Orientation and elongation of RBC in searle flow in relation to forward scattering

It is well accepted, that in whole blood as well as in blood suspensions light transmission increases, when shear stress is applied. Up to now it is not clear to what extent the changes in forward scattering are related to the orientation of the RBC in now or to their elongation. If the latter would be true, forward scattering could be used as a simple parameter for RBC deformability. For our present investigation we used the method of Laser diffraction in combination with image analysis to determine RBC elongation. Simultaneously forward scattering was measured by a photo detector, placed in the center of the non-diffracted laser beam When slowly increasing the shear stress from 0-500 dyn/cm(2) the light intensity measured by the photo detector first increased steeply, reaching a maximum of transmission at about 25 dyn/cm(2), followed by a monoexponential (elongation related) decay, reaching a "steady state" at shear stresses producing maximum elongation (450-500 dyn/cm(2)). But, the decrease of transmission was only present, if the hematocrit (HCT) of the sample was > 0.5%. At a HCT <=0.5%, only an exponential increase of transmission was detected, reaching a "steady state" at about 25 dyn/cm(2). In the apparatus used, the orientation of RBC is complete, if a shear stress of 15 to 25 dyn/cm(2) is applied. Hence, at low shear stresses the increase of transmission is a consequence of RBC orientation. At low HCT, RBC-elongation (due to shear stresses between 25- 500 dyn/cm(2)) does not influence forward scattering. The elongation related decrease of light transmission observed at high HCT (1%- 6%) may be explained by an increase of the area of deformed RBC, promoting the formation of additional cell layers. As a consequence multiple scattering will reduce transmission. Alternatively, during RBC-elongation the cross section of interaction, comprising absorption and scattering, may be altered.