Human erythrocytes are deformed upon entrance into capillaries of diamter smaller than cell's dimension. This pressure driven deformation transforms the biconcave erythocyte into an asymmetric parachute-like. The extent of deformation depends on the shear stress evoked by the driving pressure. The shear stress that exist in the capillaries under normal physiological conditions are in the range of 50 to 150dyn/cm2. We simulated these in-vivo conditions by studying the filtration of the human red blood cells (RBCs) through a millipore filter possessing uniform pores (diameter of 4.6±0.4µm (mean±S.D.) and a pore length of 12.0±1.0µm). In the first stage of the study we established the dependence of the flow on the driving pressure. We used two ranges of driving hydrostatic pressures. The first one was a low-pressure range of 8-22mm H20, which evokes in the pore physiological shear stresses of 76-210dyn/cm2. The second one was in the range of 45-250mm H20, which produces in a pore larger shear stresses of 430-2400dyn/cm2 . The filtration was measured by a micofiltrometer, constructed by us, which allows to determine the stationary flow of small quantities of cells (50-80 RBCs) through an averaged single pore, relative to the transit-time of the same volume of the physiological solution (PBS), was calculated. The relative increase of the transit time corresponds to a relative increase of cell rigidity (RR). In addition, the apparent viscosity (µap) of the RBC suspension in PBS was calculated from the stationary flow rate of RBC suspension through the pore using Poiseuille equation. The study was performed employing human RBCs from adult or neonate blood. The pressure-flow curves of both types of cells, at low pressure, showed a non-linear dependence. At low pressures the flow was convex towards the pressure axis, but at higher filtration pressures it became essentially linear. Consequently, an inverse dependence of RR and µap on the driving pressure was found in the physiological range of pressures 22-45mm H20. Thus, at 8mm H20 the calculated RR and µap were 0.37±0.12 and 3.3±1.1cP, respectively. However, at a higher pressure of 45-47mm H20, RR and µap values diminished to 0.12±0.01 and 1.9±0.3cP, respectively. This clearly demonstrates the non-Newtonian behavior of RBCs at low pressures, suggesting the apparent decrease of RBCs rigidity and viscosity upon pressure increase. The occurrence of non-Newtonian behavior of RBCs vanishes in RBCs treated by a reducing agent.
In the higher range of pressures of 45-250mm H2O, which brings about the larger non-physiological, shear stresses of 430-2400dyn/cm2 , the RR and µap values were constant, i.e. the cells possessed a flow of a simple Newtonian fluid. The transition from the non-Newtonian to the Newtonian region was observed in the range of 22-45mm H2O, i.e. at shear stresses of 430-450dyn/cm2. The modulation of cell membrane rigidity by sulfydryl reagents (diamide or DTT, 1 and 5mM) affected the RR and to a less extent the µap. The effect was stronger at lower driving pressures than at higher ones. Neonatal RBCs revealed lower RR and µap (by 2 and 1.3 fold, respectively) than adult RBCs, regardless of their larger diameter. The differences in RR and µap between RBCs of adult and neonats were larger at low pressure flows than at high ones.
