An in vitro model is 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 and orientation as well as the deformability of the red blood cells. Light transmission and reflection in human blood shows a parabolic behavior at hematocrit levels greater than 40%, when plotted against blood flow. At both low and high flow rates, the light transmission increases when compared to an intermediate flow where the transmission shows a minimum. The optical wavelength used also affects the light transmission and reflection in moving blood. The results of studies of blood in flow-through models are important for the understanding of the optical mechanisms behind the signal generation in photometrical measurement techniques.