eli5: Why is it that a heart beats in pairs of beats?
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The human heart has four distinct chambers in it, separated by tissue membranes and valves. Deoxygenated blood returns from the body via the great veins (vena cava) into the right atrium, and is then pumped into the right ventricle when the atrium contracts. When the right ventricle contracts, the blood is forced through the pulmonary artery into the lungs where it can offgas waste carbon dioxide and uptake oxygen. The freshly oxygenated blood returns from the lungs through the pulmonary vein into the left atrium, which is relaxing while the right ventricle is contracting, so the blood has somewhere to go. When the left atrium contracts, blood is forced into the left ventricle, and when the left ventricle contracts, blood is forced out to the body via the great arteries, eventually to return to the right atrium. Both atria contract simultaneously while the ventricles fill, and both ventricles contract simultaneously while the atria fill. The characteristic “lub dub” sound of a heartbeat corresponds to the relatively weak contraction of both atria (the low pressure chambers), followed by the powerful contraction of both ventricles (the high pressure chambers).
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