solutions in the steady state. The following physiologically important points emerged. (i) Steric exclusion of protein reduces the effective interstitial fluid viscosity and creates nonzero interstitial reflection coefficients, which impose a slight nonuniformity of interstitial plasma protein distribution. Together these two effects produce nonintuitive deviation of interstitial flow from Darcy's law when interstitial protein concentration is increased experimentally. (ii) Flow across fenestrae with high reflection coefficients is buffered by steep local gradients in interstitial oncotic and hydraulic pressures immediately outside the fenestrae. Buffering is underestimated by the mean interstitial Starling forces and by treatment of the capillary as if it were uniformly permeable to water. (iii) A bidirectional flow pattern can develop across the synovial lining and can contribute to fluid turnover within the joint cavity.