Difference between revisions of "Lymphatic system" - New World Encyclopedia

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The lymphatic system acts as a secondary circulatory system in the body. It drains fluid back into the venous circulation of the circulatory system from the tissues in order to prevent dehydration, it fights infections, recycles plasma proteins, and filters lymph. The lymphatic system consists of a complex capillary network which carries the lymph, a system of collecting vessels which drain the lymph back into the bloodstream, and of lymph glands, or nodes, that filter the lymph as it passes through.

Lymph, which is carried by the lymphatic system, originates as blood plasma lost from the circulatory system due to hydrostatic pressure. This pressure forces plasma to leak out of the blood vessel into the surrounding tissues. Once within the lymphatic system, this clear fluid is called lymph and has a very similiar composition to the original interstitial fluid.

The human lymphatic system

Unlike the circulatory system, the lymphatic system is not closed and has no central pump like the heart. It is a one-way system with fluid traveling from the interstitial space back to the blood. The capillaries of the lymphatic system, which consist of a single layer of endothelium, lie close to all blood capillaries (minus those in the kidney and central nervous system).

Lymphatic Circulation

Fluid in blood vessels experience osmotic as well as hydrostatic pressures. The former favors absorption into the vessel while the latter favors net filtration of fluids, mainly plasma, into the interstitial space. When filtration is favored, plasma leaks out of the vessel and into the surrounding tissues. From there, it enters the lymphatic system through the process of diffusion. Through the use of fibers, the walls of lymph capillaries are joined to the connective tissues nearby and are held open. Consequently, large gaps between the cells are formed, which allow fluid, interstitial proteins, and other matter such as bacteria to flow into the lymph capillary (Silverthorn 2004).

Once in the lymphatic system circulation, the lymph moves slowly and under low pressure due mostly to the milking action of skeletal muscles. Like veins in the circulatory system, lymph vessels have one-way valves and depend mainly on the movement of skeletal muscles to squeeze fluid through them. Rhythmic contraction of the vessel walls may also help draw fluid into the lymphatic capillaries. This 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); these ducts drain into the circulatory system at the right and left subclavian veins, which are located under the clavicle, or collar bone.

The skeletal muscle pump, discussed above, is crucial for lymph movement as illustrated by an immobilized limb, for example. Often times immobilized or injured limbs swell due to the accumulation of fluid in the interstitial space. This condition, known as edema, can be lessened by elevating the limb above the level of the heart (gravity will aid the flow of lymph back to the blood).

Lymph vessels found in the lining of the gastrointestinal tract play an important role in digestion. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain into the liver for processing, fats 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. The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation.

Secondary Lymphoid Organs

As lymph travels through the lymphatic system, it encounters the secondary lymphoid organs along the way: the spleen, lymph nodes, and accessory lymphoid tissues. The lymph nodes, which are bean- shaped glands, consist of tissues containing immunologically active cells, such as lymphocytes and macrophages (Silverthorn 2004). They filter the lymph as it passes through to make sure no pathogens, or foreign matter, are allowed to return to the venous circulation of the circulatory system.

The spleen, lymph nodes, and accessory lymphoid tissue are the secondary lymphoid organs. These organs contain a scaffolding that support circulating B- and T-lymphocytes and other immune cells like macrophages and dendritic cells. 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. In the lymph nodes, the macrophages and dendritic cells phagocytose the antigens, process them, and present the antigens to lymphocytes, which can then start producing antibodies or serve as memory cells to recognize the antigens again in the future.

External links

The Lymphatic System (MedicalEngineer.co.uk)
Lymphatic System Overview (innerbody.com)

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-The '''lymphatic system''' acts as a secondary circulatory system in the [[body]]. It drains fluid back into the circulatory system (specifically the venous circulation) from the [[tissues]] in order to prevent dehydration, it fights infections, recycles plasma proteins, and filters '''lymph'''. The lymphatic system consists of a complex capillary network which carries the lymph, a system of collecting vessels which drain the lymph back into the bloodstream, and of lymph glands, or nodes, that filter the lymph as it passes through.
+The '''lymphatic system''' acts as a secondary circulatory system in the [[body]]. It drains fluid back into the venous circulation of the circulatory system from the [[tissues]] in order to prevent dehydration, it fights infections, recycles plasma proteins, and filters '''lymph'''. The lymphatic system consists of a complex capillary network which carries the lymph, a system of collecting vessels which drain the lymph back into the bloodstream, and of lymph glands, or nodes, that filter the lymph as it passes through.
 Lymph, which is carried by the lymphatic system, originates as [[blood]] plasma lost from the [[circulatory system]] due to hydrostatic pressure. This pressure forces plasma to leak out of the blood vessel into the surrounding tissues. Once within the lymphatic system, this clear fluid is called lymph and has a very similiar composition to the original [[interstitial fluid]]. [[Image:lymphatic_system.gif|framed|200px|The human lymphatic system]]
@@ Line 11: / Line 11: @@
 Once in the lymphatic system circulation, the lymph moves slowly and under low pressure due mostly to the milking action of skeletal [[muscle]]s. Like [[vein]]s in the circulatory system, lymph vessels have one-way valves and depend mainly on the movement of skeletal muscles to squeeze fluid through them. Rhythmic contraction of the vessel walls may also help draw fluid into the lymphatic capillaries. This 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); these ducts drain into the circulatory system at the right and left subclavian veins, which are located under the clavicle, or collar bone.
-The skeletal muscle pump, discussed above, is crucial for lymph movement. An immobilized limb, for example, portrays the importance of the skeletal muscles in transporting lymph. Often times immobilized or injured limbs swell due to the accumulation of fluid in the interstitial space. This condition, known as ''edema'', can be lessened by elevating the limb above the level of the [[heart]] (gravity will aid the flow of lymph back to the blood).
+The skeletal muscle pump, discussed above, is crucial for lymph movement as illustrated by an immobilized limb, for example. Often times immobilized or injured limbs swell due to the accumulation of fluid in the interstitial space. This condition, known as ''edema'', can be lessened by elevating the limb above the level of the [[heart]] (gravity will aid the flow of lymph back to the blood).
 Lymph vessels found in the lining of the [[gastrointestinal tract]] play an important role in digestion. While most other nutrients absorbed by the [[small intestine]] are passed on to the portal venous system to drain into the [[liver]] for processing, [[fat]]s 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''. The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation.
 ==Secondary Lymphoid Organs==