Decreased Hydrodynamic Resistance in the Two-Phase Flow of Blood Through Small Vertical Tubes at Low Flow Rates

The aggregation of red blood cells in blood flowing through small tubes at very low shear rates leads to the two-phase flow of an inner core of rouleaux surrounded by a cell-depleted peripheral layer. The formation of this layer is known to be accompanied by a decrease in hydrodynamic resistance to flow. To quantitate this effect, we measured the pressure gradient, flow rate, and the radius of the red blood cell core in suspensions flowing through tubes of 172-μm radius at mean linear flow rates (Ū) from 50 to 0.15 tube diameters · sec. Washed red blood cells were suspended in 1.5% buffered dextran 110 at hematocrits of 34–52%. Using syringe pumps, blood flowed from a stirred reservoir through a vertical 12-cm length of tube in either the upward or downward direction. The pressure drop was measured with transducers. Mean values in distributions in the core radius were obtained by analyzing cine films of flow taken through a microscope with flow in the upward direction, measuring the core radius at five equally spaced axial positions of the tube in each of 100 frames. At 34% and 46% hematocrit, the hydrodynamic resistance increased as Ū decreased from 50 sec, reaching a maximum at Ū∼2 sec. It then decreased to a minimum at Ū