Observing Human Blood Cells in Flow Through Microchannels

For the rheologist, blood is essentially a concentrated suspension of biconcave 8-μm diameter red cells (40–45% by volume) that circulates within the body in vessels from 25 mm down to 5 μm diameter. Here, we describe in vitro tracking of blood cells in a traveling microtube apparatus and in a counter-rotating micro cone-plate device at low Reynolds numbers, Re. Observations of the flow behavior of individual red cells reveal a marked and continuously changing deformation and interaction of the cells in shear, and this, together with their migration away from the vessel wall accounts for the low whole blood overall viscosity compared to other concentrated suspensions and emulsions. Red cells also strongly affect the flow behavior and interactions of platelets and of white cells, which although present at much lower concentrations (0.3% by volume), play key roles in thrombosis, hemostasis, and inflammation. Studies of the kinetics of the formation and break-up of receptor-ligand bonds between membranes of platelets and of white cells in shear flow revealed single bond strengths of 50 −200 nN. Such micro particle image velocimetry (μPIV) studies have recently been considerably refined and extended to in vivo vessels such as postcapillary venules. Using submicron fluorescent latex spheres, the existence of an impermeable and hydrodynamically effective surface layer (< 0.5 μm thick) extending out from the vessel endothelium has been confirmed. The lecture is illustrated by movies of blood flow in vitro and in vivo.