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Answer: The upper chamber is called an atrium, or auricle, and the lower chamber is called a ventricle. The two atria act as receiving chambers for blood entering the heart; the more muscular ventricles pump the blood out of the heart.

Answer:

The pumping of the heart involves the alternate contraction and relaxation of the chambers of the heart. The upper chambers of the heart are called  atria , and the lower chambers of the heart are called  ventricles .