Theoretical investigation on the effects of red blood cell aggregation on rheological properties of blood flowing through narrow tubes

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The effects of red cell aggregation and sedimentation on the flow properties of blood flowing through narrow tubes are theoretically estimated using some simple models.(1) Effects of aggregation on the apparent viscosity of blood in narrow horizontal tubes : Depending on a degree of aggregation tendency of red cells we proposed two flow models that are termed the rigid-core model and the two-fluid model. The apparent viscosity is calculated as a function of a degree of sedimentation in each model. The apparent viscosity increases rapidly with increasing a degree of sedimentation. This prediction is consistent with experimental results. These models also suggest the presence of a certain condition under which the apparent viscosity of aggregating red cell suspensions becomes lower than that of non-aggregating suspensions.(2) Effects of aggregation on the tube hematocrit of blood in narrow horizontal tubes : The relative tube hematocrits are calculated by using the two models and expressed as a function of pseudo-shear rate using a relationship between the width of cell-free layr and pseudo-shear rate obtained by experiments. It is suggested that the shear dependence of the relative tube hematocrit of human blood can be qualitatively explained by the two-fluid model and that the relative tube hematocrit becomes larger than unity in the range of low shear rates.(3) Time-dependent effects of aggregation on flow properties of blood : The transient flow properties of blood flowing through narrow vertical tubes at low rates are analyzed using the three-layr model and a kinetic equation for core formation, which are proposed in our previous paper. A size of the core of aggregating red cells is predicted as a function of time. Then mean red cell velocity is calculated as a function of time. Theoretical predictions are in good agreement with experimental results except flow behavior at the beginning of the core formation.