What is Thoracic Duct?

The lymphatic system contains two larger lymph ducts. This thoracic duct is also known as the left lymphatic duct, the alimentary duct, the chyliferous duct, and Van Hoorne’s canal. The other type of lymphatic system is the right lymphatic duct. The thoracic duct course contains liquid chyle, which contains both lymph and emulsified fats rather than pure lymph. It also contains most of the lymph in the body except the right thorax, arm, head, and neck as they are drained by the right lymphatic duct. The level of the thoracic duct usually starts from the twelfth thoracic vertebrae (T12) and it extends to the root of the neck.  


Thoracic Duct Anatomy 

In adults, the thoracic duct length of 38-45cm, and the average diameter of the thoracic duct is about 5 millimetres. The vessel starts from the level of the twelfth thoracic vertebrae (T12) and it extends to the root of the neck. Thoracic duct drains into the systemic circulation at the angle of the left subclavian and internal jugular veins as a single trunk till the initiation of the brachiocephalic vein. The thoracic duct originates from the abdomen, where the right and left lumbar trunks and the intestinal trunk confluences. It forms a significant path called the cisterna chyli. It ascends the superior and posterior mediastinum between the descending thoracic aorta and transverse the diaphragm at the aortic aperture and the azygos vein. The duct gets enlarged vertically to the chest and curves posteriorly to the left carotid artery and left internal jugular vein. 


The thoracic duct drainage varies from 38 to 45 centimetre long and 2 to 5mm in diameter depends on the individual. It runs from the superior aspect of the cisterna chyli, L2 vertebral level found in the lymph sac, to the lower cervical spine. It continues superiorly from the cisterna chyli, running between the aorta and the azygous vein and anterior to the vertebral column. The thoracic duct course rises through the aortic hiatus of the diaphragm entering the posterior mediastinum, continuous to the right of the vertebral column. It is located posterior to the oesophagus at the T7 level and crosses over the midline to the left side of the thorax rough vertebral level T5.  As it moves upward, it runs behind the aorta and to the left of the oesophagus ascending 2-3cm above the clavicle. In the superior mediastinum, it crosses the left common carotid artery, the vagus nerve, and the internal jugular vein. It descends to empty into the junction of the left subclavian and internal jugular veins. 


The thoracic duct wall contains three layers, the intima, the media, and the adventitia. The media is composed of smooth muscle and connective tissue. The smooth muscle moves lymph flow and contracts regularly.  The valves of the thoracic duct may be unicuspid, bicuspid, or tricuspid. But usually bicuspid. At the junction of the lymphatic and venous system, the lymphatic system’s venous backflow prevents a bicuspid valve. 

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The image shows the thoracic and right lymphatic ducts. And the flow of lymph through the body. 


Functions of Thoracic Duct 

The lymph from the right thorax, arm, head, and neck drains to the right lymphatic duct. The thoracic duct collects the lymph from all other parts of the body. The action of breathing, aided by the thoracic duct’s smooth muscle and the internal valves takes responsibility for the lymph transport. The internal valves prevent the lymph from flowing backwards. There are two valves in the junction of the duct with the left subclavian vein, which prevent the flow of venous blood into the duct. The thoracic duct transfers 4 litres of lymph per day in adults.  

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This image shows the flow of lymph from the right lymphatic duct to the parts of the body.

 

Surgical Considerations

The thoracic cavity injury or obstructing the duct results in chylothorax. The long-term effect in the thoracic duct disposed of due to the traumatic injury during cardiac, thoracic, head, and neck surgeries. The frequent physiologic variants also avoid the duct during surgery. As a central line placement, iatrogenic duct injury causes certain noninvasive complications. Chylous extravasation is mainly due to the Occlusion of the thoracic duct. 


To treat the low chyle output, which is less than 1 litre per day, dietary changes like decrease intake of fats and increase intake of medium-chain triglycerides are recommended. One can undergo bowel rest and lipid-free total parenteral nutrition.  If the chyle output is more than 1 litre per day, one can consider thoracic duct ligation or percutaneous embolization. 


Clinical Significance 

Chyle accumulation concerning malignancy, as a central structure to lymphatic flow and movement, thoracic duct dysfunction. The lymph from various organs drains directly into the thoracic duct even without passing a lymph node. This anodal route is observed from the diaphragm, oesophagus, and other parts of the lungs. The drainage pattern plays an important role in finding cancers in these organs. This explains the presence of metastases without lymph node involvement. A lymph node located at the base of the neck is a Virchow node. Here, the duts get terminated and get enlarged in cases of malignancy and disturb the drainage of the thoracic duct.  When the obstacle is found in the thoracic duct, the lymph collected to the pleural cavity. The chylothorax formation remains a sign of malignancy. 

FAQs (Frequently Asked Questions)

Q1. Where Does the Thoracic Duct Drain Into?

Ans. The thoracic duct course level starts from the twelfth thoracic vertebrae (T12) and it extends to the root of the neck. It drains into the systemic circulation at the junction of left subclavian and internal jugular veins, at the outset of the brachiocephalic vein. The Chyle leak is uncommon but occurs due to the serious sequela of head and neck surgery. These are common during the thoracic duct injury, particularly with the resection of malignancy low in the neck.  

Q2. What Happens When There is a Blockage in the Main Thoracic Duct?

Ans. The thoracic duct helps to transport lymph back into the circulatory system. The Interstitial space present in the thoracic duct drainage collects the interstitial fluid with the help of lymph capillaries. Then the lymph moves through lymphatic vessels to lymph nodes. The Lymphatic vessels join to create the lymphatic ducts, which drain into the venous system. The blockage in the thoracic duct drainage causes chronic upper lymphedema.  The bicuspid valves present in lymphatics looks like a venous system. Metaplastic fibrosis causes obstruction of lymph drainage in the upper extremities impedes vertebral venous plexus / Batson’s plexus circulation.