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Lymphatic Vessels, Lymph Circulation and Functions of Lymphatic System

Despite its different composition from plasma, lymphoid fluid is a clear, watery fluid with comparable characteristics to interstitial fluid.
Despite its different composition from plasma, lymphoid fluid is a clear, watery fluid with comparable characteristics to interstitial fluid, except for plasma proteins. Proteins from the plasma are transported back to the bloodstream by the lymphatic system. Larger particles, including bacteria, or cells from damaged tissues are also taken away by the lymph nodes, which filter them out and destroy them. There are lymphocytes in lymph that circulate within the lymphatic system, patrolling various sections of the body. The lymph (now called chyle), which is formed in the lacteals of the small intestine, appears milky due to fats that have been absorbed into the lymphatic system.

Lymphatic vessels
1. Lymph capillaries
Originally, they are blind-end tubes found in interstitial spaces. As a result of their single-layer structure, which is characteristic of blood capillaries, their walls are more permeable to interstitial fluid, proteins, and cellular debris. Lymph vessels are formed from the tiny capillaries that join up. There are lymphatic vessels in nearly all tissues, but notable exceptions are the central nervous system, corneas of the eyes, bones, and the thinnest parts of the skin.

2. Larger lymph vessels
Lymph vessels often run alongside the vessels and arteries of the chest and abdomen. Similarly, the walls of small veins have similar thicknesses and consist of thick fibrous coverings, smooth muscles, and elastic tissue in the middle, and an endothelial lining at the inner end. There are numerous cup-shaped valves inside lymph vessels that ensure the lymph material reaches the thorax only in one direction. The large lymph vessels do not contain a mechanical pump such as the heart, but instead, contain a muscle layer that is capable of contracting rhythmically (the lymphatic pump). Lymph vessels are also compelled by activities in adjacent structures, such as muscles contracting and large arteries pulsating regularly. Lymph is propelled along by this process of 'milking' the lymph vessel wall. Lymph vessels combine and grow bigger, forming thoracic and right lymphatic ducts. Lymph flows into the subclavian veins.

Lymph circulation
The lymphatic system functions as a drainage system for the body by leaking blood plasma into tissues through thin capillaries. These nutrients contribute to tissue cell health by supplying oxygen, glucose, amino acids, etc. Interstitial fluid, however, is excluded from tissue cells, while extracellular fluid is retained. Even though most of this fluid flowed back into the bloodstream right away, some particulate matter and fluid remain behind. Lymphatic vessels return fluid and materials to the bloodstream through the lymphatic system, and so prevent an imbalance of fluid that could cause death to the organism.

Lymphatic capillaries are the tiny vessels that conduct fluid and proteins from the tissues into the bloodstream. All tissues are infused with this fluid and protein. Among the lungs, gut, genitourinary system, and dermis of the skin, only a few regions lack lymphatic capillaries, whereas other areas, such as the epidermis of the skin, are densely packed with these vessels. The extracellular fluid, called lymph, drains into larger vessels within the lymphatic system, called lymphatic vessels. A pair of lymphatic trunks form when these vessels converge connected to veins at the base of the neck by lymphatic trunks. Until recently, most of the body's lymphatic fluid was directly drained into the bloodstream by the right lymphatic duct. Blood is drained from throughout the body through the left subclavian vein via the thoracic duct. Contractions of muscles carry lymph fluid along the vessels, while valves prevent the flow of lymph backward. These nodes filter blood and remove microorganisms such as viruses from the lymph flowing through them. Lymph organs surround the lymphatic vessels and punctuate them at intervals.

A network of open-ended lymphatic capillaries collects interstitial fluid in all parenchymal organs, except for the brain. A lymphatic endothelial layer forms the capillaries, which are supported by a sparse basement membrane. Intercellular junctions are specialized connections between endothelial cells, which contain platelet endothelial cell adhesion molecule 1 (PECAM1) and vascular endothelial cadherin (VE-cadherin) and this allows proteins, fluids, macromolecules, small molecules, and immune cells to circulate. A gradually larger number of lymphatic collectors form from the lymphatic capillaries. An organized basal membrane in the collectors is made up of fibroblasts, lymphatic muscle cells, and connective tissues. One layer of lymphatic endothelial cells forms the collectors. As well as maintaining a basic vessel tone and causing the lymph to flow forward toward the draining lymph node, smooth and striatal muscle cells play an important role in the function of lymph drainage. As lymph flows in both directions, unidirectional valves, located along with the collectors, open and close simultaneously with the contraction of vessels, maintaining the directional flow.

Connective tissue overlies lymphatic endothelial cells to form the valves of the bicuspid heart. In the lymphatic system, the segment of lymphatic collectors located between the two segments of valves is known as lymphangion. By compressing the more distal lymphangion toward the one closest to the lymph nodes, in conjunction with the closure of the directional valves, a single-directional lymph flow is activated and backflow is prevented, thus making the collectors into pumps.

Under physiological conditions, studies have indicated that pre-and post-nodal lymph may flow between 1-5 ml h/1. However, lymphological conditions can significantly alter lymph flow to the draining lymph nodes, which can be accompanied by increased lymphangiogenesis (the regrowth of lymphatic vessels from pre-existing vessels), increased cellular trafficking, larger lymph volumes, and more pro-inflammatory mediators, which can impair lymphatic function. Many pathological conditions, including inflammation, lymphedema, and cancer, have been found to increase lymphangiogenesis. In addition, nuclear factor-B induces Prox1 interaction that enhances lymphatic endothelial cell proliferation.

In addition, nuclear factor-B induces Prox1 interaction that enhances lymphatic endothelial cell proliferation. Even as it decreases tissue Edema and eliminates inflammatory cells and cytokines/chemokines, it increases the spreading of pro-inflammatory factors. It is less controversial to consider lymphangiogenesis as a factor that contributes to metastasis of cancer, i.e., the growth of lymphatic vessels that are a strong negative prognostic indicator.

Functions of the lymphatic system
The functions of the lymphatic system are - Fluid homeostasis refers to keeping the balance of fluid between the body's tissues and blood. Among its functions is to fight bacteria and other intruders and forms part of the body's immune system. By facilitating fat digestion and absorption, it enhances the absorption of fat-soluble nutrients.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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