The right atrium is one of the four hollow chambers of the heart; it is located on the upper right hand side. It has relatively thin walls and receives blood returning through the veins. The right atrium receives blood from two large veins: the superior vena cava and the inferior vena cava. These veins return blood that is low in oxygen from various sites in the body. A smaller vein, called the coronary sinus, also drains blood into the right atrium from the wall of the heart.