Heart function

The heart is a pump that works in shifts: first it fills with blood, then it squeezes and pushes the blood out. When it squeezes, it sends that blood down two separate paths: the pulmonary circulation, which runs through the lungs, and the systemic circulation, which reaches the rest of the body. It's a lot like squeezing a rubber bulb full of water, except that here the squeeze repeats for a whole lifetime, with no time off.

For that squeeze to count for anything, the parts of the heart have to tighten in the right order and at the right moment, and that's handled by an electrical signal that sets the rhythm and decides who goes first: the atria, the upper chambers that take blood in, or the ventricles, the lower ones that push it out. That electricity is the conductor giving the cue, not the muscle doing the work. And between one squeeze and the next, the muscle loosens, the chambers fill with blood again, and the heart is ready for the beat that's coming.