How your heart works: two pumps, one spark, and what a heart attack really is

Seventy times a minute, a tiny patch of cells in your heart fires a spark, and no nerve from your brain asks it to. Follow one red blood cell through the two pumps of your heart, its valves and its own small arteries. Then find out what the two numbers of blood pressure measure, how an artery slowly clogs, what really happens in a heart attack, why cardiac arrest is a different thing, and why pressing hard on a chest can keep a person alive.