ANSWERS: 1
  • A heart's has four chambers that must beat in an organized manner. This is governed by an electrical impulse. A chamber of the heart contracts when an electrical impulse moves across it. Such a signal starts in a small bundle of highly specialized cells in the right atrium -- the sinoatrial (SI'no-A'tre-al) node (SA node), also called the sinus node. A discharge from this natural "pacemaker" causes the heart to beat. This pacemaker generates electrical impulses at a given rate, but emotional reactions and hormonal factors can affect its rate of discharge. This lets the heart rate respond to varying demands.

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