The answer is heartbeat or heart rate because the contracting and relaxing movement is in it parts which are the 4 chambers,,,,,,right and left atria =receiving chambers,,,,,,,right and left ventricles =pumping chambers of the heart

for more information=these are the sequence of events of the heartbeat
-blood enters the atria,the atria contracts,the valves between each atria and ventricles open,blood goes down to the ventricles,the valves between the atria and ventricles close,the ventricles contracts,valves leading out the ventricles,the blood leaves the heart,the heart muscles relaxes.

hope it helps you ^_^

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