Fahraeus effect and cell screening during tube flow of human blood. II. Effect of dextran-induced cell aggregation 1

Suspensions of human red blood cells were drawn through glass capillaries (I.D. 15–95 μm). The suspending medium contained high molecular weight dextran to induce extensive red cell aggregation. The effect of changes in flow rate on the Fahraeus effect and red cell screening was studied by determining capillary hematocrit (H_T), discharge hematocrit (H_D), feed hematocrit (H_F), and the volume flow rate through the capillaries. It was found that the flow dependence of cell screening was intensified compared to the results obtained in the absence of dextran. Furthermore the Fahraeus effect showed an inversed flow dependence: in the range of low flow rates ( u ¯ below 200 sec⁻¹) reduction of flow rate resulted in a decrease of the ratio H_T/H_D indicating more pronounced dynamic hematocrit reduction within the capillary. Calculations of the cell-free layer of suspending medium near the tube wall showed values ranging between 10 and 30% of the tube radius (corresponding to a width between 1 and 7 μm) depending on the diameter of the tube. The cell-free wall layer was found to increase both at low flow rates (due to increased cell aggregation) and at high flow rates (due to axial migration of disaggregated cells).