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The atria (singular atrium) are the upper chambers in the heart that recieve blood from large veins and deliver it to ventricles. They have thin walls compared to ventricles, as the blood is only moving a short distance down to the ventricles, therefore little muscle is needed compared to ventricles which pump the blood over larger distances. Atria are involved in the first stages of the cardiovascular cycle mediated by the Sinoatrial Node (SAN). A wave of depolarization sweeps over the the atria causing them to contract, which forces blood from the atria into the ventricles.

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