The microrheology of red blood cell suspensions

The general problem of microrheology is to predict the macroscopic flow properties of a material from a detailed description of the behavior of its constituent elements. This approach has been used to study suspensions of human red cells in plasma or Ringer's solution flowing steadily in rigid tubes 8-25 times the red cell diameter by observing individual cell motions under the microscope. The results have been compared with those previously obtained with model particles under similar conditions. In very dilute suspensions single red cells rotated in orbits similar to those of rigid discs at low flow rates, but, in common with model deformable particles, were observed to migrate away from the tube wall. Linear rouleaux of red cells rotated as rodlike particles and were flexible, bending during their rotational orbits in a manner similar to that of filaments of nylon or Dacron. Transparent concentrated suspensions were produced by preparing ghost cells reconstituted in biconcave form in plasma. In these, the motions of some unhemolyzed red cells were followed. The erythrocyte velocity profiles were blunted at concentrations above 20%; the cell paths were erratic because of frequent radial displacements, especially at the tube periphery, with the particles being markedly deformed and oriented parallel to the flow. Finally, the difference in flow pattern in large and small vessels is discussed and some relevant model experiments are described.