General Structure and Function of the Lymphatic System
As noted earlier, the lymphatic system is intimately connected with the cardiovascular system. Its major functions include the drainage and return of tissue fluid to the cardiovascular system and the protection of the body against infection.
As discussed previously, plasma fluid is forced into intercellular spaces near the capillaries by the pressure of blood, and not all of it is reabsorbed into the exiting venule. Lymphatic capillaries, which originate all over the body in tissues, collect this fluid, which is now referred to as lymph. (Lacteals are specialized lymphatic capillaries that begin in the microvilli of the small intestine and are the primary site of lipid absorption.) The lymph is transported via lymphatic vessels and ducts, which ultimately empty their contents back into the cardiovascular system at the subclavian veins, which drain the arms. As with venous circulation, lymph is not under high pressure, and must rely on smooth and skeletal muscular contractions, along with a system of valves, to ensure proper unidirectional flow. In addition to the lymphatic vessels, several lymphatic organs exist that play roles in specific lymphatic functions. The three most important of these are:
- Lymph nodes: Located at various positions in the lymphatic system, lymph nodes are masses of tissue through which lymph flows and is cleansed in the process. Two major types of white blood cells collect and function in the nodes: macrophages, which ingest foreign particles through phagocytosis, and lymphocytes, which are responsible for the immune response (covered in more detail shortly). Thus the lymph nodes act as filters that remove pathogens and foreign particles before they return to the general circulation.
- Thymus gland: Located in the lower neck, the thymus stores certain lymphocytes that were produced in the bone marrow. The thymus produces the hormone thymosin, which helps to differentiate these lymphocytes into functional T lymphocytes or T cells. These T cells then migrate into the lymph nodes and play a role in immunity.
- Spleen: Located in the abdominal cavity, the spleen structurally and functionally resembles a giant lymph node. Unlike the nodes, however, it also contains blood. Loaded with macrophages and lymphocytes, the spleen is active in destroying foreign invaders as well as damaged or nonfunctional red blood cells.