The lymphatic system is composed of a network of vessels that collects fluid and plasma proteins that leak out of capillaries and into the interstitial space. Lymphatic vessels return the lymph (fluid and plasma protein) back to the circulatory system through the veins.
The lymphatic system is a secondary system of vessels that is distinct both in anatomy and function from the blood vessel capillaries of the circulatory system. Small lymphatic vessels (or “lymphatics”) called lymphatic capillaries are found in almost all organs of the body except superficial layers of the skin, the central nervous system, endomysium of muscles, and the bone. These exceptions have a system of smaller vessels called prelymphatics. Fluid from prelymphatics returns to nearby lymphatic vessels, or the cerebrospinal fluid in the case of the central nervous system.
Lymphatic capillaries are made up of a single layer of endothelial cells. They are anchored to the surrounding connective tissue by special filaments called anchoring filaments. The system begins as a series of sacs. Each sac has a low hydrostatic pressure relative to the outside of the sac. At the end of the lymphatic capillaries there are endothelial valves. The valves form as a result of the slight overlap of the endothelial cells, and the overlapping edge has the ability to open inward. The valves open enough to allow fluid and plasma protein to pass into the lymphatic capillary.
At approximately 600 sites in the human body, lymphatic vessels converge into bundles of tissue called lymph nodes. The shape of a lymph node resembles a kidney bean and ranges in size from a few millimeters to a few centimeters. They are mostly found at the base of extremities such as the arms, legs and head. Many afferent lymphatics or vessels lead the lymph into the node at the larger curve of the bean shape and efferent lymphatics, fewer in number, take the lymph away from the node at the hilum, the depressed region of the bean shape. All nodes have a blood supply from the circulatory system running through them. The blood vessels enter and exit at the hilum. Inside the nodes are a honeycomb of lymph-filled sinuses that have macrophages and groupings of lymphocytes that produce antibodies.
Lymph is the fluid flowing through the lymphatic system and originates from the interstitial spaces of the organs and tissues. Another element of the lymph is a type of cell of the immune system called a lymphocyte, a type of white blood cell. Lymphocytes mature in either the thymus (T-lymphocytes) or the bone marrow (B-lymphocytes), which are primary lymphoid organs. The blood supply transports lymphocytes from their site of maturation (the thymus or bone marrow) to secondary lymphoid organs such as the lymph nodes, spleen, and tonsils. All lymphocytes in the adult originate in the bone marrow.
The lymphatic system has a variety of functions ranging from returning fluid from organs back to the circulatory system, to an important part in the human immune response, to absorbing lipids from the intestines. The defining role of lymphatic vessels is to return any fluid that has leaked from the capillaries and into the interstitial space back to the circulatory system through the veins. This is important because if fluid was retained in the tissues, it would result in reduced blood volume and swelling of the tissues.
Another important role of the lymphatic system is the ability of plasma proteins to fit through the lymphatic valves and into the lymphatic capillary. Since most proteins have such a high molecular weight, they are unable to be reabsorbed by venous capillaries. Without the reabsorption of the plasma proteins, humans can die within 24 hours.
The lymphatic system also has an essential role in the process of digestion. Primarily, the lymphatic capillaries in the gastrointestinal tract are one of the main routes for fats to be absorbed. Fats enter the lymphatics before entering the blood stream.
Fluid enters organs and tissues from the arterial capillaries, and is eventually reabsorbed by the venous capillaries. However, not all of the fluid is reabsorbed by blood capillaries. About one-tenth of the fluid is returned to the blood vessels via the lymphatic system. The lymphatic system reabsorbs about 2–4 qt. (1.8–3.8 L) of fluid per day. Lymph composition is different depending on the site of origin. For example, lymph collected from the gastrointestinal tract is high in fat that has been absorbed during digestion, and lymph collected from the bone marrow and thymus is high in lymphocyte concentration.
Lymph is collected when the pressure of the interstitial fluid and plasma proteins increases in the organs and tissues. Lymph pushes against the outside of the lymphatic valves and flows into the lymphatic capillary. This is called bulk flow. Valves are located throughout the lymphatic system approximately 0.15 in. (38 mm) apart. Backflow is not possible in the lymphatics because the valves open in only one direction. Therefore, the lymphatic system runs in only one direction.
There are several factors that affect the rate at which lymph is collected. Interstitial fluid pressure affects the rate of flow of fluid into the lymphatic capillaries. Elevated capillary pressure, increased interstitial fluid pressure, and increased capillary permeability all contribute to an increase in the amount of interstitial pressure and the rate of lymph flow.
Smooth muscles around the lymphatic vessels act as lymphatic pumps, and their involuntary contractions affect the rate of lymph flow. As the lymphatic vessel swells with fluid, the smooth muscle around that portion senses the stretch and automatically contracts, pushing the lymph through the valve to the next chamber. The valve prevents backwards flow as the smooth muscle in the chamber contracts to send the lymph through the next valve into the next chamber. This process continues along the entire vessel until the lymph passes through the lymph nodes and into the subclavian vein.
Factors outside of the lymphatic system can affect the rate of lymph flow by assisting in the pumping of lymph through the system. The following external factors can increase lymph flow: contraction of close skeletal muscles, movements made by other parts of the body, nearby arterial pulses, and compression of tissues by items outside of the body. Therefore, during periods of exercise, the lymphatic system is extremely active with a high flow rate.
The terminal end of the lymphatic capillary also has a pump that can affect the rate of lymph flow. When the interstitial fluid pressure is high, the surrounding tissue expands. The anchoring filaments that are attached to the endothelial cells at the terminal end of the lymphatic capillary and to the connective tissue pull the capillary valves open, allowing inward flow of interstitial fluid. Then the internal lymphatic capillary pressure causes the valves to close and the smooth muscle in the first compartment to contract and push the lymph into the next chamber.
Contractile actomyosin filaments are present in the end terminal of the lymphatic vessels. These filaments cause the rhythmic contraction of the terminal end of the lymphatic capillary. Therefore, they contribute to part of the initial pumping of lymph through the system.
High molecular weight proteins are not the only large substances that are absorbed. Micro-organisms such as bacteria can also fit between the endothelial cells of the terminal end of the lymphatic capillary. As this occurs and the bacteria are transported to the next lymph node, the meshwork of the node and sinuses within the node act as a filter, catching and trapping the foreign organisms. Once trapped, micro-organisms can be attacked by the concentrated cells of the immune system. Macrophages may consume disease-causing bacteria, B-lymphocytes may come into contact with the antigens on the surface of the micro-organism and stimulate antibodies, and T-lymphocytes called “killer” cells may attach themselves to the foreign organism and release a substance to destroy the organism. The destructive nature of the killer cells is enhanced by another T-lymphocyte called “helper” cells (T-helper cells also assist B-cells). If this system fails, then microorganisms are not destroyed, resulting in the spread of infection though the lymphatic system and extreme infection possibly leading to death.
Cancer cells that have lost adherence to, and break away from, the primary tumor are collected by the lymphatic system and filtered by latticework within the lymph nodes. Within the lymph node, T-cells release substances called lymphokines (e.g., gamma interferor and interleukin 2) that may help destroy the cancer cells. Doctors use the lymph nodes as one factor of evaluation when staging cancer. In other words, when determining how far the cancer has progressed at the time of diagnosis, the lymph nodes can be dissected to determine if cancer has spread (metastasized) from the original tumor or not. If cancer cells are present in the lymph nodes, then the cancer receives a higher stage and a less optimistic diagnosis. In cancers that metastasize via the lymphatics, the lymph nodes where cancer cells are present are often removed. This is even more common when the lymph nodes in question are adjacent to the tumor, when the lymph nodes are located on the only lymphatic vessel present in the area of the tumor, or if no other lymphatics will be damaged during the removal.
The lymphatic system carries nutrients to cells, then carries waste products away, clearing away infection and helping body fluids stay in balance. It is therefore important to keep the lymphatic system moving and flowing freely. Participating in an exercise regimen is an ideal way to do so.
The lymphatic system has no pump and relies on muscle contraction and expansion to push lymph throughout the body. It is important that individuals engage in exercise to stimulate lymphatic flow. The movement of lymph flow aids in the delivery of nutrients while helping in the removal of excess fluid and wastes. Exercise (including Pilates and yoga) may be movement related, as in walking, swimming, or climbing stairs, or may be more targeted with the use of light weights. Muscle movement associated with deep breathing and abdominal exercises is especially helpful to clear excess fluid from tissues and facilitate the movement of lymphatic fluid. Moderate exercise helps promote lymph flow, while vigorous exercise such as aerobics, greatly increases lymph flow throughout the body.
When the lymphatic system is not working properly, excess fluid and swelling (lymphedema) may occur. Lymphedema is commonly seen in breast cancer survivors, especially if lymph nodes from the axilla are involved. Exercise can significantly reduce limb swelling and promote lymph flow. Cancer survivors who are beginning an exercise regimen should first consult their doctor. When exercising they should begin with flexibility and resistive exercises. Light weights used gently, should be increased in a gradual manner. Regular exercise can stretch muscles and connective tissue, helping maintain range of motion. Individuals who have lyphedema may be directed to wear a compression bandage on the affected area while exercising.
Maintaining a healthy diet also contributes to a healthy lymphatic system. Foods that help support and build a healthy lymphatic system include those high in:
Drinking an adequate amount of water to supply fluids for movement through the vessels is also important.
Since the lymphatic system is responsible for draining excess fluid from tissues and organs, the most common symptom of diseases and disorders of the lymphatic system is swelling. For example, a disease known as elephantiasis that is caused by a filarial worm infestation involves the blockage of the lymphatics. When the lymphatics are blocked, fluid cannot be drained and swelling occurs in the affected areas. Administering ethyl-carbamazine drugs, elevating the area, and wearing a compression stocking can treat elephantiasis.
Tonsillitis is another disease of the lymphatic system. Tonsillitis usually involves a bacterial or viral infection located within the tonsils. The tonsils are swollen, and the patient experiences a fever, sore throat, and difficulty swallowing. This can be treated by the use of antibiotics or through a surgical procedure called a tonsillectomy.
A condition common among individuals following surgery for breast cancer or prostate cancer is lymphedema. It is caused by blockage of lymph vessels or lymph nodes located near the surgical site and can result in swollen arms or legs. If micro-organisms cause the swelling, then antibiotics are used as treatment. If micro-organisms are not the cause, then compression garments and message therapy are used as treatment.
Cancers called lymphosarcomas and cancers of the lymph nodes can affect the lymphatic system. The causes of these cancers are not known, and there is no consensus on what preventative measures can be taken to reduce the risk of developing these cancers. Symptoms of cancers affecting the lymphatic system include loss of appetite, energy, and weight, as well as swelling of the glands. As with many cancers, treatment includes surgical removal followed by adjuvant radiation and chemotherapy.
See also Hydration .
Fauci, Anthony S., et al. Harrison's Principles of Internal Medicine, 19th ed. New York United States: McGraw-Hill Professional, 2015.
Rizzo, Donald C. Introduction to Anatomy and Physiology. Clifton Park: Delmar, 2011.
Wittlinger, Hildegard, et al. Dr. Vodder's Manual Lymph Drainage. Clifton Park: Thieme, 2011.
American Academy of Family Physicians, PO Box 11210, Shawnee Mission, KS, 66207, (913) 906-6000, (800) 274-2237, Fax: (913) 906-6075, email@example.com, http://www.aafp.org .
Lymphatic Education & Research Network, 261 Madison Ave., 9th Fl., New York, NY, 10016, (516) 625-9675, Fax: (516) 625-9410, firstname.lastname@example.org, http://lymphaticnetwork.org .
National Institutes of Health, 9000 Rockville Pike, Bethesda, MD, 20892, (301) 496-4000, NIHinfo@od.nih.gov, http://www.nih.gov/index.html .
National Lymphedema Network, 2288 Fulton St., Ste. 307, Berkeley, CA, 94704, (510) 809-1660, (800) 541-3259, Fax: (510) 809-1699, email@example.com, http://www.lymphnet.org .
Sally C. McFarlane-Parrott
Revised by Laura Jean Cataldo, RN, EdD