First, the two upper chambers of the heart, called atria AY - tree - uh, contract.

首先, 心脏上不得两个腔室(心房)收缩.

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This contraction squeezes blood into the lower chambers of the heart, which are called ventricles ( VEN - trih - kuls ).

这种收缩会将血液压入心脏下部的两个腔室 ( 心室 ).

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