Lymphatic System.

Lymphatic System. I INTRODUCTION 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 (see Lymph). 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 (see Capillary). 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. II 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 (see Artery; Vein). 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. III 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 (see Immune System). 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. IV 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 (see Cancer; Hodgkin's Disease); and elephantiasis. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.