The heart is a muscular organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to the lungs. In humans, the heart is approximately the size of a closed fist and is located between the lungs, in the middle compartment of the chest. In humans, other mammals, and birds, the heart is divided into four chambers: upper left and right atria and lower left and right ventricles.

The atrium (Latin 'ātrium', “entry hall”) is the upper chamber through which blood enters the ventricles of the heart. There are two atria in the human heart – the left atrium receives blood from the pulmonary (lung) circulation, and the right atrium receives blood from the venae cavae (venous circulation). The atria receive blood while relaxed (diastole), then contract (systole) to move blood to the ventricles. All animals with a closed circulatory system have at least one atrium. Humans have two atria.

The atrium was formerly called the "auricle"; that term is still used to describe this chamber in some other animals, such as the Mollusca.

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