The atrium (plural: atria) is one of the two  blood collection chambers of the heart. It was previously called the auricle, but that name has now been in use as being synonymous with the right or left atrial appendage. The atrium is a chamber in which blood enters the heart, as opposed to the ventricle, where it is pushed out of the organ. It has a thin-walled structure that allows blood to return to the heart. There is at least one atrium in animals with a closed circulatory system.