The human heart beats roughly 3 billion times during the course of an average lifetime. Every single time it beats, blood is drawn into its two upper chambers, held there briefly by a network of valves, and then pumped out forcefully through its two lower chambers. This drawing-and-pumping action ensures that about six liters of freshly oxygenated blood leaves the heart and enters the bloodstream every minute—a volume that can rise to more than 35 liters per minute when someone is exercising. [time-brightcove not-tgx=”true”] But none of that can happen unless the heart’s complicated machinery—its assorted valves and muscles and electrical circuitry—is working in harmony.