A biosensor includes a PPG circuit that emits light directed at skin tissue at a plurality of wavelengths. A first and second spectral response of light reflected from the tissue is obtained around a first wavelength and a second wavelength. Using absorption coefficients for substances at the plurality of wavelengths, concentration levels of a plurality of substances may then be determined from the spectral responses. The biosensor may thus be used to determine concentrations of a plurality of substances in arterial blood flow using the spectral response.
