Robin Fåhraeus: evolution of his concepts in cardiovascular physiology.

We give an account of the work of Robin Fåhraeus over the years 1917-1938, his contribution to our understanding of blood rheology, and its relevance to circulatory physiology. Fåhraeus published few original papers on this subject, yet he clearly understood the phenomena occurring in the tube flow of mammalian blood. 1) The concentration of cells in a tube less than 0.3 mm in diameter differs from that in the larger feed tube or reservoir, the Fåhraeus effect. This is due to a difference in the mean velocity of cells and plasma in the smaller vessel associated with a nonuniform distribution of the cells. 2) In tubes less than 0.3 mm in diameter, the resistance to blood flow decreases with decreasing tube diameter, the Fåhraeus-Lindqvist effect. We define and generalize the two effects and describe how red cell aggregation at low shear rates affects cell vessel concentration and resistance to flow. The fluid mechanical principles underlying blood cell lateral migration in tube flow and its application to Fåhraeus' work are discussed. Experimental data on the Fåhraeus and Fåhraeus-Lindqvist effects are given for red cells, white cells, and platelets. Finally, the extension of the classical Fåhraeus effect to microcirculatory beds, the Fåhraeus Network effect, is described. One of the explanations for the observed, very low average capillary hematocrits is that the low values are due to a combination of the repeated phase separation of red cells and plasma at capillary bifurcations (network effect) and the single-vessel Fåhraeus effect.