The anatomy of the lymphatic system closely parallels that of the peripheral vascular system. It functions by unilaterally returning lymphatic fluid to the vascular system where it is eliminated. Almost every organ in the body has lymphatics which generate a variety of byproducts that require elimination. Examples of organs in the body that contain lymphatic tissue include the gastrointestinal (GI) tract, spleen, lymph nodes, tonsils, and thymus. Lymph nodes function to monitor the composition of lymphatic fluid/blood, engulf any pathogens, augment an immune response, and eradicate infection. The thymus serves to mature and develop T cells in response to an inflammatory process, immune response or malignancy. The absorption and transport of fats and fat-soluble vitamins from the GI system also requires lymphatics. 
The lymphatic fluid is eventually emptied at the junction of the left subclavian vein and left internal jugular veins. Lymphatic fluid is derived from plasma. It leaks out of the capillary walls because of pressure exerted by the heart or osmotic pressure at the cellular level. As the interstitial fluid accumulates, it is picked by the tiny capillary lymphatics, passes through the lymph nodes, and finally returns the fluid to the venous circulation. As the lymph passes through the lymph nodes, both monocytes and lymphocytes enter it.
In the GI tract, the lymphatic fluid has a milk-like appearance that is chiefly due to the presence of cholesterol, glycerol, fatty acids and other fat products. The vessels that transport the lymphatic fluid from the GI tract are known as lacteals.
Lymphatic capillaries are very thin vessels which are blind-ended tubes. These lacteals have a thickness of a single cell, but the cells are arranged in an overlapping manner so that the fluid from the outside can enter it with relative ease. The lymphatic capillaries tend to form a large network of tubes that are known as lymphatic vessels.
The two major lymphatics of the body include the right lymphatic duct and the thoracic duct. The right duct drains most of the right upper quadrant whereas the thoracic duct drains the lower body including the extremities and abdomen. To ensure that fluid does not flow backward, all lymphatic have one-way valves. The forward flow of lymph is due to the pressure gradient created by the muscle contractions and respiratory movements.
The key feature of lymphatic vessels is that they have thin endothelial walls and have an overlapping arrangement. This morphology allows for any fluid from the tissues to enter the cells. The lymphatic capillaries eventually merge to form much a much bigger network of vessels that are located deep within the body. These lymphatic vessels eventually form the (1) right lymphatic duct and (2) the thoracic duct.
The key functions of the lymphatics include the following:
- Defend against foreign particles and microorganisms
- Restore any excess protein molecules and interstitial fluid back to the systemic circulation
- Absorption of fat-soluble vitamins and fatty substance from the gastrointestinal tract and transport them to the venous circulation
Issues of Concern
The Thoracic Duct
The thoracic duct is a thin-walled tubular vessel measuring 2 to 4 mm in diameter. The length of the duct ranges from 36 to 45 cm. The thoracic duct begins in the abdomen at the level of the second lumbar vertebrae. Initially, it is located on the right side and slightly posterior to the aorta and by the time it enters the right crus of the diaphragm, it is side by side with the aorta. It ascends in the thoracic cavity slightly anterior and to the right of the vertebral column. At the level of the fifth thoracic vertebrae, the thoracic duct crosses over to the left of the vertebral column. From here it ascends vertically and joins the junction of the left subclavian and left jugular vein the neck.
Clinically, if a patient has an injury to the thoracic duct below T5, then fluid will collect in only the right pleural cavity. But if the injury is to the thoracic duct above T5, then fluid will appear in both pleural cavities.
Injury to the thoracic duct in the neck area can be managed with drainage alone, but if the thoracic duct is damaged in the chest cavity, drainage and surgery are usually required. It is rare for the thoracic segment of the thoracic duct to seal on its own. 
Lymphatic leaks do occur when the vessels are damaged. In the abdomen, lymphatic damage may occur during surgery, especially during retroperitoneal procedures like repair of an abdominal aortic aneurysm. These leaks are mild, and the lymph fluid eventually gets absorbed by the vessels in the peritoneum and mesentery. When the thoracic duct is injured in the chest, the leak can be extensive. In the majority of cases, conservative care with a no-fat diet (medium chain triglycerides) or total parenteral nutrition is not successful. In most cases, if the injury to the thoracic duct was surgical, then a surgical procedure is required to tie off the duct. If the thoracic duct is injured in the neck, then insertion of a drainage tube and a low-fat diet will help seal the leak.
The lymphatic system is also prone to diseases like the venous and arterial circulation. A relatively common disorder of the lymphatic system is the development of lymphedema. When this occurs, the lymphatic system is unable to drain lymphatic fluid which results in accumulation of the fluid causing swelling of the extremity. Lymphedema is classified as primary or secondary.
Primary lymphedema is an inherited disorder where the lymphatics may be missing or poorly developed. This condition usually presents soon after birth or sometimes may present later in life. There are no great treatments for primary lymphedema. Surgery procedures of the past era were found to be mutilating and not done today. The present-day treatment revolved around the use of compression stockings, pumps, and constrictive garments. 
Secondary lymphedema is an acquired disorder that has many causes including cancer, infection, trauma or following a surgical procedure. The treatment depends on the cause. In most cases, if the inciting cause is removed, the drainage will resume. However, one may need to wear compressive stockings permanently in some cases. Physical therapy may help when the extremities are involved.
Lymphomas are malignancies that arise from the cells of the lymphatic system. There is usually malignant transformation of specific lymphocytes in the lymphatics or lymph nodes that are present in the gastrointestinal tract, neck, axilla or groin. Symptoms of lymphoma may include night sweats, fever, fatigue, itching and weight loss.
Cancers of a variety of organs may commonly spread to involve regional lymph nodes.
Lymphadenitis occurs when the lymph nodes become inflamed. The cause is usually an adjacent bacterial infection. The lymph nodes usually enlarge and become tender.
Filariasis is a very common disorder caused by a parasite in Africa. The parasite rapidly divided and obstructs the lymph nodes in the groin, making it difficult for the lymphatics to drain the extremity. This often results in huge extremities and marked disability.