Why isn’t our heart tired after beating for so long?
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The muscle the heart is made of is different from regular muscle fibre. Normal muscles store energy inside them which gets used up as you exercise and (slowly) replaced by chemicals carried by the bloodstream. They get tired if you use up that energy faster than the blood can provide it. Heart muscle, on the other hand, is directly fed by the bloodstream for sustenance and oxygen. This means it never tires, but on the other hand, means things go very badly wrong if the blood supply is interrupted even briefly, as happens when you have a heart attack.
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