Capillary pore rheology of erythrocytes

A sensitive rheometer was used to obtain pressure drop-velocity data for erythrocyte suspensions in the capillary pores of Nuclepore filters. Earlier, nonlinear resistance-velocity behaviour was described for 7-μm pores at cell velocities below 0.8 mm · sec −1 . Similar behaviour is now described for cells in narrower pores (diameter of 5 μm). Two-limbed, straight-line, pressure-velocity relations are again found, which indicate a constant maximum resistance below a critical velocity and a region of decreasing resistance beyond. As previously reported, the findings occur in both the cell velocity (0–1 mm · sec −1 ) and the capillary diameter (5–7 μm) ranges for which Lighthill (1969, In “Circulatory and Respiratory Mass Transport,” Churchill, London) and Fitz-Gerald (1969, Proc. Roy. Soc. Ser. B 174, 193–227) predicted elasto-hydrodynamic resistance effects. The finding of similar behaviour in narrower pores, with a higher critical driving pressure drop, reinforces the preliminary conclusion of behaviour resulting from elastic cell distortion. The present paper examines the resistance data using simple models, showing that the resistance values are consistent with lubrication by a very thin (∼0.1 μm) fluid film, which surrounds the cells and increases in width with velocity beyond the critical point. The findings indicate tight-fitting conditions for the cells, and the changes in film width (and hence resistance) are such as might be expected from elastic bending of the cell membrane, with previously reported estimates for its Young's elastic modulus. The data are compared with hydrodynamic theory, and related experiments are discussed.