Measurement of the erythrocyte orientation in a flow by spin labeling

When a suspension of erythrocytes labeled in their membrane with a fatty acid paramagnetic molecule is allowed to flow in a flat quartz sample cell, the recorded electron paramagnetic spectra change as a function of the orientation of the cell in the magnetic field. This indicates that the red cells are themselves oriented in the flow. Such spectral variations have been reproduced by a numerical simulation procedure, which allowed us to quantify the proportion of oriented red blood cells by measuring the amplitude of some characteristic lines on the experimental spectra. Orientation rates were then measured as a function of various rheological parameters, such as shear rate, hematocrit and viscosity of the suspending medium. The kinetics of the disorientation process was determined by stopping the flow.