What are the two major ducts that return lymph to the circulatory system and where do they collect lymph from and which veins do they dump into respectively?

Which of these is the result of a parasitic infection of the lymphatic system that causes enormous swelling of the legs and scrotum?

• A. Hypertrichosis
• B. Cotard’s Syndrome
• C. Elephantiasis
• D. Wolf-Parkinson-White Syndrome

Drum roll please…..

If you answered C, congratulations! If you answered A, B, or D, keep reading.

The lymphatic system is responsible for absorbing excess interstitial fluid and transporting this fluid, called lymph, to ducts that drain into veins. The lymphatic system is also responsible for producing lymphocytes, which are the white blood cells involved in immunity.

The lymphatic system has three main roles: to transport interstitial fluid originally from blood filtrate back to the blood, to transport absorbed fat from the small intestine to the blood, and to provide immunological defense against pathogens.

As blood circulates throughout the body supplying oxygen to tissues, some fluid leaks from the blood into the surrounding tissues (interstitial fluid is formed by filtration of plasma out of blood capillaries). This leakage helps maintain an efficient movement of nutrients and salts from blood into the tissues. Because more than 3 liters of fluid leak from the circulatory system into tissues every day, some of that fluid must return to the circulatory system, otherwise a person would swell up like a balloon. Fortunately, the lymphatic system exists to remove excess fluid from our tissues.

Once fluid collects in lymphatic capillaries, it is referred to as lymph. Lymphatic capillaries are microscopic close-ended tubes that form immense networks in the intercellular spaces within most organs. These capillaries have porous junctions, therefore allowing interstitial fluid, proteins, extravasated white blood cells, microorganisms, absorbed fat, and fat-soluble vitamins to enter. The lymph is carried into larger lymph vessels called lymph ducts. Lymph ducts are similar to veins in that they contain valves to prevent backflow. The lymph moves via peristaltic waves of contraction throughout the lymph vessels until the lymph empties into either the thoracic duct or the right lymphatic duct. These ducts drain into the left and right subclavian veins, respectively. Thus, interstitial fluid is ultimately returned to the cardiovascular system.

Lymph nodes help remove pathogens from the lymph before it enters the circulatory system. Lymph nodes contain phagocytic cells which act as filters, trapping bacteria and other microorganisms that cause disease. If you have had “swollen glands”, then your lymph nodes were swollen in your neck, helping trap and destroy bacteria and other pathogens. The tonsils, thymus, and spleen—the lymphoid organs—are all sites of lymphocyte production. Certain lymphocytes, called T cells, mature in the thymus before they function in the immune system. T cells respond to antigens, which provoke an immune response from one’s body. Although the lymphatic system transports lymphocytes for immune protection, it may also transport cancer cells through the porous lymphatic capillaries, thereby helping cancer metastasize.

Now, finally to explain our trivia question… Lymphedema is excessive protein and associated fluid in the interstitial tissue, caused by inadequate lymphatic drainage. In tropical equatorial regions in the world, most commonly in Africa, a parasitic infection of the lymphatic system causes elephantiasis. Elephantiasis is a lymphedema that produces massive swelling of the legs and scrotum. The skin develops a rough appearance and usually darkens. Lymph flow also becomes obstructed. This disease is caused by a species of nematode worms, and is transmitted by mosquitoes. Chemotherapy, antibiotics, and lymphatic massage have all proven to be helpful treatments.

Take a look at this video about the lymphatic system!

References:

Fox, Stuart I. “Blood, Heart, and Circulation.” Human Physiology. 10th ed. New York, NY: McGraw-Hill, 2008. 424-25. Print.

Levine, Miller. “Circulatory and Respiratory Systems.” Biology. Boston, Massachusetts: Pearson Prentice Hall, 2008. 954-55. Print.

Lymphatic ducts empty lymph fluid into the venous system. The two lymphatic ducts of the body are the right lymphatic duct and the thoracic duct. The thoracic duct is the larger of the two and responsible for lymph drainage from the entire body except for the right sides of the head and neck, the right side of the thorax, and the right upper extremity which are primarily drained by the right lymphatic duct.[1][2]

Structure and Course

The thoracic duct is 38 to 45 centimeters long and 2 to 5 millimeters in diameter. It runs from the superior aspect of the cisterna chyli, a lymph sac at the L2 vertebral level, to the lower cervical spine. From the cisterna chyli, the duct continues superiorly, running between the aorta and the azygous vein and anterior to the vertebral column. The thoracic duct ascends through the aortic hiatus of the diaphragm entering the posterior mediastinum, still to the right of the vertebral column. It courses posterior to the esophagus at the T7 level and crosses over the midline to the left side of the thorax around the T5 vertebral level. As it continues upward, it runs behind the aorta and to the left of the esophagus ascending 2-3 cm above the clavicle. In the superior mediastinum, it passes behind the left common carotid artery, the vagus nerve, and the internal jugular vein. It then descends to empty into the junction of the left subclavian and internal jugular veins.

The wall of the thoracic duct has three layers: the intima, the media, and the adventitia. It also has a basement membrane. The media is composed of smooth muscle and connective tissue. The smooth muscle contracts regularly to move lymph flow forward. The thoracic duct also contains valves which may be unicuspid, bicuspid, or tricuspid, but are usually bicuspid. At the junction of the lymphatic and venous system, a bicuspid valve prevents venous backflow into the lymphatic system.[3]

The function of the thoracic duct is to transport lymph back into the circulatory system. Interstitial fluid is collected by lymph capillaries from the interstitial space. Lymph then moves through lymphatic vessels to lymph nodes. Lymphatic vessels merge to create 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.

During the sixth week of development of the human embryo, the early lymphatic system is composed of blunt buds near the base of the neck. Six lymph sacs form by the end of the embryonic period. These lymph sacs are the cisterna chyli, two jugular lymph sacs, two iliac lymph sacs, and a retroperitoneal lymph sac. Lymphatic vessels develop to connect these sacs and form the early lymphatic system. The thoracic duct develops from lymphatic trunks on either side of the aorta that anastomoses to form a channel from the jugular lymph sacs to the cisterna chyli. Trunks continue to anastomose and enlarge, forming embryonic right and left thoracic ducts. The adult thoracic duct is derived from both of these embryonic thoracic ducts. The right primitive thoracic duct eventually develops into the lower adult thoracic duct whereas the left primitive thoracic duct develops into the upper portion of the adult thoracic duct.

The typical anatomy described above is present in only about 50% of individuals. There are many anatomic variations of the thoracic duct. A cisterna chyli is present in about half of individuals. When embryologic lymphatic trunks converge above the T12 vertebral level, the cisterna chyli is generally absent, but there may be dilation of lower lumbar trunks.

In the embryo, portions of a bilateral system of lymphatic trunks anastomose and develop while portions atrophy. Failure of the typical pattern leads to anatomic variations of the thoracic duct. A common variation is a bifid lower aspect of the trunk caused by embryonic right and left lymphatic trunks failing to fuse. Another variation is the lower thoracic duct being replaced by a plexus of lymphatic vessels and forming a single duct higher in the mediastinum. Two rare variations include complete bilateral thoracic ducts and termination of the duct into the azygous system.

Variations in termination also exist. In the majority of cases, the duct terminates on the left side. In 2% to 3% of cases, the duct empties on the right, and bilaterally in up to 1.5% of cases. In over 95% of cases, the thoracic duct terminates in the internal jugular vein, the subclavian vein, or the angle between the two. The remaining 5% include termination in the external jugular vein, vertebral vein, brachiocephalic vein, suprascapular vein, and transverse cervical vein. The thoracic duct can also terminate as a single vessel (up to 87.5%), bilateral ducts (up to 25%), or several terminal branches (up to 7%).

The thoracic duct displays physiologic adaptation to certain disease processes by increasing in diameter. These disease states include congestive heart failure, cirrhosis or the liver, portal hypertension, and malignancy.[4][5]

Procedures within the thoracic cavity risk injuring or obstructing the duct which could result in chylothorax. The long course of the thoracic duct predisposes it to traumatic injury during thoracic, cardiac, or head and neck surgeries. Frequent physiologic variants also make avoiding the duct during surgery somewhat difficult. The thoracic duct is particularly vulnerable to trauma during esophageal surgery. Certain noninvasive procedures also risk iatrogenic duct injury such as a central line placement. Occlusion of the thoracic duct also results in chylous extravasation although this is much less common compared to injury.

Dietary changes, including decreasing intake of fat and increasing intake of medium chain triglycerides, can be used as an initial conservative approach to treating low chyle output of less than 1 L/day. Bowel rest and lipid-free total parenteral nutrition is another option. If chyle output is over 1 L/day, thoracic duct ligation or percutaneous embolization must be considered.

As a central structure to lymphatic flow and movement, thoracic duct dysfunction and resulting chyle accumulation is concerning for malignancy. Lymph from organs can drain directly into the thoracic duct without passing a lymph node. This anodal route has been observed for the diaphragm, esophagus, and parts of the lungs. The drainage pattern may play a role in the prognosis of cancers of these organs. This pattern may also explain the presence of distant metastases without lymph node involvement. 

The Virchow node, a lymph node located at the base of the neck where the duct generally terminates, can be enlarged in cases of malignancy and may obstruct drainage of the thoracic duct. When there is an obstruction of the thoracic duct, lymph collects in the pleural cavity. This formation of chylothorax can be the first sign of malignancy.

Review Questions

Thoracic Canal, Esophagus, Posterior intercostal lymph glands, cisterna chyli. Contributed by Gray's Anatomy Plates

1.

Rizvi S, Wehrle CJ, Law MA. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Jul 27, 2021. Anatomy, Thorax, Mediastinum Superior and Great Vessels. [PubMed: 30137860]

Chaudhry SR, Bordoni B. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Jul 31, 2021. Anatomy, Thorax, Esophagus. [PubMed: 29494119]

Ndiaye A, Di-Marino V, Ba PS, Ndiaye A, Gaye M, Nazarian S. Anatomical variations in lymphatic drainage of the right lung: applications in lung cancer surgery. Surg Radiol Anat. 2016 Dec;38(10):1143-1151. [PubMed: 27151087]

Sarkaria IS, Finley DJ, Bains MS, Adusumilli PS, Rizk NP, Huang J, Downey RJ, Rusch VW, Jones DR. Chylothorax and Recurrent Laryngeal Nerve Injury Associated With Robotic Video-Assisted Mediastinal Lymph Node Dissection. Innovations (Phila). 2015 May-Jun;10(3):170-3. [PMC free article: PMC4981881] [PubMed: 26165562]

Bryant AS, Minnich DJ, Wei B, Cerfolio RJ. The incidence and management of postoperative chylothorax after pulmonary resection and thoracic mediastinal lymph node dissection. Ann Thorac Surg. 2014 Jul;98(1):232-5; discussion 235-7. [PubMed: 24811982]