thoracic duct
Summary
The thoracic duct is the largest lymphatic vessel in the body, draining lymph from the lower body, left upper body, left thorax, left arm, and left head/neck into the left venous system at the junction of the left subclavian and internal jugular veins (left venous angle). It arises from the cisterna chyli at the level of L1-L2 and ascends through the diaphragm's aortic hiatus, through the posterior mediastinum, to empty into the venous system.
Detail
The thoracic duct begins as a dilated sac called the cisterna chyli, typically located anterior to L1-L2 vertebrae, which collects lymph from the lower limbs, pelvis, abdominal viscera (including intestinal lymphatics carrying chylomicrons, giving lymph a milky appearance called chyle), and lumbar lymphatic trunks. The duct ascends through the aortic hiatus of the diaphragm alongside the aorta, travels through the posterior mediastinum between the aorta and azygos vein, then crosses to the left side around T4-T5 level, ascending posterior to the esophagus and arch of aorta. It ultimately drains into the venous system at the junction of the left subclavian vein and left internal jugular vein (left venous angle), returning lymph to systemic circulation. The thoracic duct drains lymph from three-quarters of the body: both lower limbs, pelvis, abdomen, left thorax, left upper limb, and left side of the head and neck. The right lymphatic duct (much smaller) drains the remaining quarter—right upper limb, right thorax, and right side of head/neck. Clinically, thoracic duct injury (e.g., during esophageal, cardiac, or neck surgery, or trauma) can cause chylothorax—accumulation of chyle in the pleural space, presenting with milky pleural fluid rich in triglycerides. This is diagnosed by pleural fluid triglyceride levels >110 mg/dL. The thoracic duct's proximity to the esophagus makes it vulnerable during esophagectomy. Lymphatic filariasis or malignancy causing lymphatic obstruction can lead to chylous ascites or chylothorax as well. Understanding thoracic duct anatomy is essential for surgical approaches to the mediastinum and diagnosing lymphatic complications post-surgery.
Sources
- Moore's Clinically Oriented Anatomy
- Gray's Anatomy for Students
- First Aid for the USMLE Step 1
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