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the size of the right atrium is smaller that the left atrium.
well you have two atria the right atrium and left atrium but i think the right atrium has deoxygenated blood and the left atrium has oxygenated blood.
right atrium
Atria is plural for Atrium. You have your (rt)Atrium and (lt)Atrium
The two upper chambers are called the atria. A single atria is an atrium.