The lymphatic system acts as a secondary circulatory system in vertebrates and plays a crucial role in maintaining homeostasis as well as good health.
Lymph is the fluid carried by the lymphatic system. It originates as blood plasma (the liquid component of blood), after it has been lost from the circulatory system due to hydrostatic pressure. This pressure results in plasma leaking out of the blood vessel into the surrounding tissues. Once within the lymphatic system, this fluid, which may be clear or slightly yellow, is called lymph. It has a similar composition to the original interstitial fluid—the extracellular fluid that surrounds cells—but is in a more dilute form (Gray 1918).
The lymphatic system consists of three main components: (1) a complex capillary network that carries the lymph; (2) a system of collecting vessels that drain the lymph back into the bloodstream, and (3) lymph glands, or nodes, that filter the lymph as it passes through. In humans, approximately 3 quarts, or 2.83 liters, of lymph is returned to the heart everyday (Foster 2006).
Unlike the circulatory system, the lymphatic system is not closed and has no central pump (like the heart). It is a one-way system, with fluid traveling from the interstitial space back to the blood. The capillaries of the lymphatic system, which consist of a single layer of endothelium, lie close to all blood capillaries (except for those blood capillaries in the kidneys and central nervous system).
The lymphatic system serves several functions:
Because of the importance of the lymphatic system—Lemole (2001) calls it "our river of life," a variety of approaches have been recommended to keep it healthy, including exercise, diet, stress reduction, massage, and deep breathing, among others.
In humans, the lymphatic system was discovered by Olof Rudbeck in 1651. At about the same time, Thomas Bartholin made the similar discovery and published his findings first. Jean Pecquet had already noted the lymphatic system in animals.
Fluid in blood vessels experience osmotic as well as hydrostatic pressures. The former favors absorption into the vessel while the latter favors net filtration of fluids, mainly plasma, into the interstitial space. When filtration is favored, plasma leaks out of the vessel and into the surrounding tissues. From there, it enters the lymphatic system through the process of diffusion. Through the use of fibers, the walls of lymph capillaries are joined to the connective tissues nearby and are held open. Consequently, large gaps between the cells are formed, which allow fluid, interstitial proteins, and other matter such as bacteria to flow into the lymph capillary (Silverthorn 2004).
Once in the lymphatic system circulation, the lymph moves slowly and under low pressure, due mostly to the milking action of skeletal muscles. Like veins in the circulatory system, 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 also help draw fluid into the lymphatic capillaries. This fluid is then transported to progressively larger lymphatic vessels, which consist of three coats of cells and fibers instead of one layer like the capillaries, 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, which are located under the clavicle, or collar bone.
The skeletal muscle pump, discussed above, is crucial for lymph movement, as is illustrated, for example, by an immobilized limb. Often times, immobilized or injured limbs swell due to the accumulation of fluid in the interstitial space. This condition, known as edema, is caused by an imbalance between osmotic and hydrostatic pressures, with hydrostatic pressure being favored. The condition can be lessened by elevating the limb above the level of the heart (gravity will aid the flow of lymph back to the blood).
Although lymph vessels are found throughout the human body, the lymph vessels found in the lining of the gastrointestinal tract play an especially important role in digestion. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain into the liver for processing, fats are passed on to the lymphatic system through lacteals to be transported to the blood circulation via the thoracic duct. Lacteals are fingerlike extensions of the lymph system that are present in the villi of the intestine (Silverthorn 2004). The enriched lymph originating in the lymphatics of the small intestine is called chyle. The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation.
As lymph travels through the lymphatic system, it encounters the secondary lymphoid organs along the way: the spleen, lymph nodes, and accessory lymphoid tissues like the tonsils, bone marrow, and the thymus.
The lymph nodes, which are bean-shaped glands, consist of lymphoid tissues containing immunologically active cells, such as lymphocytes and macrophages (Silverthorn 2004). They filter the lymph as it passes through to make sure no pathogens, or foreign matter, are allowed to return to the venous circulation of the circulatory system. In the lymph nodes, the macrophages and dendritic cells phagocytose, or engulf, the antigens and pathogens, process them, and present them to lymphocytes (white blood cells). The lymphocytes can then start producing antibodies or serve as memory cells to recognize the antigens again in the future.
Lymph nodes are found mainly in the armpits, groin area, and neck along with some located in the abdomen, pelvis, and chest areas. The number of lymph nodes found in an individual varies, as does the number and size of the nodes found in specific areas of the body. For instance, the human armpit can contain anywhere from 15 to 30 small nodes, while the back of the abdomen can consist of 4 to 10 large nodes (Lymphatic System 2003). In general, however, the nodes increase in size as they near the thoracic duct.
The spleen, which is located in the upper left area of the abdomen close to the stomach, serves as the largest lymphoid organ in the human body. It too contains lymphocytes and macrophages and has an inner area containing lymph nodes. Together, they trap and kill foreign matter found in the blood.
The lymphatic system of humans is actually larger than the circulatory system, with twice as much lymph as blood, twice as many lymph vessels as blood vessels, and more water (Lemole 2001). As noted above, the lymphatic system is fundamentally important for combating foreign bodies, such as viruses and bacteria, as well as cancer cells. It is important for combating heart disease and arthritis as well.
At times, pathogens that enter the lymph nodes are not demolished right away. If this occurs and the bacteria or foreign matter are trapped in the node region, the lymph node will become swollen and painful if touched. Such infections can be treated with antibiotics.
On the other hand, as the lymph nodes trap and attempt to destroy pathogens, they can come across cancer cells that they cannot kill. If those cells remain in the lymph node and begin to divide, the node can become swollen; however, it usually does not cause pain upon touching. If any such swelling of the nodes occurs, a physician should be consulted as soon as possible for diagnosis and appropriate treatment (Lymphatic System 2003).
In general, cancers that originate in the lymphatic system are referred to commonly as lymphomas. Cancers can also originate outside the lymphatic system and then make their way into lymphoid tissues and glands.
Lemole (2001) claims that the key to health is a healthy lymphatic system, specifically stating "you can eliminate 70 percent of the chronic illnesses that are in part the result of that system being clogged." Among measures recommended for a healthy lympatic system are exercise, reduction of stress, massages, and a healthy diet.
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