An electronic apparatus for sensing the percentage of different blood constituents in arterial blood. Light of a plurality of separate wave lengths is sequentially made to pass through a portion of the body, either by direct transmission or scattering so that the pulsatile blood flow modulates the intensity of the light. A signal processing circuit functioning in accordance with the Lambert-Beer Law is used to determine the percentage of different blood constituents from the fluctuations component of the logarithm of the light absorption. The sampling of the separate wave lengths is time-multiplexed through a common channel, thus obviating the need for a separate channel of similar electronics for each constituent to be monitored. The signal processing circuitry is also effective to compensate for noise due to ambient light or other stray sources, thus improving the overall accuracy.