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. Before it is collected and returned to the heart, some gastrointestinal blood, which includes blood flow down the entire length of the digestive tract, is transported to the liver through veins, making up the hepatic portal system. The system joins capillaries of digestive organs to hepatic sinusoids, or a type of irregularly shaped blood vessel that is found in the liver. The liver gains nutrients from digested food as it filters bacteria and intestinal toxins out of blood. The inferior mesenteric vein carries blood from the rectum and distal large intestine and empties into the splenic vein. The superior mesenteric vein receives blood from the small intestine and colon. Together with the splenic vein, it forms the hepatic portal vein. The splenic vein receives blood from the spleen before draining blood from pancreatic veins. The splenic and superior mesenteric veins merge to form the hepatic portal vein, which moves into the liver. The liver also receives blood from the gallbladder and from gastric veins.