• The lymphatic system is a system of vessels that works closely with the circulatory system.

• Lymph is a clear, watery liquid that is formed from interstitial fluid (between cells)
• As blood flows through the blood vessels, some fluids inevitably leak out into the surrounding tissues.
• About 90% of the interstitial fluid diffuses back into the capillaries. 10% enters the lymphatic vessels.
• When interstitial fluid enters a lymph vessel, it is renamed “lymph”.

• Unlike blood vessels that are all connected, lymph vessels have “dead ends” called lymph capillaries.

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• Lymph capillaries are lined with overlapping epithelial cells that allow fluid to enter.
• These capillaries open into large lymph vessels.
• Valves within the vessels prevent the backwards flow of fluid.

• Lymph does not actively pump like blood. Instead, the surrounding skeletal muscles provide pressure, which pushes the fluid along.

• Lymph vessels carry the lymph to lymph nodes which are surrounded by a capsule of connective tissue.
• Trabeculae made of reticular fibers form the structure of the lymph node.
• Inside, the germinal centers are areas of rapid mitosis immune cells are quickly produced to fight off invaders found in the lymph.

• Lymph enters the lymph nodes through afferent vessels and leaves through one efferent vessel. This slows down the lymph so it can be filtered.
• The lymph is eventually returned through two large ducts within the chest.
• Right lymphatic duct
• Thoracic duct
• These ducts merge with veins where the fluid reenters the cardiovascular system.

• Lymph is collected from interstitial tissues between cells. It squeezes into lymph capillaries and travels to lymph nodes through larger lymph vessels. After it is filtered, it is returned to the cardiovascular system through ducts.

• Lymph nodes are one of many lymphoid organs.
• Other lymphoid organs (spleen & thymus) also protect the body from infection, but do not filter lymph.

• Lymph nodules are loosely associated tissues and are not surrounded by a capsule, like lymph nodes and lymph organs.

Spleen

• Found in the upper left part of abdomen
• Inside capsule, spleen contains 2 types of tissue:
• White pulp- composed of lymphatic tissue and lymph nodules for filtering blood
• Red pulp- Removes worn-out red blood cells and serves as a reservoir for stored blood

Thymus Gland

• Part of endocrine & lymphatic systems
• After T-lymphocytes are formed in the bone marrow, they travel to the thymus for thymic education, in which they learn to recognize specific foreign invaders.
• T-lymphocyte production is also regulated by the thymus through the release of the hormone thymosin.

• Lymph nodules are often associated with mucous membranes because foreign invaders become trapped here.
• These are called Mucosa-Associated Lymphoid Tissues (MALTs)
• MALTs can either be scattered within mucous membrane tissue or bundled together.
• Appendix & tonsils are a good example of locations where lymph nodules are found in a high concentration.

Tonsils

• An accumulation of lymphoid nodules on either side of the throat
• Along with other lymphoid nodules (lingual tonsils and adenoids), they form a protective ring around the throat to protect the body from foreign invaders entering in through the nose or mouth.

Peyer’s Patches & Appendix

• Groups of lymphocytes near the small intestine that filter bacteria from food and prevent beneficial bacteria from the intestines from causing infection in the rest of the body.

• Lymph tissue is found all over the body encapsulated in lymph organs like the spleen and thymus gland and in loosely associated lymph nodules like the tonsils, Peyer’s patches and appendix.