The atrium (plural: atria) is a blood collection chamber of the heart. It was previously called the auricle, and that term is still used to describe this chamber in, for example, the Mollusca, but in humans that name is now used for an appendage of the atrium. The atrium is an upper chamber in which blood enters the heart, as opposed to the lower ventricle, where it is pushed out of the organ. It has a thin-walled structure that allows blood to return to the heart. The atrium receives blood as it returns to the heart to complete a circulating cycle, whereas the ventricle pumps blood out of the heart to start a new cycle.