Lymphatic System

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Original Editor - Iva Top Contributors - Iva, Lucinda hampton, Shaimaa Eldib, Kim Jackson, Essam Ahmed and Candace Goh

Introduction

The lymphatic system is an important and often underappreciated component of the circulatory, immune, and metabolic systems.

It is composed of

1. Lymphatic fluid - protein rich fluid that flows through the lymphatic system and surrounds all tissues.
2. Lymphatic vessels - reabsorb interstitial fluid from the periphery to return it to the intravascular space, which prevents fluid build up in peripheral tissues
3. Lymphatic cells - include macrophages, dendritic cells, lymphocytes
4. Lymphatic organs - eg spleen and thymus.

There are three primary functions of the lymphatic system:

1. Maintenance of fluid balance
2. Facilitation of the absorption of dietary fats from the gastrointestinal tract to the bloodstream for metabolism or storage
3. Enhancement and facilitation of the immune system.

Lymphatic system 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.

• Lymph flows through lymphatic vessels and lymph nodes and is part of the immune system that is responsible for removing foreign microbes to lymph nodes and returning proteins to the blood.
• Unlike blood, which flows around the body in a loop by force created by the pumping mechanism of the heart and calf muscle, lymph flows in only one direction.
• The direction of the lymph flow is controlled by one-way valves and the flow is controlled by intrinsic contractions of lymph collectors created by the activity muscles surrounding the vessels.
• Problems that lymph may cause are primarily because there is no pump in the system.
• Lymph fluid consists of proteins, water, cells (RBCs, WBCs, lymphocytes which help to fight infections in the body), waste products, and fat in the intestinal lymph. 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 Transport System

5 components: capillaries, collecting vessels, lymph nodes, trunks, and ducts.

1. The initial point of entry into the lymphatic system is through the capillaries - a single layer of partially overlapping endothelial cells creating 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.
2. The capillaries subsequently feed fluid into the collecting vessels, in which an endothelial layer with many tight junctions forms 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.
3. Next point in the system are the lymph nodes - which possess a medulla, paracortex, and cortex which in combination, form the house of lymphocytes, antigen-presenting cells, and macrophages. There are approximately 450 lymph nodes in the human body. After visiting the lymph nodes, the lymphatic fluid flows into efferent lymphatic collecting vessels
4. Next to larger lymphatic trunks and
5. Finally into the lymphatic ducts. The lymphatic ducts 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.

Key Points

• 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.

Lymphatic Organs and Tissues

1. 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.

2. Secondary lymphoid organs

The secondary lymphoid organs, which include lymph nodes and the spleen, 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.

3. Other lymphoid tissue

Lymphoid tissue associated with the lymphatic system is concerned with immune functions in defending the body against infections and the spread of tumours. It consists of connective tissue formed of reticular fibers, with various types of leukocytes (white blood cells), mostly lymphocytes enmeshed in it, through which the lymph passes. Regions of the lymphoid tissue that are densely packed with lymphocytes are known as lymphoid follicles. Lymphoid tissue can either be structurally well organized as lymph nodes or may consist of loosely organized lymphoid follicles known as the mucosa-associated lymphoid tissue (MALT).  Mucosa-associated lymphoid tissue is an umbrella term for extranodal aggregates of lymphoid tissue in the bronchus (BALT), gut (GALT) and 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

Diseases of the Lymphatic System

The lymphatic system is also prone to diseases like the venous and arterial circulation. Eg

• 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.
• 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.

Physiotherapy

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.