The endothelial glycocalyx: Barrier functions versus red cell hemodynamics: A model of steady state ultrafiltration through a bi-layer formed by a porous outer layer and more selective membrane-associated inner layer.

Background: Ultrastructural investigations of the endothelial glycocalyx reveal a layer adjacent to the cell surface with a structure consistent with the primary  ultrafilter of vascular walls. Theory predicts this layer can be no greater than 200-300 nm thick, a result  to be reconciled with observations that red cells and large macromolecules are excluded  from a region 1 micrometer or more from the cell membrane.

Objective: To determine whether this apparent inconsistency might be accounted for by a model of steady state water and protein transport through a glycocalyx bi-layer formed by a porous outer layer in series with a more selective inner layer.

Methods: Expressions for coupled water and albumin fluxes through the two layers were used to describe steady state ultra-filtration though the bi-layer model.

Results: Albumin accumulates at the interface between the porous layer and the selective inner layer. The osmotic pressure of accumulated albumin significantly modifies the observed permeability properties of the microvessel wall by an effective unstirred layer effect.

Conclusions: The model places significant constraints on the outer layer permeability properties . The only outer layer properties that are consistent with measured steady state filtration rates and models of red cell flux through microvessels are an albumin permeability coefficient and hydraulic conductivity more than an order of magnitude larger than the those of the inner layer.