Mechanical haemolysis in a constant shear field.

Exposure of blood to a foreign environment will normally result in alterations to the blood constituents. The variables governing the extent and type of this damage are still somewhat ill defined, despite a large investigative effort over the last twenty years. Investigation of the interaction of the governing variables has been largely neglected. This work has sought to study the interaction of three parameters, viz the shear rate to which the blood is exposed, the time of exposure, and some characteristics of the surface to which the blood is exposed. Bovine blood was sheared in a constant shear field, produced by a modified cone and plate viscometer. Blood damage, as measured by haemolysis, increased both with increasing shear rate and with time. No critical shear rate was found at any time interval, but rate of haemolysis decreased with time, especially at higher shear rates. This decrease was thought to be due to deposition of plasma proteins onto the viscometer plate surface, forming a protective coating. The presence of this coating was verified by using fluorescein conjugated antibodies. The coating consisted chiefly of fibrinogen and albumen. Some immunoglobulins and complement were also present. The fibrinogen and albumen coats are not homogenous but areas of sparseness are visible. The effect of surface roughness upon blood damage was also investigated using very well defined surfaces as the viscometer plates. The most damaging surface had a peak to peak distance of 2.5 microns, the damaging effect decreasing at shorter and longer distances. In this experimental system, the haemolysis observed was entirely due to red cell inter-action with the plate surface. Red cell/foreign surface interaction was studied with light, scanning electron and transmission electron microscopy. Red cells can tether to adjacent foreign surfaces during flow, and this may be a mechanism of haemolysis. It was also found that blood can become oxygenated when being sheared in a cone and plate viscometer, and that this was caused by gas exchange at the cone periphery, probably facilitated by secondary radial flows under the cone