Atrium (of the heart)

The atrium (plural: atria) is a portion of the blood collection chamber of the heart. 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.

The right atrium receives deoxygenated blood from the superior vena cava, inferior vena cava and coronary sinus. The left atrium receives oxygenated blood from the left and right pulmonary veins. The atria do not have valves at their inlets. As a result, a venous pulsation is normal and can be detected in the jugular vein as the jugular venous pressure. Internally, there is the rough musculae pectinati and crista terminalis, which act as a boundary inside the atrium and the smooth walled part derived from the sinus venosus. There is also a fossa ovalis in the interatrial septum, which is used in the fetal period as a means of bypassing the lung. The atrium is depolarized by calcium. 

There are two atria, one on either side of the heart. On the right side is the atrium that holds blood that is deoxygenated. It sends blood to the right ventricle, which sends it to the lungs for oxygen. After it comes back, it is sent to the left atrium. The blood is pumped from the left atrium and sent to the left ventricle where it is sent to the aorta, which takes it to the rest of the body.