Which vein is the largest in the human body and returns blood from below the diaphragm?

The largest vein in the human body is the inferior vena cava, which is responsible for returning deoxygenated blood from the body regions below the diaphragm back to the heart. It runs parallel to the aorta and collects blood from several large veins that drain the lower limbs, pelvis, and abdominal organs.

The inferior vena cava's significance lies in its function of systemic circulation; it is critical for ensuring that blood is efficiently returned to the right atrium of the heart, where it will then be sent to the lungs for oxygenation. This process is vital for maintaining effective blood flow and supporting the body’s metabolic needs.

Other veins mentioned serve different roles in the circulatory system. The superior vena cava returns blood from the upper body, while the femoral and radial veins drain blood from specific limbs (the leg and the forearm, respectively). Understanding the role of the inferior vena cava in systemic circulation is key for recognizing the overall function of the venous system.