The superior and inferior venae cavae are the major veins that return blood from the thoracic and abdominal regions back to the heart. The superior vena cava receives blood from all areas above the diaphragm (with the heart wall as the exception). The inferior vena cava receives blood from all areas below the diaphragm. The blood from the abdominal and thoracic walls moves through a group of ascending veins called the azygos system, which directs blood to the superior vena cava. From the abdominal wall, two lumbar veins receive blood from smaller veins before they move into the thoracic cavity. In the thorax the right lumbar vein becomes the azygos vein, which receives blood from intercostal, mediastinal, pericardial, esophageal, and right bronchial veins. The superior vena cava then receives blood from the azygos vein before it empties into the heart's right atrium. The left ascending lumbar vein moves into the thorax as the hemiazygos vein. The vessel drains some intercostal, esophageal, and mediastinal veins before crossing right and emptying into the azygos vein.
The inferior vena cava drains blood only from the abdominal cavity, not the thoracic. It forms where the right and left common iliac veins meet in the lumbar region and ascends at the right of the aorta. The largest body vessel (diameter = 3.5 cm), it collects blood from six networks as it ascends. First to drain are the lumbar veins, followed in order by the gonadal veins that drain ovaries and testes and the renal veins from the kidneys. The remaining vessels emptying into the inferior vena cava are suprarenal veins from adrenal glands, hepatic veins from the liver, and inferior phrenic veins from the diaphragm. At this point the inferior vena cava moves through the diaphragm and delivers blood to the right atrium from below.