The right atrium serves as the receiving chamber for blood returning to the heart from the systemic circulation. The two major systemic veins are the superior and inferior venae cavae. The superior vena cava drains blood from regions superior to the diaphragm: the head, neck, upper limbs, and the thoracic region. It empties into the superior and posterior portions of the right atrium. The inferior vena cava drains blood from areas inferior to the diaphragm: the lower limbs and abdominopelvic region of the body. It, too, empties into the posterior portion of the atria but inferior to the opening of the superior vena cava.