What is the main function of the thoracic duct?

The primary function of the thoracic duct is to drain lymph fluid from the body and transport it into the left subclavian vein. The thoracic duct is the largest lymphatic vessel in the body and plays a crucial role in the immune system and maintaining fluid balance. It collects lymph from various regions, including the lower extremities, abdomen, left side of the chest, and left arm. Once the lymph reaches the thoracic duct, it is ultimately delivered to the venous circulation via the left subclavian vein, which allows for proper return to the bloodstream and helps maintain homeostasis.

The other options focus on functions that are not associated with the thoracic duct. For instance, transporting oxygenated blood pertains to the function of the cardiovascular system, specifically the arteries, while filtering red blood cells is a function of the spleen rather than a lymphatic vessel. Similarly, the production of white blood cells occurs primarily in the bone marrow and lymph nodes, not in the thoracic duct itself. Thus, the correct answer emphasizes the thoracic duct's vital role in lymphatic drainage and its connection to the circulatory system.