Answers

2016-07-10T08:29:33+08:00
Because, the heart is filled with blood which is squeezed out to circulate through the whole body.

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2016-07-10T08:29:52+08:00
Our heart is just a single organ, but then, it acts as a double pump. The first pump carries oxygen-poor blood to your lungs, where it unloads carbon dioxide and picks up oxygen, then delivers oxygen-rich blood back to your heart. The second pump deliver oxygen-rich blood to every part of our body.
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Summary....because heart drives blood along two separate pathways in the circulatory system.