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.

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.

Structure

Humans have a four-chambered heart consisting of the right atrium, left atrium, right ventricle, and left ventricle. The atria, are the two upper chambers. The right atrium receives and holds deoxygenated blood from the superior vena cava, inferior vena cava, anterior cardiac veins and smallest cardiac veins and the coronary sinus, which it then sends down to the right ventricle (through the tricuspid valve) which in turn sends it to the pulmonary artery for pulmonary circulation. The left atrium receives the oxygenated blood from the left and right pulmonary veins, which it pumps to the left ventricle (through the mitral valve) for pumping out through the aorta for systemic circulation.