The lymphatic system is a vascular network of tubules and ducts that collect, filter, and return lymph to blood circulation. Lymph is a clear fluid that comes from blood plasma, which exits blood vessels at capillary beds. This fluid becomes the interstitial fluid that surrounds cells. Lymph contains water, proteins, salts, lipids, white blood cells, and other substances that must be returned to the blood. The primary functions of the lymphatic system are to drain and return interstitial fluid to the blood, to absorb and return lipids from the digestive system to the blood, and to filter fluid of pathogens, damaged cells, cellular debris, and cancerous cells.
Lymphatic System Structures
The major components of the lymphatic system include lymph, lymphatic vessels, and lymphatic organs that contain lymphoid tissues.
Lymphatic vessels are structures that absorb fluid that diffuses from blood vessel capillaries into surrounding tissues. This fluid is directed toward lymph nodes to be filtered and ultimately re-enters blood circulation through veins located near the heart. The smallest lymphatic vessels are called lymph capillaries. Lymphatic capillaries come together to form larger lymphatic vessels. Lymphatic vessels from various regions of the body merge to form larger vessels called lymphatic trunks. Lymphatic trunks merge to form two larger lymphatic ducts. Lymphatic ducts return lymph to the blood circulation by draining lymph into the subclavian veins in the neck.
Lymphatic vessels transport lymph to lymph nodes. These structures filter lymph of pathogens, such as bacteria and viruses. Lymph nodes also filter cellular waste, dead cells, and cancerous cells. Lymph nodes house immune cells called lymphocytes. These cells are necessary for the development of humoral immunity (defense prior to cell infection) and cell-mediated immunity (defense after cell infection). Lymph enters a node through afferent lymphatic vessels, filters as it passes through channels in the node called sinuses, and leaves the node through an efferent lymphatic vessel.
The thymus gland is the main organ of the lymphatic system. Its primary function is to promote the development of specific cells of the immune system called T-lymphocytes. Once mature, these cells leave the thymus and are transported via blood vessels to the lymph nodes and spleen. T-lymphocytes are responsible for cell-mediated immunity, which is an immune response that involves the activation of certain immune cells to fight infection. In addition to immune function, the thymus also produces hormones that promote growth and maturation.
The spleen is the largest organ of the lymphatic system. Its primary function is to filter the blood of damaged cells, cellular debris, and pathogens. Like the thymus, the spleen houses and aids in the maturation of lymphocytes. Lymphocytes destroy pathogens and dead cells in the blood. The spleen is rich in blood supplied via the splenic artery. The spleen also contains efferent lymphatic vessels, which transport lymph away from the spleen and toward lymph nodes.
Tonsils are arrays of lymphatic tissue located in the upper throat region. Tonsils house lymphocytes and other white blood cells called macrophages. These immune cells protect the digestive tract and lungs from disease-causing agents that enter the mouth or nose.
Bone marrow is the soft, flexible tissue found inside bone. Bone marrow is responsible for the production of blood cells: red blood cells, white blood cells, and platelets. Bone marrow stem cells play an important role in immunity as they generate lymphocytes. While some white blood cells mature in bone marrow, certain types of lymphocytes migrate to lymphatic organs, such as the spleen and thymus, to mature into fully functioning lymphocytes.
Lymphatic tissue can also be found in other areas of the body, such as the skin, stomach, and small intestines. Lymphatic system structures extend throughout most regions of the body. One notable exception is the central nervous system.
Lymphatic System Summary
The lymphatic system plays a vital role in the proper functioning of the body. One of the major roles of this organ system is to drain excess fluid surrounding tissues and organs and return it to the blood. Returning lymph to the blood helps to maintain normal blood volume and pressure. It also prevents edema, the excess accumulation of fluid around tissues. The lymphatic system is also a component of the immune system. As such, one of its essential functions involves the development and circulation of immune cells, specifically lymphocytes. These cells destroy pathogens and protect the body from disease. In addition, the lymphatic system works in conjunction with the cardiovascular system to filter blood of pathogens, via the spleen, before returning it to circulation. The lymphatic system works closely with the digestive system as well to absorb and return lipid nutrients to the blood.
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"Introduction to the Lymphatic System." SEER Training Modules, National Cancer Institute, National Institutes of Health, U.S. Department of Health and Human Services.