The thoracic duct is the largest lymphatic vessel in the human body that is responsible for the passage of around 75% of the lymph from the whole of the body except the right upper limb, right breast, right lung and right side of the head and neck. It drains into the venous angle or Pirgoff’s angle between the left subclavian vein and left the internal jugular vein.
The thoracic duct is also known by names like
- Left lymphatic duct
- Alimentary duct
- Chyliferous duct
- Van Hoorne’s canal.
Gross Anatomy of Thoracic Duct
The thoracic duct is a tubular structure that extends from L2 vertebra to the root of the neck.
It length varies between 38-45 cm and it is 2-3 mm in diameter.
The thoracic duct begins in the abdomen by a triangular dilatation, the cisterna chyli, in front of L2 vertebral body.
It lies behind and slightly to the right of the aorta adjoining the right crus of the diaphragm.
Thoracic duct enters the thorax via the aortic opening of the diaphragm between the aorta and the azygos vein.
It lies in the posterior mediastinum in the thorax, anterior to the vertebral column, the right intercostal arteries, and the hemiazygos veins [When these cross to open into the azygos vein] and is posterior to the diaphragm, esophagus, and pericardium.
A recess of the right pleural cavity separates it from the pericardium.
At the level of the fifth thoracic vertebra, it enters the superior mediastinum to ascend behind the aortic arch and the thoracic part of the left subclavian artery, between the left side of the esophagus and the left pleura, to the thoracic inlet.
Thoracic duct forms an arch in the neck, goes about 3-6 cm above the clavicle. Then it crosses anterior to the subclavian and vertebral arteries, veins, and thyrocervical trunk with its branches.
It is anterior to the phrenic nerve and scalenus anterior muscle [medial border] but is separated from these structures by the prevertebral fascia and the carotid tubercle.
The carotid tubercle is the anterior tubercle of the transverse process of the sixth cervical vertebra.
Anteriorly, the structures are the left common carotid artery, vagus nerve, and internal jugular vein.
The duct ends by opening at the junction of the left subclavian vein with the internal jugular vein, also called venous angle or Pirgoff’s angle.
The thoracic duct is narrow in caliber in the middle of the thorax but is broader at the beginning and before its termination. It is flexuous and constricted at intervals in its course.
It might divide into two before reuniting and create a plexus like structure by branches of these divisions.
Tributaries of the thoracic duct include the following
- Left and right trunks combine to form cisterna chyli
- Lumbar trunks are themselves formed by the union of the efferent vessels from the lateral aortic lymph nodes.
- They receive the lymph from
- Lower limbs
- Walls and viscera of the pelvis
- Kidneys and adrenal glands
- Majority of the deep lymphatics of the abdominal wall.
Intestinal lymphatic trunk
- Drains into cisterna chyli
- Receives the lymph from
- The lower and front part of the liver.
Descending trunk from intercoastal lymph
- Drains lower 6 intercostal spaces
- Opens into the beginning of the duct on either side
Trunk draining upper lumbar lymph nodes
- Joins in thorax
- One on each side
- Enters thorax by piercing the crus of the diaphragm
- Left jugular
- Left subclavian trunks
- Efferents from the posterior mediastinal lymph nodes
- From posterior intercostal lymph glands of the upper 6 left spaces
- Sometimes by the left bronchomediastinal trunk
- Usually opens independently into the junction of the left subclavian and internal jugular veins
Thoracic duct valves
Three types of valves are known –
- Venous valves at lymphovenous junction
- At the internal jugular vein- 2 big semilunar valves
- Subclavian vein – unicuspid
- Cusps of these valves close when the refluxing blood fills them
- Ostial valves
- Extend obliquely across the junction
- Prevent blood from entering the thoracic duct
- Valves of the thoracic duct
- Found in rest of the thoracic duct
- Cusps closed by the refluxing lymph
The last 5 mm of the duct does not have any valves.
Microscopic Anatomy of Thoracic Duct
Structurally, the thoracic duct is similar to the vein. It is more muscular and its outer and middle layer adventitia and media are less demarcated.
The muscles and internal elastic lamina that is more prominent in the thoracic portion helps in peristaltic movements.
Cervical part contains less muscle and resembles vein more.
It is the innermost layer and is found on the luminal and valvular surfaces of the duct and cisterna chyli.
It is formed by endothelial cells which are thicker at the perinuclear region.
It is the thickest layer. It is formed by 4-9 layers of smooth muscle cells arranged in three layers.
- Inner longitudinal
- Middle longitudinal and oblique
- Outer circular layers
This layer is thickest in the abdominal part and thinnest in the cervical region.
It is the outermost layer formed by connective tissues including fibroblasts, collagen fibrils, nerves, and vasa vasorum.
Natural Variation of Thoracic Duct Anatomy
- There are two thoracic ducts, one duct on each side of the aorta, each of which opens into the venous system of the corresponding side.
- The thoracic duct does not travel to the left side and instead open in the right venous system.
- The thoracic duct may lie on the left side of the aorta in its entire length to open in the left venous system in 4% of cases.
- The thoracic duct divides at its upper part into 2 branches, right and left: The left ends in the usual manner, whereas the right opens into the right subclavian vein.
- The termination may be at a different site –
- Left internal jugular vein
- Left subclavian vein
- Left innominate
- Multiple nodes of entry at the above locations
Development of Thoracic Duct
It develops during the seventh and eighth week of fetal life from two vessels anterior to the aorta. These become the left and right embryonic thoracic ducts.
The left one gives rise to the upper third of the adult thoracic duct and the lower two-thirds of the adult duct are formed by the right embryonic thoracic duct.
The duct connects with lymph nodes over time.
Functions of Thoracic Duct
The task of the thoracic duct is to transport lymph back into the circulatory system.
Lymphatic fluid is part of the vascular system that functions for the return of the extracellular body fluid and also carries the cells of the immune system.
Lymphatic vessels join to form the lymphatic ducts which drain into the venous system.
The thoracic duct delivers an estimated 1.38 mL/kg/hour of lymph to the venous system.
In addition, products of digestion, like fats, first need to be absorbed into the lymphatic system, and then are carried to the systemic circulation through the venous system.
The thoracic duct drains almost all regions except for the right arm, right breast, right lung, the right side of the head and neck. These are drained by the right lymphatic duct which is about 1.25 cm in length, courses along the medial border of the scalenus anterior muscle at the root of the neck and ends in the right subclavian vein. It also receives lymph from the right side of the heart, and part of the convex surface of the liver.
Clinical Significance of Thoracic Duct
Chylothorax refers to the leakage of lymphatic fluid into the pleural cavity. Chylothorax is caused by traumatic and nontraumatic causes. Trauma could be due to penetrating or blunt injury, or iatrogenic [radiation, surgery].
Malignancies and diseases such as tuberculosis, sarcoidosis, superior vena cava obstruction can cause this condition. It could also be congenital as in Down or Noonan syndrome.
It is diagnosed by a chest radiograph which shows pleural effusion. The study of the effusion fluid would show raised cholesterol and triglyceride measurements.
Thoracic Duct Fistula
This occurs after surgery in the lower part of the left side of the neck. Its presentation is as leakage of milky white fluid from the neck following the incision.