The Fahraeus effect in narrow capillaries (i.d. 3.3 to 11.0 μm)

Abstract

Measurements of the Fahraeus effect were performed in glass capillaries (i.d. 3.3 to 11.0 μm) during perfusion with suspensions of human red blood cells in Ringer's solution at reservoir hematocrits ( H F) varying between 0.1 and 0.6. The flow velocity of the red cells ( v c) and that of the suspending fluid ( v p) were determined by dual slit photometry. The tube hematocrit ( H T) was obtained from microphotographs and/or by analysis of photoelectric signal recordings; the discharge hematocrit ( H D) was calculated from these measurements. The ratio of cellular and suspending medium flow velocity ( v c v p ) was seen to vary between 1.65 in the 11.0-μm capillary and 1.09 in the 3.3-μm capillary at H F = 0.35. The Fahraeus effect was observed to decrease with capillary diameter: the ratio H T H D increased from 0.67 in the 11.0-μm to 0.94 in the 3.3-μm capillary at H F = 0.35. Furthermore the Fahraeus effect increased with decreasing hematocrit in the two larger capillaries used (i.d. 9.5 and 11.0 μm) whereas it was independent of hematocrit in the smaller capillary tubes. Likewise, calculations of the thickness of the cell-free marginal zone demonstrated an increase with decreasing hematocrit in the larger tubes, but no hematocrit dependence in capillaries < 6.3 μm. Significant effects of variation in flow rate on the Fahraeus effect were not detected although shear-dependent shape changes of the flowing red blood cells were observed.