Lymph originates as blood plasma lost from the circulatory system, which leaks out into the surrounding tissues. The lymphatic system collects this fluid by diffusion into lymph capillaries, and returns it to the circulatory system. Once within the lymphatic system the fluid is called lymph, and has almost the same composition as the original interstitial fluid. The human lymphatic system Contents 1 Lymphatic circulation 2 Secondary lymphoid organs 3 External links 4 See also Lymphatic circulation The lymphatic system acts as a secondary circulatory system, except it collaborates with white blood cells in order to protect people from being infected by viruses or bacteria. Unlike the circulatory system, the lymphatic system is not closed and has no central pump; the lymph moves slowly and under low pressure due mostly to the milking action of skeletal muscles. Like veins, lymph vessels have one-way valves and depend mainly on the movement of skeletal muscles to squeeze fluid through them. Rhythmic contraction of the vessel walls may also help draw fluid into the lymphatic capillaries. This fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body); these ducts drain into the circulatory system at the right and left subclavian veins. Lymph vessels are present in the lining of the gastrointestinal tract. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain, via the portal vein, into the liver for processing, fats are passed on to the lymphatic system, to be transported to the blood circulation via the thoracic duct. The enriched lymph originating in the lymphatics of the small intestine is called chyle (not chyme). The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation. The lymph system is a one-way system (interstitial fluid back to blood). Secondary lymphoid organs The spleen, lymph nodes, and accessory lymphoid tissue are the secondary lymphoid organs. These organs contain a scaffolding that support circulating B- and T-lymphocytes and other immune cells like macrophages and dendritic cells. When micro-organisms invade the body or the body encounters other antigens (such as pollen), the antigens are transported from the tissue to the lymph. The lymph is carried in the lymph vessels to regional lymph nodes. In the lymph nodes, the macrophages and dendritic cells phagocytose the antigens, process them, and present the antigens to lymphocytes, which can then start producing antibodies or serve as memory cells to recognize the antigens again in the future. External links The Lymphatic System (MedicalEngineer.co.uk) Lymphatic System Overview (innerbody.com) See also Thomas Bartholin and Olaus Rudbeckius, the discoverers of the lymphatic system in humans. History of anatomy in the 17th and 18th centuries Lymph Node Lymphedema, a condition of localized fluid retention caused by a compromised lymphatic system American Society of Lymphology Immune system - edit Humoral immune system | Cellular immune system | Immunological tolerance | Lymphatic system | White blood cells | Antibodies | Antigen (MHC) | Complement system | Inflammation  Lymphatic system - edit Lymph nodes | Lymph | Lymphocytes | Lymph vessels | Thoracic duct | Immune system | Bone marrow | Spleen | Thymus | Tonsils Human organ systems v·d·e Cardiovascular system | Digestive system | Endocrine system | Immune system | Integumentary system | Lymphatic system | Muscular system | Nervous system | Reproductive system | Respiratory system | Skeletal system | Urinary system Retrieved from "http://en.wikipedia.org/wiki/Lymphatic_system" Categories: Lymphatic system