When you listen to a recording of a heart, you hear a soft double thump. That’s because the heart pushes out blood and right away pulls other blood in. Put your ear up to someone’s chest and listen very closely. You’ll hear thump-thump…thump-thump…thump-thump. It’s easy to understand what happens as some blood goes out and some blood comes in. But what happens in the … The rest of the story…