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 cerebral spinal 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.
Inside the lymph vessels are valves that prevent the backflow of lymph, a general name for the slightly opalescent fluid picked up by the lymphatics. Surrounding the lymphatics are smooth muscles that contract involuntarily to assist in the movement of lymph through the system. The lymphatic capillaries converge into larger lymph vessels. The larger lymph vessels pass through swellings called lymph nodes and then empty into one of two large lymph ducts. The lymph ducts empty into the venous circulatory system through either the right or left subclavian veins. Lymph from the right side of the head, arm and chest empties into the right subclavian vein. Lymph collected from the lower part of the body, and lymph from the left side of the head, arm and chest empties into the left subclavian vein. Both subclavian veins are located within the thorax underneath the clavicles, the thin bones located on the top part of the chest.
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 lymphfilled sinuses that have macrophages and groupings of lymphocytes that produce antibodies.
As mentioned, 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, which is 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.
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 (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 the 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 also affect the rate of lymph flow by assisting in the pumping of lymph through the system. The following eternal 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 and the flow rate is high.
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 also 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.
Interstitial space—The spaces found within organs and tissues.
Lymph—The slightly opalescent fluid found within the lymphatic system.
Lymph nodes—Bean shaped swellings along the lymphatic vessels that contain macrophages and lymphocytes.
Lymphatics—The system of lymphatic vessels.
T-lymphocytes (T-cells)—A type of white blood cell that originates in the thymus and attaches themselves to foreign organisms, secreting lymphokines that kill the foreign organisms.
Tonsil—A collection of lymphocytes that form a mass in the back of the pharynx.
Role in human health
The lymphatic system has a variety of roles in human health 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 the 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, the 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. With out 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.
High molecular weight proteins are not the only large substances that are absorbed. Microorganisms 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 with in the node act as a filter, catching and trapping the foreign organisms. Once trapped, microorganisms can be attacked by the concentrated cells of the immune system. Macrophages may consume disease-causing bacteria, Blymphocytes may come into contact with the antigens on the surface of the microorganism and stimulate antibodies, and T-lymphocytes called "killer" cells that 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 lymphokine (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 determining the stage of the 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 (metastisized) 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.
Common diseases and disorders
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, which 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
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 microorganisms cause the swelling, then antibiotics are used as treatment. If microorganisms are not the cause, then compression garments and message therapy are used as treatment.
There are also cancers called lymphosarcomas and cancers of the lymph nodes that can affect the lymphatic system. The causes of these cancers are not known and there is not a 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.
Braunwald, Eugene, et al. Harrison's Principles of Internal Medicine, 15th ed. New York: McGraw-Hill, 2001.
Lee, Richard G. M.D., et al. Wintrob's Clinical Hematology, 10th ed. Philadelphia: Lippincott Williams & Wilkins, 1999.
Vander, Arthur. Human Physiology: The Mechanisms of Body Function, Seventh ed. New York: WBC McGraw-Hill, 1998.
Lymphatic Research Foundation. 941 N.E. 19th Avenue, Suite 305, Ft. Lauderdale, Florida, 33304-3071. (954)525-3510. <http://www.lymphaticresearch.org/>.
Sally C. McFarlane-Parrott