The lymphatic system is an important and often under appreciated component of the circulatory, immune, and metabolic systems. It is is considered as a part of both the circulatory and immune systems. The functions of the lymphatic system complement the bloodstream functions, as it regulates the balance of fluids in the body and filters the pathogens from the blood.
It is composed of
- Lymphatic fluid - protein rich fluid that flows through the lymphatic system and surrounds all tissues . It carries about 75-100 grams of protein circulating in the body per day. Lymph is composed of white blood cells, triglycerides, bacteria, cell debris, water, and protein. It has a composition comparable to blood plasma.
- Lymphatic vessels - reabsorb interstitial fluid from the periphery to return it to the intravascular space, which prevents fluid build up in peripheral tissues
- Lymphatic cells - include macrophages, dendritic cells, lymphocytes
- Lymphatic organs - eg spleen and thymus.
There are three primary functions of the lymphatic system:
- Maintenance of fluid balance
- Facilitation of the absorption of dietary fats from the gastrointestinal tract to the bloodstream for metabolism or storage
- Enhancement and facilitation of the immune system.
Lymphatic Transport System
The lymph drainage system is complex and comprises initial lymphatics (lymph capillaries), pre-collectors, collectors, lymphatic trunks, and lymph nodes. Topographically, the lymph system is distinguished as superficial (subcutaneous) and deep (subfascial). The superficial system drains the skin and subcutis areas. The deep system drains muscles, joints, tendon sheaths, and nerves. Both systems are connected via the perforating vessels, which conduct lymph fluid from the subfascial areas to the surface.
- Capillaries - The initial point of entry into the lymphatic system, a single layer of partially overlapping endothelial cells that create a valve. These overlapping junctions form a "button-like" opening, which allows fluid into the capillary when the pressure outside of the vessel is greater than the pressure inside of the vessel. The capillaries subsequently feed fluid into the collecting vessels.
- Collecting vessels - composed of an endothelial layer with many tight junctions forming a "zipper-like" structure. These collecting vessels also have intraluminal valves as well as pericytes. The pericytes contain alpha-smooth muscle actin, which functions to contract the vessel and pump the fluid further through the system. The valves prevent the backflow of the lymphatic fluid and ensure the unidirectional flow of fluid.
- Lymphatic ducts - Final collecting vessels. Allow the entry of the lymphatic fluid into the venous system bilaterally, via the opening found at the intersection of the subclavian and internal jugular vein.
Left Duct (or thoracic duct) collects most of the lymph in the body other than from the right thorax, arm, head, and neck which are drained by the right lymphatic duct. It is 38–45 cm in length
Right Duct drains (1.25 cm in length) lymph fluid from:
- Upper right section of the trunk, (right thoracic cavity, via the right bronchomediastinal trunk),
- Right arm (via the right subclavian trunk),
- Right side of the head and neck (via the right jugular trunk),
Image at R - in white lymphatic ducts. Note much larger L duct.
Lymphatic Organs and Tissues
- Primary lymphoid organs
The primary lymphoid organs generate lymphocytes from immature progenitor cells. The thymus and the bone marrow constitute the primary lymphoid organs involved in the production and early clonal selection of lymphocyte tissues.
- Bone marrow - soft, organic, spongelike material in the cavities of bones. It is a network of blood vessels and special connective tissue fibers that hold together a composite of fat and blood-producing cells. Chief function is to manufacture erythrocytes, leukocytes, and platelets. These blood cells normally do not enter the bloodstream until they are fully developed, so that the marrow contains cells in all stages of growth. If the body's demand for leukocytes is increased because of infection, the marrow responds immediately by stepping up production.
2. Secondary lymphoid organs
The secondary lymphoid organs may be encapsulated to have a specific shape or may be in the form of diffused tissues. Spleen, lymph nodes, adenoids, tonsils are the encapsulated secondary lymphoid organs. They maintain mature naive lymphocytes and initiate an adaptive immune response. The peripheral lymphoid organs are the sites of lymphocyte activation by antigens. Activation leads to clonal expansion and affinity maturation. Mature lymphocytes recirculate between the blood and the peripheral lymphoid organs until they encounter their specific antigen.
- Lymph node - Any of the accumulations of lymphoid tissue organized as definite lymphoid nodes along the course of lymphatic vessels. Lymph nodes are the main source of lymphocytes of the peripheral blood and, as part of the reticuloendothelial system, serve as a defense mechanism by removing noxious agents such as bacteria and toxins, and probably play a role in antibody formation.
- Adenoids - A normal collection of unencapsulated lymphoid tissue in the nasopharynx. Also called pharyngeal tonsils.
- Tonsils - Oval-shaped masses of glandular tissue located on both sides at the back of the throat. Tonsils act like filters to trap bacteria and viruses.
3. Other lymphoid tissue (or diffuse lymphoid regions not encapsulated)
- skin (SALT), as well as breast and uterine cervix. MALT is the arm of the immune defence in closest contact with exogenous antigens, thus differing from the compartmentalised peripheral somatic lymphoid tissues, which include the lymph nodes, thymus and spleen. It constitutes about 50 percent of the lymphoid tissue in the human body
At R image - Mucosa Associated Lymphoid Tissue (MALT) Nodule. Mucosa and Peyer's patches (any of numerous large oval patches of closely aggregated nodules of lymphoid tissue in the walls of the small intestine especially in the ileum that partially or entirely disappear in advanced life and in typhoid fever become the seat of ulcers which may perforate the intestines).
Diseases of the Lymphatic System
- 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 abnormally developed (eg. aplasia, hypoplasia, hyperplasia). This condition usually presents at birth or sometimes may present later in life.
- Secondary lymphedema is an acquired disorder that has a known cause such as cancer, infection, trauma or following a surgical procedure.
- 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. This cancer can arise anywhere in the body and is broadly divided into Hodgkin and non-Hodgkin lymphoma types
- When germs enter the body, they are commonly picked up by the lymphatic system. They often then infect nearby lymph nodes, causing enlargement and tenderness. The medical term for this condition is lymphadenitis, but it is more commonly known as "swollen glands." Many common bacterial and viral infections can cause swollen glands, which go back to normal after the infection clears. The lymphatic vessels can also become infected and inflamed, a condition known as lymphangitis. Red streaks appear on the skin along the route of the inflamed lymphatic vessels, typically accompanied by fever and chills. Streptococcal bacteria are the most common cause of lymphangitis.
- Structural Conditions The lymphatic vessels sometimes form incorrectly or grow abnormally. For example, a lymphangioma is a noncancerous, lymph-filled growth that arises when small lymphatic vessels do not connect normally with the rest of the lymphatic system. These growths are most commonly seen during infancy and childhood, but adults can also be affected. Lymphangiectasia is another type of structural abnormality of the lymph system that occurs when the lymphatic vessels become damaged and lymph backs up. The affected vessel expands like a balloon, causing a noncancerous growth, which may leak lymph fluid. People with this condition also commonly have lymphedema.
- 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.
Physiotherapy has a crucial part in helping to resolve problems with the lymphatic system such as using manual lymphatic drainage and exercise which are components of Complete Decongestive Therapy (CDT) ,,  . Exercise induces transport of lymph. Movement of muscles and arterial pulses cause the transport of lymph through the lymphatic system. Being active therefore has a positive effect of lymphatic system and the movement of lymph through our body. Some activities that are recommended are swimming, brisk walking, tennis, jumping, gymnastic, deep breathing, etc  
Manual lymph drainage has shown to be of benefit in examples below
- Hind foot operations:the application of lymph drainage techniques after hindfoot (the posterior part of the human foot that contains the calcaneus, talus, navicular, and cuboid bones) operations, in combination with standard physiotherapy exercises, achieves greater limb volume reduction than exercise alone. 
- Breast cancer related lymph edema: 
- Sports Medicine and Rehabilitation.
- Lymph nodes function to monitor the composition of lymphatic fluid/blood, engulf any pathogens, augment an immune response, and eradicate infection. They are like filtering stations for the lymph fluid.
- 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.
- 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. The lymphatic capillaries tend to form a large network of tubes that are known as lymphatic vessels.
- 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. 
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