Abstract: A method and device for the continuous real-time monitoring of relative blood volume change, based on registration of blood hemoglobin concentration, during long periods of time, such as dialysis session. A method and device for cardiac output measurement, based on optical dilution technique, during dialysis, surgeries, intensive care procedure. The effects of blood electrolyte composition change which result in a change of light beam geometry are eliminated by the relative orientation between a light source and a single light or photodetector. An illumination axis and a detection axis are oriented in an offset, non collinear configuration at a sufficient angle to substantially eliminate the scattering properties of the blood. A primary implementation of the device is extracorporeal paths, such as hemodialysis tubing systems, artery-vein extracorporeal artificial shunts, other extracorporeal systems. It also may be applied to vessels, tissues or to body parts being capable of transillumination.