Final answer:
The thoracic duct is the large lymphatic vessel that collects lymph from the lower half of the body and discharges it into the blood circulation via the left subclavian vein.
Step-by-step explanation:
The large lymphatic vessel that collects lymph from the lower half of the body and dumps it into the blood circulation is called the thoracic duct. Lymph from the lower body flows into the cisterna chyli, which is the starting point of the thoracic duct positioned just beneath the diaphragm. The thoracic duct receives lymph from the left and right lumbar trunks and the intestinal trunk, which collect lymph from the lower abdomen, pelvis, and lower limbs. This major duct then continues upward to drain the lymph into the left subclavian vein, effectively returning it to the blood circulation.
The lymphatic system doesn't operate under pressure from the heart like the circulatory system. Instead, it relies on vessel contractions, skeletal muscle actions, and one-way valves to transport lymph. Unlike the right lymphatic duct, which drains lymph from only the upper right side of the body, the thoracic duct covers all other areas, demonstrating the asymmetric nature of the lymphatic drainage system.