eli5: Why is it that a heart beats in pairs of beats?
In: 3
First some anatomy/physiology: The heart has four chambers and four major valves. Starting with deoxygenated blood returning to the heart, it takes the following path: right atrium, right ventricle via the tricuspid valve, pulmonary artery via the pulmonic valve, lung, left atrium via the pulmonary vein, left ventricle via the mitral valve, and finally the aorta via the aortic valve. Blood at this point is oxygenated and flows to tissues and organs throughout the body. After delivering oxygen it then returns to the right atrium and the cycle repeats itself.
The heart is pump and like any pump, it’s effectiveness is dependent on forward flow of blood and preventing back-flow (or regurgitation in medical speak). This why the heart valves are so important; when certain chambers are contracting, some valves need to be closed to prevent backflow while others need to be open to allow for forward flow.
The “lub-dub” of a heart beating is actually the valves snapping shut. The first sound (or “lub”) is the tricuspid and mitral valves snapping shut to prevent backflow into the right and left atria, respectively. The second sound (or “dub”) is the pulmonic and aortic valves snapping shut to prevent backflow into the right and left ventricles, respectively.
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