How does the heart work?

The heart is a fist-sized muscle that works as a double pump with four chambers: two atria on top and two ventricles below. The right side receives oxygen-poor blood from the body and sends it to the lungs to pick up oxygen; the left side receives the oxygen-rich blood from the lungs and pumps it out to the rest of the body. Valves between the chambers keep blood flowing in one direction, producing the familiar “lub-dub” sound. An electrical signal from the heart’s natural pacemaker triggers each beat. Beating roughly 60–100 times a minute — about 100,000 times a day — the heart pumps around 7,500 liters of blood daily.