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How can your heart keep working while other muscles tire?

Holding a heavy object still is a sustained muscular effort. A heartbeat is a cycle: contraction is followed by relaxation, allowing the heart to fill again. Cardiac muscle also depends on a continual oxygen and energy supply. Its unusual electrical timing and cellular machinery help explain repeated pumping, without making it an inexhaustible muscle that works independently of its conditions.

3–7 min

Questions you could explore

  • Why is the interval between contractions essential to the heart’s job rather than wasted time?

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