Thoracic duct

The thoracic duct is the principal lymphatic vessel responsible for draining lymph from the majority of the body (excluding the right thoracic limb, right side of the head and neck, and right thorax) into the venous system. It originates from the cisterna chyli, a dilated lymphatic sac located ventral to the bodies of the first few lumbar vertebrae, typically between L1 and L4 in dogs, and then ascends cranially through the aortic hiatus of the diaphragm into the thoracic cavity.

Within the thorax, the thoracic duct generally courses dorsally and to the right of the aorta, then crosses to the left side at the level of the fifth to seventh thoracic vertebrae in most species. In dogs, the duct is most often found dorsal and right-sided in the caudal thorax, and it may have multiple branches or accessory ducts, especially in the cranial thorax. The duct runs cranially along the dorsal mediastinum, typically lying between the aorta and the azygos vein, and dorsal to the esophagus.

At the thoracic inlet, the thoracic duct arches laterally and ventrally, terminating most commonly at the left jugulosubclavian angle (the junction of the left subclavian and internal jugular veins), but anatomical variations are frequent (the duct may terminate at the left venous angle, left internal jugular vein, left subclavian vein, or even the right venous angle, and ring formations or multiple terminations are common).

The thoracic duct receives lymph from intercostal, mediastinal, bronchomediastinal, and other regional lymphatic trunks as it ascends.