Atrium (heart)

Atrium (heart)

In anatomy, the atrium (plural: atria), sometimes called auricle (although in the technical sense, the auricle is a separate part of the main atria known as the atrial appendages), refers to a chamber or space. For example, the term is used for a portion of the lateral ventricle in the brain and the blood collection chamber of the heart. The atrium most commonly refers to 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. In fish, the circulatory system is very simple: a two-chambered heart including one atrium and one ventricle. In other vertebrate groups, the circulatory system is much more complicated. Their circulatory systems are divided into two types: a three-chambered heart, with two atria and one ventricle, or a four-chambered heart, with two atria and two ventricles. 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.