less than the hematocrit in the feed reservoir. The ratio of these two hematocrits, the tube relative hematocrit, , is defined as Explanation of phenomena These initially confusing results can be explained by the concept of a plasma cell-free layer, a thin layer adjacent to the capillary wall that is depleted of red blood cells. Because the cell-free layer is red cell-poor, its effective viscosity is lower than that of whole blood. This layer therefore acts to reduce flow resistance within the capillary. This has the net effect that the effective viscosity is less than that for whole blood. Because the cell-free layer is very thin (approximately 3 μm) this effect is insignificant in capillaries whose diameter is large. This explanation, while accurate, is ultimately unsatisfying, since it fails to answer the fundamental question of why a plasma cell-free layer exists. There are actually two factors which promote cell-free layer formation. For particles flowing in a tube, there is a net hydrodynamic force that tends to force the particles towards the center of the capillary. This has been cited as the Segré–Silberberg effect, although the named effect pertains to dilute suspensions, and may not operate in the case of concentrated mixtures. There are also effects associated with deformability of red blood cells that might increase this force. It is clear that red blood cells cannot pass through the capillary wall, which implies that the centers of red blood cells must lie at least one red blood cell half-thickness away from the wall. This means that, on average, there will be more red blood cells near the center of the capillary than very near the wall. Cell-free marginal layer model is a mathematical model which tries to explain Fåhræus–Lindqvist effect mathematically. See also Cell-free marginal layer model Fåhræus effect Blood viscosity hemodynamics References Further reading Blood Robin Fåhræus