Fig. 4. Illustration of loops of arterial pressure (transmural)  versus the photoplethysmogram (PPG). Note that the vessels are more compliant (  i.e. , Δvolume/Δpressure) interbeat than intrabeat. This is explained by dynamic compliance, a property of blood vessels. Also note that physiologic fluctuations in the baseline of the waveform must be removed by some algorithm to reproduce this image.  32 The interbeat pressure–volume loops reveal a portion of the sigmoid curve illustrated in  figure 3 . The data above were collected in our laboratory with institutional human use committee approval and subject consent. The photoplethysmogram was recorded overlying a digital artery. V = photodetector voltage, which is a function of light intensity.