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The lymphatic system is a complex network of lymphoid organs, lymph nodes, lymph ducts, lymphatic tissues, lymph capillaries and lymph vessels that produce and transport lymph fluid from tissues to the circulatory system. The lymphatic system is a major component of the immune system.|
The lymphatic system has three interrelated functions: (1) removal of excess fluids from body tissues, (2) absorption of fatty acids and subsequent transport of fat, as chyle, to the circulatory system and, (3) production of immune cells such as lymphocytes (e.g. antibody producing plasma cells) and monocytes.
Lymph originates as blood plasma that leaks from the capillaries of the circulatory system, becoming interstitial fluid, and filling the space between individual cells of tissue. Plasma is forced out of the capillaries (called filtration) and forced back in (called absorption) due to interactions of hydrostatic pressure (favoring movement out of the capillaries) and oncotic pressure (favoring movement into the capillaries). While out of the capillaries, the fluid mixes with the interstitial fluid, causing a gradual increase in the volume of fluid. Most of the fluid is returned to the capillaries. The proportion of interstitial fluid that is returned to the circulatory system by osmosis is about 90% of the former plasma, with about 10% accumulating as overfill. The excess interstitial fluid is collected by the lymphatic system by diffusion into lymph capillaries, and is processed by lymph nodes prior to being returned to the circulatory system. Once within the lymphatic system the fluid is called lymph, and has almost the same composition as the original interstitial fluid.
Unlike the blood system, the lymphatic system is not closed and has no central pump. Lymph movement occurs slowly with low pressure due to peristalsis, valves, and the milking action of skeletal muscles. Like veins, lymph travels through vessels in one way only, due to semilunar valves. This depends mainly on the movement of skeletal muscles to squeeze fluid through them, especially near the joints. Rhythmic contraction of the vessel walls through movements may also help draw fluid into the smallest lymphatic vessels, capillaries. Tight clothing can restrict this, thus reducing the removal of wastes and allowing them to accumulate. If tissue fluid builds up the tissue will swell; this is called oedema. As the circular path through the body's system continues, the fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body); both ducts drain into the circulatory system at the right and left subclavian veins. The system collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or bacteria. This is known as a secondary circulatory system.
Function of the fatty acid transport system
Lymph vessels, called lacteals, are present in the lining of the gastrointestinal tract. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain, via the portal vein, into the liver for processing, fats (lipids) are passed on to the lymphatic system, to be transported to the blood circulation via the thoracic duct. The enriched lymph originating in the lymphatics of the small intestine is called chyle (not chyme). The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation. The lymph system is a one-way system, transporting interstitial fluid back to blood.
The thymus, spleen, lymph nodes, peyer's patches, tonsils, vermiform appendix, and red bone marrow are accessory lymphoid tissues that comprise the lymphoid organs. These organs contain a scaffolding that support circulating B- and T-lymphocytes and other immune cells like macrophages and dendritic cells. Another sub-component of the lymphatic system is the reticuloendothelial system. When micro-organisms invade the body or the body encounters other antigens (such as pollen), the antigens are transported from the tissue to the lymph. The lymph is carried in the lymph vessels to regional lymph nodes. The lymph nodes filter the lymph fluid and remove foreign material, such as bacteria and cancer cells. Specialized cells called macrophages and dendritic cells phagocytose pathogens, process them, and present antigens to lymphocytes. When these pathogens are recognized, the lymph nodes will enlarge as additional immune cells are produced to help fight the infection.
In elephantiasis, infection of the lymphatic vessels cause a thickening of the skin and enlargement of underlying tissues, especially in the legs and genitals. It is most commonly caused by a parasitic disease known as lymphatic filariasis.
Lymphedema also causes abnormal swelling, especially in the appendages (though the face, neck, and abdomen can also be affected). It occurs if the lymphatic system is damaged, or underdeveloped in some way. An estimated 170 million suffer with the disorder. There are three stages:
Stage 1: Pressing the swollen limb leaves a pit that takes a while to fill back in. Because there is little fibrosis (hardening) it is often reversible. Elevation reduces swelling.
Stage 2: Pressure does not leave a pit. Elevation does not help. If left untreated, the limb becomes fibrotic.
Stage 3: This stage of lymphedema is often called elephantiasis. It is generally only in the legs after lymphema that has gone long untreated. While treatment can help a little, it is not reversible.
In prenatal development, the lymphatic system begins to develop by the end of the fifth week of gestation. Six primary lymph sacs are formed: two jugular, two iliac, one retroperitoneal and one cisterna chyli. Numerous channels connect the sacs with each other. Eventually, the thoracic duct is formed and lymphatic vessels replace the lymph sacs.