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The Sino-Atrial Node (SAN), often referred to as the heart's natural pacemaker, plays a crucial role in the cardiac cycle by generating electrical impulses that stimulate the contraction of the heart's atria. When the SAN depolarizes, it sends out signals that prompt the atrial muscles to contract. This contraction is responsible for pushing blood from the atria into the ventricles, setting the pace for the rest of the heart's activity.

This function of the SAN is integral to maintaining a regular heart rhythm, as it helps to coordinate the contraction of the atria, thus ensuring effective blood flow throughout the heart. The electrical impulses generated by the SAN establish the heart rate and rhythm, ultimately influencing the entire cardiac cycle.

The other options do not accurately describe the primary function of the SAN. While the ventricles do contract in response to electrical signals, this stimulation is primarily initiated by the Atrioventricular (AV) node, not directly by the SAN. Blood pressure regulation mainly occurs through mechanisms involving blood vessel constriction and heart output, which are controlled by different systems. Similarly, the regulation of respiratory rate is managed by the respiratory centers in the brain and is unrelated to the functions of the SAN.