Final answer:
The lumbar veins, azygos vein, hemiazygos vein, and the inferior vena cava are the main systemic veins responsible for draining the abdominal wall, with most blood flowing into the inferior vena cava which then returns it to the heart.
Step-by-step explanation:
The systemic veins that drain the abdominal wall are primarily the lumbar veins, which draw blood from the lumbar portion of the abdominal wall and the spinal cord. These veins are part of the large network of vessels that transport deoxygenated blood back towards the heart. Specifically, the superior lumbar veins drain into the azygos vein on the right and the hemiazygos vein on the left, which in turn return blood to the superior vena cava, leading to the right atrium of the heart.
Most of the blood from areas inferior to the diaphragm, however, enters the inferior vena cava, the largest vein in the abdominal region, before being transported back to the heart. The remaining lumbar veins that do not join the azygos or hemiazygos system drain directly into the inferior vena cava. This system ensures that blood from the abdominal wall and lower parts of the body is efficiently returned to the heart for reoxygenation.
Furthermore, the adrenal or suprarenal glands are drained by adrenal veins; the right adrenal vein enters the inferior vena cava directly, while the left adrenal vein joins the left renal vein before reaching the inferior vena cava.