Which vein returns blood to the right atrium from bodily parts below the diaphragm?

The inferior vena cava is the major vein responsible for returning deoxygenated blood from the lower half of the body to the right atrium of the heart. It gathers blood from various structures and organs located below the diaphragm, including the abdomen and lower limbs. This process is essential for maintaining the circulation of blood, allowing the heart to pump it to the lungs for oxygenation.

The superior vena cava, while crucial for returning blood from the upper body to the right atrium, does not serve the lower regions of the body. The femoral vein, being a major vein of the thigh, primarily drains blood from the lower limb but ultimately connects to the inferior vena cava. The radial vein, which is located in the forearm, returns blood from the arm but does not play a role in returning blood from areas below the diaphragm. Thus, the inferior vena cava is the correct answer, as it specifically addresses the return of blood from the lower body to the heart.