In anatomy, the atrium (plural: atria) refers to a chamber or space. As such it may for example be the atrium 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 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 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.
Humans have a four-chambered heart. The right atrium receives de-oxygenated 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.
Internally, there is the rough musculae pectinati and crista terminalis, which acts 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.
There are two atria, one on either side of the heart. On the right side is the atrium that holds blood that needs oxygen. 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 ventricle where it is sent to the aorta which takes it to the rest of the body.