Britannica Sources used in this answer

Which parts of the body drain into the thoracic duct?

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Diagram of the human lymphatic system highlighting lymphatic vessels and major lymphoid organs, including lymph nodes, adenoids, tonsils, thymus gland, spleen, appendix, bone marrow, and ducts. — lymphatic system The human lymphatic system, showing the lymphatic vessels and lymphoid organs.

The thoracic duct serves as the primary channel for the lymphatic system, responsible for draining lymph from the majority of the body.1

While the right lymphatic duct is responsible for draining the upper right portion of the body, the thoracic duct collects and returns lymph to the bloodstream from all other regions, including:1

The lower body
The left side of the head and neck
The left arm
The left side of the thorax

The duct originates near the small of the back and ascends along the front of the backbone, eventually emptying into the venous system at the junction of the left subclavian and left internal jugular veins.213