The lymphatic system in vertebrates is a network of conduits that carry a clear fluid called lymph. It also includes the lymphoid tissue through which the lymph travels. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils. The system also includes all the structures dedicated to the circulation and production of lymphocytes, which includes the spleen, thymus, bone marrow and the lymphoid tissue associated with the digestive system. The lymphatic system as we know it today was first described independently by Olaus Rudbeck and Thomas Bartholin.
The blood does not directly come in contact with the parenchymal cells and tissues in the body, but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body. Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system. The lymph is then moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic vessels or by extrinsic compression of the lymphatic vessels via external tissue forces (e.g. the contractions of skeletal muscles).
The lymphatic system has three interrelated functions: it is responsible for the removal of interstitial fluid from tissues; it absorbs and transports fatty acids and fats as chyle to the circulatory system; and it transports immune cells to and from the lymph nodes. The lymph transports antigen-presenting cells (APCs), such as dendritic cells, to the lymph nodes where an immune response is stimulated. The lymph also carries lymphocytes from the efferent lymphatics exiting the lymph nodes.
The study of lymphatic drainage of various organs is important in diagnosis, prognosis, and treatment of cancer. The lymphatic system, because of its physical proximity to many tissues of the body, is responsible for carrying cancerous cells between the various parts of the body in a process called metastasis. The intervening lymph nodes can trap the cancer cells. If they are not successful in destroying the cancer cells the nodes may become sites of secondary tumors.
Diseases and other problems of the lymphatic system can cause swelling and other symptoms. Problems with the system can impair the body's ability to fight infections.
Organization
The lymphatic system can be broadly divided into the conducting system and the lymphoid tissue.
The conducting system carries the lymph and consists of tubular vessels that include the lymph capillaries, the lymph vessels, and the right and left thoracic ducts.
The lymphoid tissue is primarily involved in immune responses and consists of lymphocytes and other white blood cells enmeshed in connective tissue 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).
Formation of lymph
Blood supplies nutrients and important metabolites to the tissues, and collects back the waste products that they produce, which requires exchange of respective constituents between the blood and tissues. This exchange is not direct, however, and is effected through an intermediary called interstitial fluid or tissue fluid that the blood forms. Interstitial fluid (ISF) is the fluid that occupies the spaces between the cells and acts as their immediate environment. As the blood and the surrounding cells continually add and remove substances from the ISF, its composition keeps on changing. Water and solutes can freely pass (diffuse) between the ISF and blood, and thus both are in dynamic equilibrium with each other; exchange between the two fluids occurs across the walls of small blood vessels called capillaries.
ISF forms at the arterial (coming from the heart) end of the capillaries because of higher pressure of blood, and most of it returns to its venous ends and venules; the rest (10—20%) enters the lymph capillaries as lymph. Thus, lymph when formed is a watery clear liquid with the same composition as the ISF. As it flows through the lymph nodes, however, it comes in contact with blood and tends to accumulate more cells (particularly lymphocytes) and proteins.
The two primary lymph systems are the thymus gland and the bone marrow, where the immune cells form or mature. The secondary lymph system is made up of encapsulated and unencapsulated diffuse lymphoid tissue. The encapsulated tissue includes the spleen and the lymph nodes. The unencapsulated tissue includes the gut-associated lymphoid tissues and the tonsils.
Lymphoid tissue
Lymphoid tissue associated with the lymphatic system is concerned with immune functions in defending the body against the infections and spread of tumors. It consists of connective tissue with various types of white blood cells enmeshed in it, most numerous being the lymphocytes.
The lymphoid tissue may be primary, secondary, or tertiary depending upon the stage of lymphocyte development and maturation it is involved in. Primary (central) lymphoid tissues serve to generate mature virgin lymphocytes from immature progenitor cells. Secondary (peripheral) lymphoid tissues provide a place where lymphocytes can talk to each other; an environment for antigen focusing, where lymphocytes can 'study' an antigen and sharpen up the immune response by clonal expansion and affinity maturation; and provide a home for lymphocytes, where they can be available when they are needed.
The thymus and the bone marrow constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes. Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes. It is exemplified by the lymph nodes, and the lymphoid follicles in tonsils, Peyer's patches, spleen, adenoids, skin, etc. that are associated with the mucosa-associated lymphoid tissue (MALT). The tertiary lymphoid tissue typically contains far fewer lymphocytes, and assumes an immune role only when challenged with antigens that result in inflammation. It achieves this by importing the lymphocytes from blood and lymph.
Lymph nodes
For more details on this topic, see lymph node.A lymph node is an organized collection of lymphoid tissue, through which the lymph passes on its way to returning to the blood. Lymph nodes are located at intervals along the lymphatic system. Several afferent lymph vessels bring in lymph, which percolates through the substance of the lymph node, and is drained out by an efferent lymph vessel.
The substance of a lymph node consists of lymphoid follicles in the outer portion called the "cortex", which contains the lymphoid follicles, and an inner portion called "medulla", which is surrounded by the cortex on all sides except for a portion known as the "hilum". The hilum presents as a depression on the surface of the lymph node, which makes the otherwise spherical or ovoid lymph node bean-shaped. The efferent lymph vessel directly emerges from the lymph node here. The arteries and veins supplying the lymph node with blood enter and exit through the hilum.
Lymph follicles are a dense collection of lymphocytes, the number, size and configuration of which change in accordance with the functional state of the lymph node. For example, the follicles expand significantly upon encountering a foreign antigen. The selection of B cells occurs in the germinal center of the lymph nodes.
Lymph nodes are particularly numerous in the mediastinum in the chest, neck, pelvis, axilla (armpit), inguinal (groin) region, and in association with the blood vessels of the intestines.
Lymphatics
Tubular vessels transport back lymph to the blood ultimately replacing the volume lost from the blood during the formation of the interstitial fluid. These channels are the lymphatic channels or simply called lymphatics .
General structure of Lymphatics
The general structure of lymphatics is based on that of blood vessels. There is an inner lining of single flattened cells composed of a type of epithelium that is called endothelium, and the cells are called endothelial cells . This layer functions to mechanically transport fluid and since the basement membrane on which it rests is discontinuous; it leaks easily. The next layer is that of smooth muscles that are arranged in a circular fashion around the endothelium, which by shortening (contracting) or relaxing alter the diameter (caliber) of the lumen. The outermost layer is the adventitia that consists of fibrous tissue. The general structure described here is seen only in larger lymphatics; sm
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