The Lymphatic System
The lymphatic system is an extensive network of vessels and nodes that picks up a fluid, called lymph, from the spaces between the cells and empties it into the venous circulation just before it enters the heart. It is a one-way system returning to the bloodstream substances not taken up by the venous capillaries.
The lymphatic system also acts as a filter. It contains many specialized cells, chiefly in the lymph nodes, that trap bacteria and other foreign particles, preventing them from reentering the blood. The lymph nodes play another role in protecting the body against infection because they contain large numbers of lymphocytes and are important sites of lymphocyte formation. Lymphocytes are the cells that manufacture antibodies, and during periods of active antibody formation the lymph nodes often become swollen and tender. See also immunity.
Lymph vessels are found in all vertebrate, or backboned, animals. In most amphibians and reptiles and in some fishes, the lymph is propelled by pulsating lymph hearts. In other vertebrates, including all mammals, lymph movement depends primarily on external forces, such as the action of neighboring skeletal muscles, pulsations of nearby arteries, intestinal contractions, alterations of pressure within the chest during breathing, and the force of gravity. Lymph nodes are found only in mammals; similar structures occur in most young birds.
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With the possible exception of bone, cartilage, the sclera (the white of the eyeball), lymphatic vessels are found in all body tissues. Lymph nodes generally occur in clusters and are most abundant in the neck, armpit, groin, and around the large blood vessels in the abdomen and chest.
The narrowest lymphatic vessels are the capillaries, which originate in the spaces between the cells and converge to form collecting systems of larger vessels. At various points along the inner walls of these vessels are bicuspid (two-flap) valves that prevent the lymph from flowing backward.
The larger lymphatic vessels leading from the lower limbs and pelvis join with others from the intestines and other abdominal organs to form a dilated region called the cisterna chyli. From the cisterna chyli, the largest lymph vessel, the thoracic duct, passes upward through the chest cavity to join the venous circulation. It usually enters the venous circulation at the junction of the left subclavian vein and the left internal jugular vein. Sometimes the thoracic duct consists of several channels that enter the venous circulation through as many as five branches. Before joining the venous circulation, the thoracic duct receives lymph from the left side of the head and neck, the left arm, and the left side of the chest. Lymph vessels from the right side of the head and neck, right arm, and right portion of the chest empty into the right lymphatic duct, which joins the venous circulation at the junction of the right subclavian vein and the right internal jugular vein.
Numerous communications occur between lymph vessels of all sizes. There are also accessory communications between the lymph vessels and the venous circulation. Most of these connections are between the lymphatics and the inferior vena cava, the major vein that carries blood to the heart. These communications are most abundant in the region where the renal veins from the kidneys join the inferior vena cava. The connections between the lymphatics and the venous circulation appear to function as alternate routes of lymph return when there is an obstruction of the major lymph vessels or when there is an increase in the lymph volume or pressure within the lymph vessels.
Lymph nodes are oval or bean-shaped masses of tissue located along the lymph vessels so that the lymph flows through them on its way to the venous circulation. Each node has an indentation, called a hilum, on one side. It is at the hilurn that blood vessels enter and leave the node and that lymphatic vessels leave the node. Lymph vessels leading to the node enter it at various places on the surface opposite the hilum. Covering the entire node is a capsule, and extensions, or trabeculae, from the capsule roughly divide the node into compartments.
The tissue of a lymph node is mostly divided into an outer portion, or cortex, and an inner portion, or medulla-. A cortex is missing only at the hilum. In the cortex are the masses of tightly packed lymphocytes- the cortical nodules, and the areas of lymphocyte production- the germinal centers. In the medulla the masses of lymphocytes form cords called medullary cords.
Between the outer capsule and the cortex is a channel, the peripheral sinus, which receives lymph from the lymphatic vessels entering the node. From this sinus the lymph flows through a number of cortical and medullary sinuses and finally leaves the node through the lymph vessels emerging from the hilum. All the sinuses contain many fine fibers. Attached to these fibers and along the walls of the sinuses are the cells that trap particulate matter.
Formation and Composition of Lymph
During the normal process by which the blood provides the body cells with food materials and removes their wastes, there is continuous exchange of fluids, salts, and nutrients between the arterial capillaries and the spaces between the cells. The balance maintained between the loss of these substances from the arterial capillaries and the return of these materials to the venous capillaries depends on several factors. These include the pressure in the arterial capillaries, the capillary permeability, the concentration of proteins in the fluid, and local tissue conditions.
Large molecules that leave the arterial capillaries or that are produced or released by the cells do not enter the venous capillaries but are taken up by the lymphatic system. Thus, the lymph drained from different parts of the body varies considerably in its composition. For example, lymph from the intestines, called chyle, has a high fat content, especially after a meal. The end products of protein and carbohydrate digestion are absorbed directly into the bloodstream, but the end products of fat digestion enter small lymphatic vessels, called lacteals, which carry them to the cisterna chyli.
Diseases and Disorders
The lymphatic system, like other parts of the body, is subject to a variety of diseases and disorders. A number of bacteria and other infectious agents can cause inflammations of the lymph vessels (lymphangitis) or the lymph nodes (lymphadenitis). Obstructions of the lymph vessels, as caused by scar tissue or tumor cells, as well as abnormalities interfering with the normal uptake and transport of lymph, cause an accumulation of fluid and protein within the affected tissue. As a result, the tissue becomes swollen, a condition known as lymphedema.
The lymphatic system may also develop malignant tumors. Hodgkin's disease is a malignant tumor of the lymph nodes. The lymphatic system may also spread malignant cells from cancerous organs to healthy ones.