At every heartbeat the arterial blood pressure raises (systolic period) and the dimension of the tips of the fingers slightly increases, while the higher oxygenation causes the decrease of the optical density of the skin tissue. During the cardiac relaxation period (diastolic period) the pressure decreases, the density increases and the physical dimension of the tips of the fingers decreases. Since these cyclical variations follow the cardiac rhythm, they can be used to measure the frequency of the peripheral pulsations.
Theoretical topics of Pulse Rate
Circuit blocks of Pulse Rate