Final answer:
The thoracic duct is the largest lymphatic vessel in the body, draining the left side of the head, neck, thorax, abdominal cavity, pelvic cavity, both legs, and the left arm into the left subclavian vein.
Step-by-step explanation:
The thoracic duct is the large duct that drains lymph from the lower limbs, left thorax, left upper limb, and the left side of the head and neck. This includes drainage from the abdominal cavity and pelvic cavity, effectively serving the majority of the body. The thoracic duct begins beneath the diaphragm in the cisterna chyli, receiving lymph from the lower abdomen, pelvis, and lower limbs via the left and right lumbar trunks and the intestinal trunk, before draining into the left subclavian vein. Unlike the right lymphatic duct, which drains the upper right portion of the body, the thoracic duct drains a much larger portion.