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For several years, the formation and evolution of thrombi in small arteries of rats has been quantitatively studied at the laboratory of PHYSIOLOGY and PHYSIOPATHOLOGY at the V.U.B. Global size parameters can be determined by projecting the image of a small arterial segment onto photosensitive cells. The transmitted light intensity is a measure for the thrombotic phenomenon. This unique method permitted extensive in vivo study of the platelet-vessel wall interaction and local thrombosis. We actually attempt to refine the spatial resolution of these measurements in order to get information on texture and form of the thrombotic mass at any stage of its evolution. Therefore a thorough understanding of how light propagates through non hemolyzed blood is essential. Application of results from Twersky's multiple scattering theory, combined with appropriate border conditions and parameter values was attempted. It is well known that the erythrocytes are mostly aligned in the direction of the blood flow. In order to explain the measured intensity profiles, we had to postulate alignment in the plane perpendicular to the flow as well. The theoretical predictions are in good agreement with the experimental values if we assume almost perfect alignment of the erythrocytes such that their short axes are pointing in the direction of the center of the artery. Conclusive evidence of the interaction between local flow properties and light transmission could be found by observing arteries with perturbated flow.