Described is an apparatus and method of optoacoustic monitoring of blood concentrations of one or more constituents by directing a flow of a patients blood through a substantially transparent vessel to optoacoustically detect a concentration of one or more constituents. To detect constituents, pulses of laser light can be passed through the blood flow at one or more frequencies in order to generate an altered laser emission from the exposed blood, and/or induce detectable optoacoustic responses from the constituents. The detectable responses can be detected and measured by analyzing an alteration of the laser emissions and/or the frequency, slope and/or amplitude of the optoacoustic responses for different constituents in the blood.
