The interaction of leukocytes and erythrocytes in capillary and postcapillary vessels

Abstract

In capillaries white blood cells tend to flow with a lower velocity than red blood cells. This is due to the larger volume of the white cells and their spherical shape as compared to that of the red cells with their biconcave disk shape, as well as to the smaller deformation of the white cell during flow in narrow blood vessels. As a result, red blood cells often accumulate upstream of a white cell in a capillary with a single file of cells, whereas downstream of the white cell a red cell depleted region is formed. When the white cell enters a postcapillary vessel with increased diameter, the following red cells will pass the slower white cell and thereby displace it away from the vessel axis toward the wall. Then interaction between the white cell and the endothelium leads to adhesion, and the white cell starts rolling along the endothelium. We have investigated the detailed flow field in these small vessels with a large-scale hydrodynamic model of a capillary and a postcapillary vessel based on geometric and kinematic similarity. The capillary is simulated by a straight or divergent rigid axisymmetric tube, and white cells are simulated by spheres, red cells by flexible disks, and plasma by a Newtonian fluid. The experiments show that, under the proper circumstances, hydrodynamic collision of a disk and a sphere in a narrow tube results in the disks passing the sphere, leading to the displacement of the sphere toward the tube wall. The model reproduces qualitatively the in vivo flow field, and the results indicate that the attachment of white blood cells in postcapillaries is augmented as a result of hydrodynamic interaction with red cells.