blood returns to the heart due to skeletal and gaseous exchange muscles as you move and breath.
blood under low pressure flows into the left and right atria from the pulmonary veins adn vena cava.
As the atria fill , the pressure of blood against the atrioventricular valves pushes them open and blood begins to leak into the ventricles.
Then the atria walls contract forcing more blood into the ventricles.
This is known as the atrial systole.
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