Michael Matisse/Photodisc/Thinkstock Your heart may be only slightly larger than your fist, but it's one tough muscle. Take an inside look at the heart, and learn interesting facts and tips for keeping it healthy. Up Next What makes your heart beat? Blood Quiz: Do you know what's flowing through your veins? How and why does the heart pump blood to itself? What determines the rhythm of your heart? What causes the sound of a heartbeat?