为供应充足的氧气来维持你的身体之需，你的心脏是怎样努力工作的呢？或者它跳动得有多快呢？
How hard does your heart have to work-and how fast does it have to pump-to get oxygen-rich blood throughout your body?
为供应充足的氧气来维持你的身体之需，你的心脏是怎样努力工作的呢？或者它跳动得有多快呢？
How hard does your heart have to work-and how fast does it have to pump-to get oxygen-rich blood throughout your body?
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