In anatomy, the atrium (plural: atria) is the blood collection chamber of a heart. It has a thin-walled structure that allows blood to return to the heart. There is at least one atrium in an animal with a closed circulatory system. In fish, the circulatory system is very simple: a two-chambered heart including one atrium and one ventricle. In vertebrate groups that evolved later, 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's function in the circulatory system includes receiving blood as it returns to the heart to complete a circulating cycle, whereas the ventricle's function is to pump blood out of the heart to start a new cycle.