Right Atrium

Internally, the heart is divided into four hollow chambers, two on the left and two on the right. The upper chambers, called "atria," have relatively thin walls and receive 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.