The thoracic duct is a crucial part of the human body's lymphatic system - in fact, it is the largest lymphatic vessel in the body. The thoracic duct is about 40 cm in length and 5 mm in diameter, and extends from the second lumbar vertebra to the root of the neck. Its job is to collect lymph fluid from throughout the body and carry it upward to the neck, where the fluid drains into the left subclavian vein.
In healthy adults, the thoracic duct transports upto 4 liters of lymph every 24 hours - all of the body's lymph, in fact, except that from the right arm and right side of the upper chest, head, and neck (which is transported by the right lymphatic duct). The lymph is transported upward mostly by the action of breathing, although smooth muscle inside the duct and a series of one-way valves help keep it moving in the right direction.
If for some reason the thoracic duct gets damaged or blocked, a condition known as cyclothorax develops, in which large amounts of lymph collect in the pleural cavity.
The thoracic duct has also been known at various times by several other names, including "alimentary duct", "chyliferous duct", "duct of Pecquet", the "left lymphatic duct", and "Van Hoorne's canal", but "thoracic duct" is the most common variation in modern clinical use.