Fluid drop-like behaviour of erythrocytes—Disturbance in pathology and its quantification

Abstract

A number of well established peculiarities of erythrocytes in flow (e.g. cell deformation, cell aggregation, diapedesis of cells) have recently been related to the equally well known “anomalous viscosity” of whole blood. By applying microrheological techniques, the relevance of the above mentioned phenomena to blood flow has been studied. It has been shown that the erythrocyte can act either as the basic unit of a three dimensional structure of rouleaux which greatly inhibit flow, or like a fluid drop thereby fully participating in the flow.

Cell fluidity depends both on having a flexible membrane and liquid in the cell interior. Membrane flexion is greatly helped by the surplus surface area the erythrocyte possesses compared to that of an isovolaemic sphere. The biconcave resting shape is also a consequence of this surplus: it should be noted, however, that under most flow conditions the red cell is being continuously deformed into a variety of shapes. In bulk blood flow, the membrane is continuously rotating around the cell content, the shear stresses are transmitted into the cell interior and the cell behaves much like a fluid drop.

Cell fluidity can disappear either because the internal content becomes abnormally viscous or when the membrane becomes much stiffer. This is signalled by deviations from the normal biconcave shape such as occur in structural membrane defects or in unstable hemoglobins. Cell fluidity can also functionally disappear when the erythrocytes are immobilized into rouleaux or agglutinates of as few as two cells. Under all these conditions, the cells offer a much higher resistance to flow, especially in the restricted channels of the microcirculation. The cells then behave as solid particles and they can only pass channels larger than their resting diameter; any smaller pore is likely to trap the cell, leading to prolonged stagnation and eventual lysis.