In anatomy, the<unk> (plural: atria) refers to a chamber or space. As such it may for example be the<unk> of the lateral ventricle in the brain or, popularly, 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<unk> in an animal with a closed circulatory system.
In fish, the circulatory system is very simple: a two-chambered heart including one<unk> and one ventricle.
In other vertebrate groups, the circulatory system is much more complicated such that 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<unk> 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.
Atrium 
Atrioventricular node
Sinoatrial node
Interventricular septum
Coronary sinus
Atrioventricular nodal branch
Atrioventricular canal
Right atrium