Humans have always been fascinated by the primary machine that sustains life: the heart. Sure, we haven't always known what it does or how it works, but it's hard to ignore its ever-present beating. In fact, it's hard to ignore even the sound once you've tuned in. It's fortunate that we do manage to ignore those rhythmic beats, because consciously listening to a few billion lub-dubs over the course of a lifetime would drive us batty.
If you look at your closed fist, you will see fairly accurately exactly how big your heart is. It weighs somewhere between 7 and 15 ounces (198 and 425 grams), functions essentially as a pump, and pushes blood through a network of blood vessels that -- placed end to end -- extend 60,000 miles. Consisting of four chambers and four valves, it receives blood from two different sources -- oxygenated blood from the lungs, and oxygen-depleted blood from the body -- and sends it two different places at once. Oxygen-depleted blood goes to the lungs for a recharge, and oxygen-rich blood from the lungs goes out to the entire body to replenish every cell and tissue. Once this process stops, so do you.
But no matter how many times you've doodled one on a piece of paper or touched your chest when expressing affection for someone, there's probably a lot you don't know about it. So what are five things you don't know about your heart?