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  • In a nutshell, the lymphatic system is a drainage system that removes excess fluid from body tissues and returns it to the bloodstream.

  • It's actually a subsystem of both the circulatory and immune system.

  • The major purpose of the circulatory system is to bring oxygen and nutrients to body tissues and remove wastes.

  • This exchange happens in the smallest blood vessels called the capillaries.

  • Blood plasma containing nutrients moves out of capillaries at the arterial end of capillary beds, while tissue fluid containing wastes re-absorbs back in at the venous end.

  • However, not all of the fluid is drawn back to the bloodstream at this point.

  • About 15% of it is left in the tissues and would cause swelling if accumulated.

  • This is where the lymphatic system comes into play. It picks up the excess fluid and returns it to the circulatory system.

  • Unlike the blood circulatory system, which is a closed loop, the lymphatic system is a one-direction, open-ended network of vessels.

  • Lymphatic vessels begin as lymphatic capillaries made of overlapping endothelial cells.

  • The overlapping flaps function as a one-way valve.

  • When fluid accumulates in the tissue, interstitial pressure increases, pushing the flaps inward, opening the gaps between cells, allowing fluid to flow in.

  • As pressure inside the capillary increases, the endothelial cells are pressed outward, closing the gaps, thus preventing backflow.

  • Unlike blood capillaries, the gaps in lymphatic capillaries are so large that they allow bacteria, immune cells, such as macrophages, and other large particles to enter.

  • This makes the lymphatic system a useful way for large particles to reach the bloodstream.

  • It is used, for example, for dietary fat absorption in the intestine.

  • Once inside lymphatic vessels, the recovered fluid is called lymph.

  • Lymph flow is enabled by the same forces that facilitate blood flow in the veins.

  • It goes from lymphatic capillaries to larger and larger lymphatic vessels and eventually drains into the bloodstream via the subclavian veins.

  • On the way, it passes through a number of lymph nodes, which serve as filters, cleansing the fluid before it reaches the bloodstream.

  • Lymph nodes are small bean-shaped structures scattered throughout the lymphatic network.

  • They are most prominent in the areas where the vessels converge.

  • Lymph nodes contain macrophages and dendritic cells that directly swallow up any pathogens, such as bacteria or viruses, that may have been taken up from an infected tissue.

  • They also contain lymphocytes: T-cells and B-cells, which are involved in adaptive immune response, a process that produces activated lymphocytes and antibodies specific to the invading pathogen.

  • These are then carried by the lymph to the bloodstream to be distributed wherever they are needed.

  • The lymphatic system also includes lymphoid organs.

  • Primary lymphoid organs, the thymus and bone marrow, are the sites of lymphocyte production, maturation and selection.

  • Selection is the process in which lymphocytes learn to distinguish between self and non-self, so they can recognize and destroy pathogens without attacking the body's own cells.

  • Mature lymphocytes then leave the primary for the secondary lymphoid organs, the lymph nodes, spleen, and lymphoid nodules, where they encounter pathogens and become activated.

In a nutshell, the lymphatic system is a drainage system that removes excess fluid from body tissues and returns it to the bloodstream.

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