The heart pumps blood by ____. It does this in ____ stages. First the ____ contract. Then, about a tenth of a second later, the ventricles contract. 1. When the heart is relaxed, both sides are filled with blood from the ____. 2. The atria contract. The veins contract where they join the atria, so blood from the atria is forced into the ____. 3. Then the ventricles contract. The valves between the ventricles and atria close. The blood is forced out of the heart and into the ____.

How the heart pumps blood

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