Hematocrit fluctuations within capillary tubes and estimation of Fåhraeus effect

Experimental and theoretical approaches were used to study hematocrit fluctuations in blood flowing along a uniform microvessel. In the experimental studies, human blood cell suspensions were passed along glass tubes with inside diameters 9.8 micron to 16.8 micron. A characteristic pattern of hematocrit fluctuation was observed in the neighborhood of white blood cells, the cell being preceded by a 'plasma gap' with reduced hematocrit and followed by a 'train' of increased hematocrit. The passage times of trains and plasma gaps and the hematocrits within the plasma gaps were determined by microphotometry. From these data, train hematocrits were deduced, expressed as equivalent discharge hematocrits. They ranged from the feed hematocrit to a value of more than 0.8 and were found to vary inversely with white cell velocity at a given flow rate. A theoretical model was developed which relates train formation to the Fåhraeus effect. The Fåhraeus effect is the reduction of tube hematocrit (HT) below discharge hematocrit (HD) which occurs in capillary tubes because the mean velocity of the red blood cells (VRBC) is higher than the mean bulk flow velocity (VB). The ratio of these velocities decreased with increasing hematocrit, and it is shown that train hematocrit is sensitive to this hematocrit-dependence. Increased hematocrit in trains behind slowly moving white cells is associated with reduced red cell velocity in the trains. From the dependence of train hematocrit on white cell velocity, the variation of Fåhraeus effect with hematocrit was deduced. The results were shown to be consistent with a model for the Fåhraeus effect in which VRBC/VB varies linearly with discharge hematocrit HD. In addition, the Fåhraeus effect was found to be approximately independent of vessel diameter over the range examined.