the change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a light-emitting diode (led) and then measuring the amount of light either transmitted or reflected to a photodiode. each cardiac cycle appears as a peak, as seen in the figure. because blood flow to the skin can be modulated by multiple other physiological systems, the ppg can also be used to monitor breathing, hypovolemia, and other circulatory conditions. additionally, the shape of the ppg waveform differs from subject to subject, and varies with the location and manner in which the pulse oximeter is attached.