An in vitro model was developed for application in studies of the optical and physical characteristics of flowing blood in rigid and flexible tubes (artificial vessels). The results indicate that both transmission and reflection of light are dependent on blood volume changes, orientation as well as deformability of the red blood cells. Light transmission and reflection in human blood showed a parabolic behavior at hematocrit levels. > 38% when plotted against blood flow. At both a low and high flow rate, the light transmission increased when compared to an intermediate flow where the transmission showed a minimum. The optical wavelength used also affected the light transmission and reflection in moving blood. The results from studies of blood in flow-through models are of importance for the understanding of the optical mechanisms behind the signal generation in photometrical measurement techniques.