Each heartbeat begins with a single, electrical impulse, or "spark." The distinctive sound we hear through a stethoscope, or when we place our head on a loved one's chest, is the sound of the heart valves opening and closing in perfect synchronicity with each other. It is a two-party rhythm - a delicate dance of systole and diastole, which propels the heart's electrically charged particles through its chambers roughly every second of the day, every day of our lives.