Final answer:
The veins of the abdomen, specifically the ascending lumbar veins, connect to the azygos system which in turn drains into the superior vena cava, while most venous blood from below the diaphragm enters the inferior vena cava, leading to the heart.
Step-by-step explanation:
The area where veins of the abdomen meet the azygos system primarily involves the ascending lumbar veins and the inferior vena cava (IVC). The ascending lumbar veins, which drain the lumbar portion of the abdominal wall and spinal cord, connect to the azygos vein on the right or the hemiazygos vein on the left. These veins contribute to returning blood to the superior vena cava. In contrast, most of the venous blood from areas below the diaphragm, including the adrenal or suprarenal glands, enters the inferior vena cava, which then empties into the right atrium of the heart.
The azygos vein itself is a component of the thoracic region's venous system and plays a significant role in draining blood into the superior vena cava after passing through the diaphragm from the thoracic cavity on the right side of the vertebral column. Together with the hemiazygos vein, it drains a small amount of blood from the thoracic esophageal veins and the left intercostal veins.