eli5: Why is it that a heart beats in pairs of beats?
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Human hearts have 4 chambers to them: two top ones, called atria, and two bottom ones, called ventricles. Your atria receive blood and pass it along to your ventricles, your ventricles take the blood from the atria and push it hard enough to get all the way to where it’s going and back. At the exit of each chamber there is a valve. Think of them as one way doors; the blood can keep moving the way it’s supposed to, but it can’t go the wrong way (unless the valve is damaged).
The left atrium gets blood from your lungs, freshly filled with oxygen, and passes it along to the left ventricle, which sends it all over your body. The blood then reaches the right atrium, at which point it needs to go to the lungs to leave it’s CO2 and get more oxygen, so your right ventricle sends it there. In order to keep blood exiting and entering the heart at the same rate, both atria squeeze blood to the ventricles at the same time, and both ventricles squeeze blood out of the heart at the same time. Your atria squeeze first, your ventricles squeeze second.
The sound you can hear is the sound made by the valves at each exit closing. You hear a “lub” when the valves between the atria and ventricles close, as the ventricles start to squeeze. You then hear a “dub” when the valves at the exit if the ventricles close, as the ventricles start to relax.
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