What Is The Lymphatic System & How Does It Work?
The Human Body’s Mechanism For Immunity Against Viruses and Bacteria
Jun 1, 2009
The lymphatic system is the body's defense system. It is a series of vessels that transport waste products and foreign invaders through the interstitial fluid. Interstitial fluid is crucial to the maintenance of cell and tissues. When interstitial fluid travels through the lymphatic system, it is then called lymph.
What is Lymph?
Lymph is a colorless fluid that is full of lymphocytes (white blood cells) which are essential to fighting foreign invaders such as viruses and bacteria. It also contains salts, protein molecules, glucose and urea.
What Are Lymph Vessels?
Interstitial fluid from the cells and tissue filters in the lymphatic system. The lymphatic system runs parallel to arteries and veins throughout the body. Lymph vessels have their endothelial (epithelium composed of a single layer of smooth cells) cells overlapping. This gives them slight gaps between cells in addition to pores that some cells have. The purpose of these gaps and pores is so interstitial fluid can flow through vessels.
Once interstitial fluid (now called lymph) fluid has entered the lymph vessels, the vessels become larger as they carry lymph to the lymph nodes. Larger vessels are called lymphatics. Lymphatics have valves to prevent backflow of fluid. These valves open and close in turn, with contraction and relaxation of surrounding muscles, to pump fluid to lymph nodes.
What Are Lymph Nodes?
Lymph nodes or lymph glands occur in clusters in the lymphatic system throughout the body. Afferent (bringing to an organ or part) vessels transport fluid to the nodes where it is filtered and cleaned.
Macrophages (large white blood cells that engulf foreign particles), and lymphocytes engulf foreign bodies, debris and bacteria, removing all unwanted material. Efferent (away from organ or part) vessels carry the protein-rich lymph to two large lymph vessels that drain into the venous system at the neck.
Lymph Circulation
The lymphatic system has vessels that converge. Lymphatic vessels of the head and neck converge at the internal jugular nodes. Lymphatic vessels of the upper limbs, chest wall and back, converge at the axilla (armpit).
Lymphatic vessels of the lower limbs drain into inguinal nodes in the groin, then empty into the cisterna chyli (dilated lymph channel), which then empties into the thoracic duct. Lymph from thoracic organs, upper left limb and left half of the head and neck, drains into the thoracic duct. This duct empties into the left jugular vein.
Lymph nodes from the right upper limb, the right side of the head and neck and thorax, drain into the right lymphatic duct, which empties into the right jugular vein.
Lymphatic Organs
- Thymus – located in the upper chest between the heart and sternum. The thymus produces white blood cells called T-lymphocytes, which can recognize antigens of foreign invaders.
- Spleen – located in the upper left part of the abdomen, the spleen has a very high concentration of lymphatic tissue. It performs blood filtering and production and storage of lymphocytes.
- Tonsils – are aggregations (a group or mass) of lymphatic tissue and are the first line of defense against bacteria entering the respiratory system.
Resource and further reading:
Campbell and Reece, 2002, 'The Body’s Defenses', in Biology 6th edition, ed. Benjamin Cummings, San Francisco, chap 43.
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