Ba-Dum-Bum Goes Your Heart!

The human heart is astonishingly simple for an organ that works so hard. It is a hollow, cone-shaped muscle about the size of a fist, weighing around 10 ounces in men and 9 ounces in women—just 0.5 percent of total body weight. It sits between the lungs behind the sternum, with about two-thirds of its mass tilting to the left. Despite its small size, it is a relentless pump: at an average resting rate of 72 beats per minute, it beats roughly 100,000 times a day, 38 million times a year, and over 2.5 billion times by age 70.

But that simplicity is deceptive. The heart has only three layers—the smooth inner endocardium, the muscular middle myocardium, and the fluid-filled outer pericardium—and four chambers: right atrium, right ventricle, left atrium, and left ventricle. The right side handles oxygen-poor blood returning from the body; the left side receives oxygen-rich blood from the lungs and pumps it back out. "The atria is incoming, the ventricles is outgoing," Josh explains. One-way valves at each chamber's exit prevent backflow, which is why the heartbeat sounds like "lub-dub"—the lub and dub are the valves closing, not the heart contracting.