The inferior vena cava is a large vein, fed by many smaller tributaries, that returns blood from the abdominopelvic organs to the heart. Lumbar veins, draining the posterior abdominal wall, empty into both the inferior vena cava and the ascending lumbar veins of the azygos system. The inferior phrenic vein drains the inferior surface of the diaphragm. The right gonadal vein drains the right-side gonads into the inferior vena cava, while the left gonadal vein drains into the left renal vein, which, along with the right renal veins, drains the kidneys. Similarly, the suprarenal veins drain the adrenal glands — the right drains into the inferior vena cava and the left into the left renal vein. Formed by the union of the internal and external iliac veins, the common iliac vein drains the pelvis, external genitalia, lower abdominal wall, and limbs. Interestingly, the inferior vena cava doesn't receive veins directly from the gastrointestinal tract, spleen, pancreas, and gallbladder. These organs pass their blood into the hepatic portal vein, then into the hepatic veins, and finally reach the inferior vena cava.