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.