lymphatic system

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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 also Spleen; 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 also Cellular immunology; Phagocytosis.

This comprises a body-wide network of branching lymphatic vessels (carrying the fluid lymph) ; the lymph nodes and other lymphoid tissue; the lymphocytes (in the circulating blood) ; the spleen, the bone marrow, and, in early life, the thymus.

That blood circulates is common knowledge. The less dramatic circulation of lymph is less well known, but although its interruption has less immediately hazardous consequences, it is physiologically essential. Also, the lymphoid tissue is a vital component of the body's immune system.

All blood capillaries are to some extent leaky, though the leakiness varies between different organs and tissues. The hydrostatic pressure inside the capillaries is greater than that in the tissues, pushing fluid out. The fluid that leaks out into the interstices among the cells is a little of the watery part of the blood plasma, with all the substances it contains except for the larger proteins. Conversely, some water moves into the capillaries because of the osmotic pull of those proteins. Normally, there is a small net loss from the blood. This fluid movement has the effect of refreshing the tissue fluid in the immediate environment of the cells. Fluid does not accumulate in the tissues, but moves away by entering the blind ends of microscopic lymphatic channels, which are present in all organs and tissues except the central nervous system. These vessels drain into progressively larger ones, and they have valves which maintain flow towards the chest. En route, lymph vessels encounter lymph nodes or other lymphoid tissue such as the patches which lie behind the lining of the large intestine, and the tonsils and adenoids at the back of the throat and nose. The lymph passing through these is exposed to phagocytes which pick up any foreign material, notably bacteria, and take part in the processes of the immune system mediated by the lymphocytes which populate the lymphoid tissue. Thus any invader which gets further than the point of entry, and travels in the lymph, will be ‘challenged’ at the first lymph node it reaches; for this reason an infection for example in a finger may give rise to an inflamed lymph node at the elbow, or if it gets past there, in the armpit; or a sore throat or mouth ulcer can cause tender swollen lymph nodes in the neck. The lymphoid tissue in the wall of the large intestine performs a similar function for any bacterial or other invasion from the faeces.

The lymph nodes are outposts of the immune system, well placed to challenge bacterial invaders. Cancer cells gain access to lymphatic vessels in tumour tissue, and spread by this route; further spread may be initially forestalled at the lymph nodes. For this reason, surgical removal of a malignant tumour may also involve a clearance of the lymph nodes to which its vessels drain. If lymph vessels draining a part of the body (such as an arm) are blocked or removed, there will inevitably be a collection of excess fluid in the catchment area of those vessels: this is one cause of oedema.

The lymph from the whole body (except the central nervous system) finally drains into vessels at the base of the neck (the major one is the thoracic duct) and flow through these into the venous blood stream, on its way to the heart. Thus the fluid lost from the blood capillaries is recycled. Overall, the rate of flow is about 4 litres in 24 hours.

The lymph drained from the small intestine has an additional function: it carries the fats absorbed from the food. Because of this, the lymph in the thoracic duct (known as chyle) has a high fat content; it also has a high protein content because although a very small fraction of the plasma proteins escape from blood capillaries in the tissues, the amount collected from the whole body is significant.

Illustration

Lymphatic system
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— Sheila Jennett

See also blood; immune system; oedema; thymus.

A system of blind-ending vessels that drain excess fluid from the extracellular spaces. The lymphatic system contains lymph nodes, and produces macrophages and lymphocytes. Groups of nodes occur in most parts of the body, but particularly in the groin, armpits, and behind the ears and neck. They often become inflamed during an infection.

The network of small vessels and tissue spaces that move lymph throughout the body. The lymphatic system has several functions, including filtering out harmful bacteria; manufacturing white blood cells (white blood cells are produced by the lymph nodes); distributing nutrients to the cells of the body; helping to maintain the body's fluid balance by draining off excess fluids so that tissues do not swell; and assisting in the digestion of fats.

A vast, complex network of capillaries, thin vessels, valves, ducts, nodes, and organs that helps to protect and maintain the internal fluid environment of the entire body by producing, filtering, and conveying lymph and by producing various blood cells.

• Heart and Circulatory and Lymphatic Systems - lymphatic system: system of small vessels by which lymph is circulated through body