lymphatic system, network of vessels carrying lymph, or tissue-cleansing fluid, from the tissues into the veins of the circulatory system. The lymphatic system functions along with the circulatory system in absorbing nutrients from the small intestines. A large portion of digested fats are absorbed via the lymphatic capillaries. Like the blood circulatory system, the lymphatic system is composed of fine capillaries that lie adjacent to the blood vessels. These merge into larger tributaries known as trunks, and these in turn merge into two still larger vessels called ducts. The thoracic and right lymphatic ducts empty into the venous system in the region of the collarbones. Lymph, a colorless fluid whose composition is similar to that of blood except that it does not contain red blood cells or platelets, and contains considerably less protein, is continuously passing through the walls of the capillaries. It transports nutrients to the cells and collects waste products. Most of the lymph returns to the venous capillaries; however, a small amount (about 10%) enters the terminal lymphatic capillaries and is returned to the blood via the lymphatic system. The fluid that flows through the lymphatic system is functionally important because it contains substances having large molecules (such as proteins and bacteria) that cannot enter the small pores of the venous capillaries. Along the lymphatic network in certain areas of the body (neck, armpit, groin, abdomen, chest) are small reservoirs, the lymph nodes, which collect bacteria and other deleterious agents from the lymph which passes through them, and act as a barrier against the entrance of these substances into the bloodstream. In a disease state, therefore, the lymph nodes may become filled with harmful material to the degree where they can be seen or felt; therefore, enlarged lymph nodes are of diagnostic importance. Such enlargement of the lymph nodes can be a warning sign of various kinds of cancer, including breast cancer and Hodgkin's disease. In cases where a cancerous growth has developed, removal of lymph nodes may help to prevent its further spread. However, such a procedure also slows the flow of lymph and may thus render some of the body vulnerable to infection. See also lymphoma, non-Hodgkin's.

System of lymph nodes, vessels, and nodules and lymphoid tissue, including the thymus, spleen, tonsils, and bone marrow, through which lymph circulates and is filtered. Its primary function is to return proteins, waste products, and fluids to the blood; molecules too big to enter the capillaries pass through the more permeable walls of lymphatic vessels. Valves keep lymph flowing in one direction, more slowly than blood and at a lower pressure. The lymphatic system also has a role in the immune system. Nodes filter bacteria and foreign matter from lymph. Smaller nodules, which often produce lymphocytes, form in areas more exposed to such materials. They can merge and become permanent, as in the tonsils. Blockage of a lymph vessel may cause fluid to collect in the tissues, producing lymphedema (tissue swelling). Other lymphatic system disorders include lymphocytic leukemias and lymphoma. Seealso reticuloendothelial system.

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