Atrium generally stands for a chamber in the heart that is used for collecting the blood. It is a thin walled structure which allows the flow of blood back to heart.

In a human heart, there are four chambers which include the right atrium, left atrium, right and left ventricles. The right atrium receives de-oxygenated or impure blood to the heart from vena cava. The left atrium receives the pure and oxygenated blood which is sent from the lungs to the heart. The blood from the left atrium is transferred to the left ventricle and then is pumped to the blood.

Atrium does not have any valves. A rough muscular structure acts as the boundary inside the atrium and smooth walled part which is derived from the Sinus venous. There are two atria each on other sides of the heart.

When it comes to other vertebrate group other than humans, the circulatory system is much more complicated and they are divided into a three chambered heart, with two aortas and one ventricle or four chambered heart with two atria and two ventricles. The atria take the responsibility of allowing blood inside the heart again while the valves stop this exercise.