lymphatic system(redirected from Lymph system)
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lymphatic system (lĭmfătˈĭk), network of vessels carrying lymph, or tissue-cleansing fluid, from the tissues into the veins of the circulatory system. The lymphatic system functions along with the circulatory system in absorbing nutrients from the small intestines. A large portion of digested fats are absorbed via the lymphatic capillaries. Like the blood circulatory system, the lymphatic system is composed of fine capillaries that lie adjacent to the blood vessels. These merge into larger tributaries known as trunks, and these in turn merge into two still larger vessels called ducts. The thoracic and right lymphatic ducts empty into the venous system in the region of the collarbones. Lymph, a colorless fluid whose composition is similar to that of blood except that it does not contain red blood cells or platelets, and contains considerably less protein, is continuously passing through the walls of the capillaries. It transports nutrients to the cells and collects waste products. Most of the lymph returns to the venous capillaries; however, a small amount (about 10%) enters the terminal lymphatic capillaries and is returned to the blood via the lymphatic system. The fluid that flows through the lymphatic system is functionally important because it contains substances having large molecules (such as proteins and bacteria) that cannot enter the small pores of the venous capillaries. Along the lymphatic network in certain areas of the body (neck, armpit, groin, abdomen, chest) are small reservoirs, the lymph nodes, which collect bacteria and other deleterious agents from the lymph which passes through them, and act as a barrier against the entrance of these substances into the bloodstream. In a disease state, therefore, the lymph nodes may become filled with harmful material to the degree where they can be seen or felt; therefore, enlarged lymph nodes are of diagnostic importance. Such enlargement of the lymph nodes can be a warning sign of various kinds of cancer, including breast cancer and Hodgkin's disease. In cases where a cancerous growth has developed, removal of lymph nodes may help to prevent its further spread. However, such a procedure also slows the flow of lymph and may thus render some of the body vulnerable to infection. See also lymphoma, non-Hodgkin's.
A system of vessels in the vertebrate body, beginning in a network of exceedingly thin-walled capillaries in almost all the organs and tissues except the brain and bones. This network is drained by larger channels, mostly coursing along the veins and eventually joining to form a large vessel, the thoracic duct, which runs beside the spinal column to enter the left subclavian vein at the base of the neck. The lymph fluid originates in the tissue spaces by filtration from the blood capillaries. While in the lymphatic capillaries it is clear and watery. However, at intervals along the larger lymphatic vessels, the lymph passes through spongelike lymph nodes, where it receives great numbers of cells, the lymphocytes, and becomes turbid.
The lymph nodes of mammals vary in number, size, form, and structure in different species. The amount of connective tissue of the lymph nodes, that is, the degree of development of the capsule and trabeculae, also varies in different mammals. Other lymphoid organs include the tonsils, thymus gland, and spleen, and in certain classes and groups of animals, structures which are confined to such groups, for instance, the bursa of Fabricius in the birds, a diverticulum from the lower end of the alimentary canal. See Spleen, Thymus gland, Tonsil
The functions of the lymphatics are to remove particulate materials such as molecular proteins and bacteria from the tissues; to transport fat from the intestine to the blood; to supply the blood with lymphocytes; to remove excess fluid; also to return to the bloodstream the protein which has escaped from the blood capillaries. Basically, the composition of lymph closely resembles that of the plasma; lymph contains all of the types of protein found in plasma, but in lower concentration. The composition of lymph varies to some extent from one part of the body to another. Thus, the lymph from the liver contains more protein than that from the skin.
The lymph nodes serve as filtering-out places for foreign particles, including microorganisms, because the lymph comes into intimate contact with the many phagocytic cells of the sinusoids. These macrophages are of both the fixed and free wandering types. In addition to the phagocytic function, lymphoid tissue produces antibodies, although the actual process of antibody formation is not well understood. See Cellular immunology, Phagocytosis
the complex of capillaries and other vessels that collect lymph from the tissues and organs and carry it to the venous system in vertebrates and man. The system also comprises the lymph nodes (“glands”), located along the course of the vessels. The lymphatic system returns to the circulatory system the fluid that filters from the capillaries into the tissues, transmits nutrients absorbed in the intestine, and plays a protective role by filtering off lymph in the lymph nodes. From the tissues, the lymph infiltrates the lymphatic capillaries, which originate with blind ends in the tissues and collect in small (and then larger) vessels and trunks. These flow into the veins (as a rule, in the areas of the body with the lowest blood pressure).
In accordance with the low pressure of lymph, all lymphatic vessels are very thin-walled. In fish, amphibians, and reptiles, these vessels form thin-walled dilations called lymphatic sinuses (for example, the subcutaneous sinuses of the frog). In amniotes and man, there are thick-walled dilations (called lymphatic cisterns) and lymph nodes, in which the vessels lose their own walls and turn into sinuses of the nodes. In man, as in all mammals, there is a large number of lymph nodes, located singly or in groups, mainly at the base of the tongue, in the vicinity of the pharynx, neck, and bronchi, in the axillary and inguinal areas, and, especially, in the mesentery and walls of the intestines.
The movement of lymph is accomplished by periodic pressure put on the vessels of the skeletal muscles and viscera, by the pulsation of the vessels themselves, and by the pressure of the fluid that is continuously entering the capillaries. In fish, amphibians, reptiles, and birds, lymph is actively pumped over by lymph hearts; in mammals, this is fostered by the suction effect of the thoracic cavity upon inhalation. In birds, mammals, and man, the vessels have pocketlike valves that allow the lymph to flow in one direction only.
The lymphatic system became distinct from the venous system during the course of evolution. In cyclostomes and sharks, the lymphatic system carries both lymph and blood. Extensively linked with the venous system, it is called the hemolymphatic system to distinguish it from the true lymphatic system of more highly organized vertebrates. The superficial lateral veins of the trunk in sharks became, in other fishes, the superficial lymphatic vessels, which pass along the sides of the body and into the tail and fins. In addition, fish have deep ducts that accompany the main circulatory trunks and discharge anteriorly into the anterior cardinal veins and posteriorly into the caudal vein (where, in some fishes, the lymphatic heart develops).
The lymphatic system of amphibians is similar to that of fish, but the development of lymph hearts (from two pairs in acaudates to 20 in caudates and 100 in apods) makes lymph circulation more active. Highly developed subcutaneous vessels and sinuses ensure against possible dehydration. In reptiles, lymph from the intestinal vessels and posterior extremities is collected in the lymphatic cistern, from which flow paired thoracic ducts. These, together with vessels from other parts of the body, flow into the jugular veins. The lymph hearts are preserved only at the base of the tail, and there are no lymph nodes (with the exception of one in the mesentery of crocodiles).
There are few nodes in birds; in some species, a lymph heart that pumps lymph into the veins of the kidneys is preserved at the base of the tail. Paired thoracic ducts open into the anterior venae cavae. Valves appear in the vessels for the first time in birds.
In mammals, the entry of lymph into the circulatory system is concentrated anteriorly: paired thoracic ducts originate at the chyle cistern of the mammary sinus (where lymph from the intestine and posterior half of the body is collected), receiving vessels from the rest of the trunk and flowing into the superior venae cavae.
In man only the left thoracic duct, located asymmetrically, is preserved, beginning at the chyle cistern and discharging into the left angulus venosus (the junction of the left subclavian and interior jugular veins). Lymph from the right side of the upper half of the body flows into the circulatory system through the right lymphatic duct.
REFERENCESZhdanov, D. A. Funktsional’naia anatomiia limfaticheskoi sistemy. Gorky, 1940.
Zhdanov, D. A. Obshchaia anatomiia i fiziologiia limfaticheskoi sistemy. [Leningrad] 1952.
Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.