Why the Thoracic Duct Matters in the Human Body
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 on Thoracic Duct: Structure, Function & Importance
1. What exactly is the thoracic duct?
The thoracic duct is the largest lymphatic vessel in the human body. Its main job is to collect most of the lymph, a clear fluid containing white blood cells, from the body and return it to the bloodstream. You can think of it as the main drainage pipe for the lymphatic system.
2. What is the main function of the thoracic duct in our body?
The primary function of the thoracic duct is to transport lymph and a special fatty fluid called chyle from the digestive system back into the circulatory system. This process is essential for maintaining fluid balance, supporting immune function, and absorbing fats from the food we eat.
3. Where is the thoracic duct located in the body?
The thoracic duct starts in the abdomen from a sac called the cisterna chyli. From there, it extends upwards through the chest (thorax), running alongside the aorta and vertebral column. It ends in the neck, where it drains its contents into the bloodstream at the junction of the left subclavian vein and the left internal jugular vein.
4. How is the thoracic duct different from the right lymphatic duct?
The main difference between the two ducts is the area of the body they are responsible for draining:
- The thoracic duct is much larger and collects lymph from the entire left side of the body, the abdomen, and both legs.
- The right lymphatic duct is significantly smaller. It only drains lymph from the right side of the head, neck, chest, and the right arm.
5. Why is the thoracic duct so important for fat digestion?
When we digest fatty foods, the fats are absorbed in the small intestine and converted into a milky fluid called chyle. Unlike other nutrients that go directly into the blood, this chyle enters the lymphatic system first. The thoracic duct is the primary vessel that collects this fat-rich chyle and delivers it to the bloodstream, allowing the body to use these fats for energy.
6. What could happen if the thoracic duct gets blocked or injured?
A blockage or injury to the thoracic duct can cause serious health issues. If it's blocked, lymph cannot drain properly, which can lead to a type of swelling called lymphedema. If the duct is injured, chyle can leak into the chest cavity, a condition known as chylothorax, which can cause breathing problems and the loss of vital fats and proteins.
7. Does the thoracic duct carry blood like an artery or a vein?
No, the thoracic duct does not carry blood. It is a key part of the lymphatic system, not the circulatory system. It only carries lymph and chyle. Its job is to empty these fluids *into* the bloodstream, but it is a one-way channel and does not transport blood itself.
