Answer:
Blood is prevented from flowing back into the atria by heart valves. Blood leaves the heart in the body's main artery - the aorta - from the left side, and the pulmonary artery, from the right. Humans have a double circulatory system.The atria contracts when they are around 50% empty.This ensures all the blood is emptied from the atria and enters the ventricles.
Explanation:
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