Inversion of Fahraeus effect and effect of mainstream flow on capillary hematocrit

The well known Fahraeus effect (1929) states that if whole blood is allowed to flow from a large reservoir into a small circular cylindrical tube, the hematocrit in the tube is smaller than that in the reservoir, and the smaller the tube, the smaller will be the tube hematocrit. This is interpreted as a feature of particulate flow. We find that this relationship is not monotonic in a model experiment in which gelatin particles (circular disks) are suspended in a silicone fluid to simulate blood. When the diameter of the underformed cell is equal to or greater than the tube diameter, the volume fraction of the cells in the tubes increases to a value equal to or greater than that in the reservoir. Thus the Fahraeus effect has a point of inversion. Additional experiments show that the hematocrit in the tube could be greatly influenced by the flow condition outside the entrance of the tube. If the tube is perpendicular to the main direction of flow in the reservoir (as is the case of Barbee and Cokelet's experiment, or in most arteriole-capillary junctions), the velocity gradient and the velocity of flow in the reservoir just outside the entrance to the tube affects the hematocrit in the tube.