A mechanism for erythrocyte-mediated elevation of apparent viscosity by leukocytes in vivo without adhesion to the endothelium

In spite of the relatively small number of leukocytes in the circulation, t hey have a significant influence on the perfusion of such organs as skeleta l muscle or kidney. However, the underlying mechanisms are incompletely und erstood. In the current study a combined in vivo and computational approach is presented in which the interaction of individual freely flowing leukocy tes with erythrocytes and its effect on apparent blood viscosity are explor ed. The skeletal muscle microcirculation was perfused with different cell s uspensions with and without leukocytes or erythrocytes. We examined a three -dimensional numerical model of low Reynolds number flow in a capillary wit h a train of erythrocytes (small spheres) in off-axis positions and single larger leukocytes in axisymmetric positions. The results indicate that in o rder to match the slower axial velocity of leukocytes in capillaries, eryth rocytes need to position themselves into an off-axis position in the capill ary. In such off-axis positions at constant mean capillary velocity, erythr ocyte axial velocity matches on average the axial velocity of the leukocyte s, but the apparent viscosity is elevated, in agreement with the whole orga n perfusion observations. Thus, leukocytes influence the whole organ resist ance in skeletal muscle to a significant degree only in the presence of ery throcytes.