Andrew, Warren Department of Anatomy, School of Medicine, Indiana University, Bloomington, Indiana.
Last reviewed:June 2019
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- Comparative anatomy
- Lymph vessels
- Lymph nodes
- Lymph flow
- Links to Primary Literature
- Additional Readings
A system of vessels in the vertebrate body, beginning in a network of exceedingly thin-walled capillaries in almost all the organs and tissues, except the brain and bones. The lymphatic system is a one-way vascular network in vertebrates that works in parallel with the blood circulation (Fig. 1). It drains interstitial fluid from most of the body's organs, with the exceptions of bone marrow, the retina, the central nervous system, and some avascular tissues (the cornea, cartilage, hair, nails, and epidermis). The interstitial fluid, which is rich in protein, must be removed, or swelling (edema) will occur. When the drainage function of the lymphatics is disturbed, serious swelling can occur; this condition is called lymphedema. When working normally, though, the lymphatic network is drained by larger channels, mostly coursing along the veins and eventually joining to form a large vessel (the thoracic duct), which runs beside the spinal column to enter the left subclavian vein at the base of the neck. The lymph fluid originates in the tissue spaces by filtration from the blood capillaries. While in the lymphatic capillaries, it is clear and watery. However, at intervals along the larger lymphatic vessels, the lymph passes through spongelike lymph nodes, where it receives great numbers of cells—the lymphocytes—and becomes turbid. With the exception of the capillaries, the lymphatic vessels contain numerous valves preventing backflow of the lymph (Fig. 2). In addition to fluid drainage, the functions of the lymphatic system are to remove particulate materials (for example, molecular proteins and bacteria) from tissues, to transport fat from the intestine to the blood, and to supply the blood with lymphocytes (the chief cells of the immune system, including T cells arising from the thymus and B cells produced in the bone marrow). The formation of new lymphatic vessels is termed lymphangiogenesis. See also: Blood vessel; Circulation; Edema; Immunology; Lymphangiogenesis; Vascular disorders
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