Developing red cell flow orientation shown by changes in blood reflectivity

Abstract

The rate of onset of erythrocyte flow orientation at normal concentration has not previously been established. Reflectivity of blood and resuspended non-aggregated red cells at normal (41%) and elevated (60%) hematocrit has been used to examine this process. Video recordings were made before, during, and after shearing by bob motion in a cylinder-in-cylinder viscometer at shear rates ranging from 4 to 100 inverse seconds. Unaggregated erythrocytes in PBS, already more reflective than blood before shearing, became even more reflective during shearing even at the lowest shear rate studied. The time required for the increased reflectivity to stabilize was observed to be inversely proportional to shear rate for both blood and resuspended red cells. Reflection became constant after a total shear strain (bob shear rate x time) from 4 to 10 at all shear rates studied. Onset of increased reflection expressed in total shear strain units (an index of overall bob movement) was independent of shear rate in the absence of aggregation. When red cells were studied in native plasma, reflectivity increased as much as 30% during shearing, but always remained below unaggregated red cell reflectivity for the same hematocrit and shear rate. Greater reflectivity of unaggregated red cells persisted after cessation of shearing, while blood’s reflectivity dropped progressively over several seconds to its pre-shearing value. The geometry chosen for study and insensitivity to light source composition indicate that specular reflection by red cells near the cup’s inner surface is responsible for the increased light return during flow. Maximal rate of rise in reflectivity at all shear rates studied was observed to coincide with blood’s previously reported transient shear stress overshoot and to correspond with an overall bob motion that would rotate suspended particles approximately 45°. The close relation of both reflectivity increase and shear stress overshoot to such modest overall bob movement indicates that an efficient flow-mediated rotation of either individual or aggregated erythrocytes from initially random positions toward an orientation in the shear plane characterizes flow onset.