Lymphatic System, common name for the circulatory vessels or ducts in which the fluid bathing the tissue cells of vertebrates is collected and carried to join the bloodstream proper. The lymphatic system is of primary importance in transporting digested fat from the intestine to the bloodstream; in removing and destroying toxic substances; and in resisting the spread of disease throughout the body.The portions of the lymphatic system that collect the tissue fluids are known as lymphatic capillaries and are similar in structure to ordinary capillaries. The lymphatic capillaries that pick up digested fat in the villi of the intestine are known as lacteals. The lymphatic capillaries are more permeable than ordinary capillaries and allow passage of larger particles than would ordinarily pass through capillary walls; large-molecule proteins, produced as a result of tissue breakdown, pass into the lymphatics for transport away from the tissues.
LYMPHATIC CAPILLARIES
These vessels are found in all body tissues except the central nervous system, which has a circulatory system known as the cerebrospinal system. The lymphatic capillaries run together to form larger ducts that intertwine about the arteries and veins. The lymph in these larger ducts, which are similar to thin, dilated veins, is moved along by the muscular movements of the body as a whole; it is prevented from moving back through the ducts by valves located along them at intervals. The ducts from the lower limbs and abdomen come together at the dorsal left side of the body to form a channel, known as the cisterna chyli, that gives rise to the chief lymphatic vessel of the body, the thoracic duct. This vessel receives lymph from the left side of the thorax, the left arm, and the left side of the head and neck; it empties into the junction of the left jugular and left subclavian veins. Another, smaller vessel, known as the right lymphatic duct, receives lymph from the right side of the thorax, the right arm, and the right side of the head and neck and empties its contents into the right subclavian vein.
LYMPH NODES
Along the course of the lymphatic vessels are situated the lymph nodes, more commonly called the lymph glands. These nodes are bean-shaped organs containing large numbers of leukocytes, embedded in a network of connective tissue. All the lymph being returned along the lymphatics to the bloodstream must pass through several of these nodes, which filter out infectious and toxic material and destroy it. The nodes serve as a center for the production of phagocytes, which engulf bacteria and poisonous substances. During the course of any infection, the nodes become enlarged because of the large number of phagocytes being produced; these nodes are often painful and inflamed. The swollen glands most often observed are located on the neck, in the armpit, and in the groin. Certain malignant tumors tend to “travel” along the lymphatics; surgical removal of all nodes that are suspected of being involved in the spread of such malignancies is an accepted therapeutic procedure.
OTHER ORGANS
In addition to the lymph nodes that occur in the lymphatic vessels, several organs, composed of similar tissue, are included in the lymphatic system. The largest and most important of these organs is the spleen.
Embryologically, the lymphatic vessels arise as outbuddings from several veins, especially from the internal jugular and iliac veins. The buds spread throughout the body and separate from the venous system at many points.
Among the abnormal conditions affecting the lymphatic system are inflammation of the lymphatics or of the lymph nodes, seen in infections; tuberculosis of the lymph nodes; malignancies in the lymphatic system (Cancer; Hodgkin's Disease); and elephantiasis.
DISEASES OF THE LYMPHATIC SYSTEM
Filariasis : a group of infectious disorders caused by threadlike nematodes of the superfamily Filarioidea, that invade the subcutaneous tissues and lymphatics of mammals, producing reactions varying from acute inflammation to chronic scarring. In the form of heartworm, it may be fatal to dogs and other mammals.
In the human body the female nematode gives birth to elongated embryos, the microfilariae, which migrate through the peripheral blood and skin, from which they are taken by bloodsucking insects. Within the insect carrier, the microfilariae grow into motile, infective larvae that, at the insect's next blood meal, are introduced into the human host, where they reach maturity in about a year. The term filariasis is commonly used to designate bancroftian filariasis, caused by Wuchereria bancrofti, organisms that are widely distributed in tropical and subtropical regions of the world and are transmitted to man by mosquitoes, usually Culex fatigans. The nematode lives principally in the lymph nodes and lymph vessels, notably those draining the legs and genital area, where the adult worms induce allergic reactions in the sensitized tissues.
The initial inflammatory stage is characterized by granulomatous lesions, swelling, and impaired circulation; this stage is followed by enlargement of the lymph nodes and dilation of the lymph channels, which, over the years, harden and become infiltrated and clogged with fibrous tissue elements, resulting in some of the untreated cases in the condition known as elephantiasis, which is typically associated with the gross expansion of the tissues of the legs and scrotum. The most effective therapeutic drugs are diethylcarbamazine and sodium caparsolate, which kill the adult worms and microfilariae.The form of filariasis known as filariasis malayi closely resembles bancroftian filariasis in its symptoms and pathological changes; it is caused by Brugia malayi, found chiefly in the Far East. Onchocerciasis (river blindness) is caused by Onchocerca volvulus, which is transmitted to man by flies of the genus Simulium, which breed along fast-moving streams; the condition is widespread in southern Mexico and Guatemala and is common in Central Africa. Characteristic lesions are nodules beneath the skin, usually in the head region; the infection may also invade the eyes, causing blindness in about 5 percent of the infected individuals. Treatment consists of the surgical excision of the nodules and the administration of chemotherapeutics. Loiasis, prevalent in West and Central Africa, especially along the Congo River, is caused by Loa loa and transmitted by flies of the genus Chrysops. It is characterized by transient areas of allergic inflammation in the tissues beneath the skin, called calabar swellings; adult worms may sometimes be visible beneath the conjunctiva (the delicate membrane lining the eyelids and covering the exposed surface of the eyeball). Loiasis produces irritation but seldom permanent damage. Treatment includes surgical removal of the worms from the conjunctiva and drug therapy. Other forms of filariasis are caused by Acanthocheilonema perstans and Mansonella ozzardi and are not in most cases associated with specific symptoms. The prevention of filariasis relies heavily on insecticides and insect repellents.
Hodgkin disease : an uncommon cancer of the lymphatic system (malignant lymphoma) that usually strikes young adults and people 55 years of age or older. Most patients can be cured if the disease is detected in its early stages, but even those with advanced Hodgkin disease have a significant chance of recovery. The overall cure rate is approximately 75 percent.
In its early stages the disease is characterized by local, painless swelling of one or more lymph nodes and sometimes by swelling of the spleen, liver, or other organs. In addition to swollen lymph nodes, symptoms may include fever and itching followed later by weight loss and fatigue. A microscopic examination of affected tissue, usually obtained from a lymph node, is required to confirm diagnosis.
The cause of Hodgkin disease remains unknown, but numerous infectious agents, including bacteria, protozoa, and viruses, have been suggested. Previous infection with the Epstein-Barr virus, the causative agent of mononucleosis, has been linked to many cases of Hodgkin disease. Hodgkin disease tumours develop from B lymphocytes. Treatment consists of chemotherapy, radiation, or a combination of both, depending on the stage of development of the disease.