The thoracic duct is the largest lymphatic vessel within the human body, and plays a key role in the lymphatic system. It is also called the left lymphatic duct or the alimentary duct. A large portion of the body’s lymph is collected by this duct and then drained into the bloodstream near the brachiocephalic vein between the internal jugular and the left subclavian veins.

The typical length of this duct in an adult averages between 38 and 45cm, while the diameter is about 5 to 7 mm. It originates from the second lumbar vertebra level and goes to the neck’s root. The duct arises from the combination of the left and right lumbar trunks and the intestinal trunk in the abdomen. The thoracic duct gets extended in the chest area and from there it curves toward the internal jugular vein and the left carotid artery at the C7 vertebra. It travels through the aortic aperture diaphragm and rises along the posterior mediastinum.

It transports up to four liters of lymphatic fluid each day. This process is primarily caused by the breathing action and is assisted by the smooth muscle of the duct.