Atria Structure
The right atrium contains the sinoatrial node which sends an impulse throughout the heart causing the cardiac muscle of the atrium to contract in a coordinated, wave-like manner. Since the atria are thin-walled and much less muscular than the ventricles, they are able to easily expand with the addition of blood. The forces of contraction are much weaker within the atria as a result. The contraction is strong enough, however to force 1/4 of the total blood volume entering into the ventricles. The rest of the blood enters the atria passively.
Read more about this topic: Heart Chamber
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