Human erythrocyte filterability at low driving pressure

In this study, the human RBC capillary flow has been modeled by passing 11 μl of RBC suspension (Hematocrit = 6%) in phosphate buffer solution (PBS) of a viscosity of 1 and 2.6 cP (in the presence of 2% Dextran) through 5 μm pore diameter polycarbonate Nuclepore filters. We have developed a digitally controlled experimental system for measuring the RBC filterability at a constant driving pressure, in the range of 10-400 Pa, producing a wall shear stress range of 1-50 Pa. The RBC filterability was evaluated by measuring the cell suspension flow rate normalized by the PBS flow rate. The RBC filterability has been found to be a nonlinear function of the driving pressure, having a single minimum locus at 25 Pa. Lowering the driving pressure below 25 Pa revealed an unexpected increase of the RBC filterability. The maximal RBC filterability (near unity) was detected at the lowest driving pressure (10 Pa) and the corresponding estimated RBC linear velocity while traveling through the capillary pore was as high as 800 μm/s. Increasing the driving pressure above 25 Pa confirmed previous results, where RBC filterability is monotonically and asymptotically increasing. Increasing the PBS medium viscosity from 1 to 2.26 cP significantly attenuated the RBC filterability and led to the anomalous increase of RBC deformability at the 10 Pa pressure range. We propose that the anomalous increase in RBC deformability was caused by RBCs undergoing spontaneous mechanical fluctuations.