In adults, the thoracic duct is typically 38–45 cm in length and has an average diameter of about 5 mm. The vessel usually commences at the level of the twelfth thoracic vertebra (T12) and extends to the root of the
neck before descending to terminate at the
venous angle.
Origin The thoracic duct commences at the upper extremity of the
cisterna chyli at the level of the T12 vertebra. and is situated posterior to the esophagus at the T7 vertebral level. It crosses the midline to the left side at about the T5 level, continuing to ascend. It then passes posterior to the aorta, and to the left of the oesophagus. In the superior mediastinum, the thoracic duct is situated posterior to and to the left of the esophagus. It is situated between the visceral and
alar fascia. It passes posterior to the left
common carotid artery,
vagus nerve (CN X), and
internal jugular vein. At C7 level, it lies posterolaterally to the
carotid sheath. From here, it passes anteroinferiorly to the
thyrocervical trunk, and
phrenic nerve. It descends until reaching and draining at the
venous angle.
Fate The thoracic duct usually drains into the
systemic (blood) circulation at the left
venous angle where left subclavian and left internal jugular veins unite to form the left brachiocephalic vein.
Variation The characteristic anatomy of the thoracic duct is present in only about half of individuals.
Origin A cisterna chyli is absent in about half of individuals; the cisterna chyli fails to develop when the fusion of
lumbar trunk during embryologic development occurs above the vertebral level of T12. In such cases, dilation of the lumbar trunks may be present instead.
Number of ducts A bifid inferior portion of the thoracic duct (due to a failure of fusion during embryonic development) is not uncommonly observed; a plexus of lymphatic vessels replacing the thoracic duct inferiorly and only coalescing into a single duct in the mediastinum may also occur. Rarely, the thoracic duct may be entirely bilaterally paired.
Termination In over 95% of individuals, the thoracic duct ends by draining either at the venous angle, or into the
internal jugular vein, or the
subclavian vein, but - in the minority of cases - empties into either the
brachiocephalic vein,
external jugular vein,
suprascapular vein,
transverse cervical vein, or
vertebral vein. In a vast majority of cases, the thoracic duct terminates on the left side, but may rarely terminate on the right side of the body, or bilaterally. It usually terminates as a single vessel, but it sometimes ends in bilateral vessels or as several terminal branches. Rarely, the thoracic duct terminates "prematurely" by emptying into the azygous system. ==Function==