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In anatomy, the atrium is the blood collection chamber of a heart. It has a thin-walled structure that allows blood to return to the heart. There is at least one atrium in an animal with a closed circulatory system. In fishes, their circulatory system is very simple; the two-chambers heart includes one atrium, and a ventricle; but in vertebrate groups that evolved later, their circulatory systems are much more complicated. Their circulation systems were divided into two types: three-chambers heart, with two atrium chambers and a ventricle chamber, and a four-chambers heart, including two atria and two ventricles. The atrium's function in the circulatory system includes receiving blood as it returns to the heart to complete a circulating cycle, whereas the ventricle's function is to pump blood out of the heart to start a new cycle.

In architecture, an atrium may refer to the entry room to a home.

It may also mean a large open space, often several storeys high and having large windows, within an office building, usually located immediately inside the main entrance. Atria are popular with companies because they give their buildings "a feeling of space and light", but have been criticised by fire inspectors as they could allow fire to spread to a buildings upper storeys more quickly.

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