The significance of red cell surface area to the permeability of fibrin network.

Thrombus formed under low shear conditions in the intravascular compartment consists of red cells enmeshed within a fibrin network framework. The permeability of the network determines the rate of molecular transport by convection. The effect of red cells on the permeability of the fibrin network was examined in networks developed from red cell suspensions in platelet-rich (PRP) and platelet-poor plasma (PPP). Red cell-rich networks developed in PRP were significantly more permeable than those developed in PPP. Network permeability decreased linearly with increase in hematocrit due to volume exclusion and cell surface hydraulic drag. The hydraulic resistance of the red cells was similar in order of magnitude to that of the fibrin fibers. The hydraulic resistance was calculated to be dependent on the surface area of the red cells, as well as on red cell concentration. Calculation of red cell surface area from permeability (45-90 microns2) was found to be lower than the known surface area (145 microns2). From these studies, it is suggested that red cells entrapped within the network are aggregated. Aggregation is promoted by low shear plasma conditions during fibrin polymerization and by fibrin polymerization intermediaries. The degree of red cell aggregation regulates the hydraulic resistivity of the red cells, and the fibrin fiber structure regulates the hydraulic resistivity of the fibrin network. Both are significant determinants of the network permeability and, therefore, of molecular transport in thrombi.