Partitioning Resistance to Plasma Fluid within Endothelial Glycocalyx

Abstract

We tested the hypothesis that the resistance to fluid movement within the endothelial glycocalyx is better described by models with an inner semi‐periodic structure and an outer less porous layer, likely stabilized by hyaluronic acid. Previous detailed hydrodynamic modeling has demonstrated that periodic structures close to the membrane, characteristic of membrane‐bound glycoproteins in a quasi‐periodic array extending 200–300nm from the endothelial membrane surface and having a significant resistance to water flow (Darcy coefficient (Kw) in the range 10−13 to 10−14cm2 s) are sufficient to describe both the selectivity and permeability of the glycocalyx to macromolecules and an effective lubrication layer mechanism for red cells. However, a thicker glycocalyx of similar uniform composition does not provide the same consistent description of both permeability and red cell mechanics (Annals of Biomedical Eng. 40: 828,2011). The problem is resolved if the outer layers of the glycocalyx (thickness L) have less resistance to water flows. For example, the criterion for an effective lubrication layer (L/Kw0.5 >100) is met with a outer glycocalyx thickness up to 1 micron but with a hydraulic resistance more that an order of magnitude less than the inner core. These observations are consistent with the conclusion that the fraction of the circulating plasma volume that partitions within the glycocalyx is much smaller than estimates based on the characteristics of red cell flows, total glycocalyx thickness, and glycocalyx volume (for review and clinical significance see Chapters 2 and 3 of Perioperative Fluid Management (E. Farag and A. Kurz Eds) Springer 2016)