Electrical signals from a natural pacemaker cause the heart muscle to contract. Valves in the heart keep Blood flowing in one direction. Their snapping shut after each contraction causes the sounds heard as the heartbeat.

• 从自然心律调节器产生的电子信号导致心脏肌肉收缩。心脏里的瓣膜使血液往同一方向流动,每次收缩后,它们的闭合产生了听起来像是心跳的声音。