An atrium, formerly called an atrium, is an upper chamber of the heart, bounded by a thin wall, that receives blood from the bloodstream. There are two of them in the human heart, allowing the passage of blood from the veins (vena cava and pulmonary) to the ventricles (right and left). The atrium is a blood-filled cavity whose wall is composed of cardiac muscle, it communicates with the ventricle through the atrioventricular orifice which allows the passage of blood. The atria are smaller than the ventricles and their wall is much thinner. We distinguish the right atrium from the left atrium. The right atrium has an irregular ovoid shape with an approximately vertical long axis, it has 6 walls. The left atrium has an irregularly rounded shape, it arrives there the 4 pulmonary veins and is composed of 6 walls also. The left atrium receives oxygenated blood from the lungs. The right atrium receives deoxygenated blood from other parts of the body.