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Thelymphatic vessels(orlymph vesselsorlymphatics) are thin-walled vessels (tubes), structured likeblood vessels,that carrylymph.As part of thelymphatic system,lymph vessels are complementary to thecardiovascular system.Lymph vessels are lined byendothelial cells,and have a thin layer ofsmooth muscle,andadventitiathat binds the lymph vessels to the surrounding tissue. Lymph vessels are devoted to thepropulsionof the lymph from thelymph capillaries,which are mainly concerned with the absorption ofinterstitial fluidfrom the tissues. Lymph capillaries are slightly bigger than their counterpartcapillariesof the vascular system. Lymph vessels that carry lymph to alymph nodeare called afferent lymph vessels, and those that carry it from a lymph node are called efferent lymph vessels, from where the lymph may travel to another lymph node, may be returned to a vein, or may travel to a largerlymph duct.Lymph ducts drain the lymph into one of thesubclavian veinsand thus return it to generalcirculation.
| Lymphatic vessel | |
|---|---|
 Lymph capillariesin the tissue spaces | |
 | |
| Details | |
| System | Lymphatic system |
| Identifiers | |
| Latin | vas lymphaticum |
| MeSH | D042601 |
| TA98 | A12.0.00.038 |
| TA2 | 3915 |
| TH | H3.09.02.0.05001 |
| FMA | 30315 |
| Anatomical terminology | |
The vessels that bring lymph away from the tissues and towards the lymph nodes can be classified asafferent vessels.These afferent vessels then drain into the subcapsular sinus.[1] Theefferent vesselsthat bring lymph from the lymphatic organs to the nodes bringing the lymph to the right lymphatic duct or the thoracic duct, the largest lymph vessel in the body. These vessels drain into the right and leftsubclavian veins,respectively. There are far more afferent vessels bringing in lymph than efferent vessels taking it out to allow for lymphocytes and macrophages to fulfill their immune support functions. The lymphatic vessels contain valves.
Structure
editThe general structure of lymphatics is based on that ofblood vessels.There is an inner lining of single flattened epithelial cells (simple squamous epithelium) composed of a type ofepitheliumthat is called theendothelium,and the cells are calledendothelial cells.This layer functions to mechanically transport fluid and since thebasement membraneon which it rests is discontinuous; it leaks easily.[2]The next layer is that ofsmooth musclesthat are arranged in a circular fashion around the endothelium, which by shortening (contracting) or relaxing alter the diameter (caliber) of thelumen.The outermost layer is theadventitiawhich consists of fibrous tissue. The general structure described here is seen only in larger lymphatics; smaller lymphatics have fewer layers. The smallest vessels (lymphaticorlymph capillaries) lack both the muscular layer and the outer adventitia. As they proceed forward and in their course are joined by other capillaries, they grow larger and first take on an adventitia, and then smooth muscles.
The lymphatic conducting system broadly consists of two types of channels—theinitial lymphatics,theprelymphaticsorlymph capillariesthat specialize in collection of the lymph from the interstital fluid, and the largerlymph vesselsthat propel the lymph forward.
Unlike the cardiovascular system, the lymphatic system is not closed and has no central pump. Lymph movement occurs despite low pressure due toperistalsis(propulsion of the lymph due to alternate contraction and relaxation ofsmooth muscle), valves, and compression during contraction of adjacent skeletal muscle andarterialpulsation.[3]
Lymph capillaries
editThe lymphatic circulation begins with blind ending (closed at one end) highly permeable superficial lymph capillaries, formed by endothelial cells with button-like junctions between them that allow fluid to pass through them when the interstitial pressure is sufficiently high.[4]These button-like junctions consist ofprotein filamentslikeplatelet endothelial cell adhesion molecule-1,or PECAM-1. A valve system in place here prevents the absorbed lymph from leaking back into the interstital fluid. This valve system involves collagen fibers attached to lymphatic endothelial cells that respond to increased interstitial fluid pressure by separating the endothelial cells and allowing the flow of lymph into the capillary for circulation.[5]There is another system of semilunar valves that prevents back-flow of lymph along the lumen of the vessel.[4]Lymph capillaries have many interconnections (anastomoses) between them and form a very fine network.[6]
Rhythmic contraction of the vessel walls through movements may also help draw fluid into the smallest lymphatic vessels,capillaries.If tissue fluid builds up the tissue will swell; this is callededema.As the circular path through the body's system continues, the fluid is then transported to progressively larger lymphatic vessels culminating in theright lymphatic duct(for lymph from the right upper body) and thethoracic duct(for the rest of the body); both ductsdraininto the circulatory system at the right and leftsubclavian 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.
Lymph vessels
editThe lymph capillaries drain into largercollecting lymphatics.These arecontractile lymphaticswhich transport lymph using a combination of smooth muscle walls, which contract to assist in transporting lymph, as well as valves to prevent the lymph from flowing backwards.[3]As the collecting lymph vessel accumulates lymph from more and more lymph capillaries along its length, it becomes larger and eventually becomes anafferent lymph vesselas it enters a lymphs node. The lymph percolates through the lymph node tissue and exits via anefferent lymph vessel.An efferent lymph vessel may directly drain into one of the (rightorthoracic) lymph ducts, or may empty into another lymph node as its afferent lymph vessel.[6]Both the lymph ducts return the lymph to the blood stream by emptying into thesubclavian veins
Lymph vessels consist of functional units known aslymphangionswhich are segments separated by semilunar valves. These segments propel or resist the flow of lymph by the contraction of the encircling smooth muscle depending upon the ratio of its length to its radius.[7]
Function
editLymph vessels act as reservoirs for plasma and other substances including cells that have leaked from the vascular system and transport lymph fluid back from the tissues to the circulatory system. Without functioning lymph vessels, lymph cannot be effectively drained andlymphedematypically results.
Afferent vessels
editTheafferent lymph vesselsenter at all parts of the periphery of thelymph node,and after branching and forming a dense plexus in the substance of the capsule, open into thelymph sinusesof thecorticalpart. It carries unfiltered lymph into the node. In doing this they lose all their coats except theirendotheliallining, which is continuous with a layer of similar cells lining the lymph paths.
Afferent lymphatic vessels are only found in lymph nodes. This is in contrast to efferent lymphatic vessel which are also found in thethymusandspleen.
Efferent vessels
editTheefferent lymphatic vesselcommences from thelymph sinusesof themedullaryportion of the lymph nodes and leave the lymph nodes at the hilum, either to veins or greater nodes. It carries filtered lymph out of the node.
Efferent lymphatic vessels are also found in association with the thymus andspleen.This is in contrast to afferent lymphatic vessels, which are found only in association with lymph nodes.
Clinical significance
editLymphedemais the swelling of tissues due to insufficient fluid drainage by the lymphatic vessels. It can be the result from absent, underdeveloped or dysfunctional lymphatic vessels. In hereditary (or primary) lymphedema, the lymphatic vessels are absent, underdeveloped or dysfunctional due to genetic causes. In acquired (or secondary) lymphedema, the lymphatic vessels are damaged by injury or infection.[8][9]Lymphangiomatosisis a disease involving multiple cysts or lesions formed from lymphatic vessels.
Additional images
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Lymphatic system -
Section across portal canal of pig. X 250.
See also
editReferences
edit- ^"19.2B: Distribution of Lymphatic Vessels".Medicine LibreTexts.22 July 2018.Retrieved28 November2021.
- ^Pepper MS, Skobe M (October 2003)."Lymphatic endothelium: morphological, molecular and functional properties".The Journal of Cell Biology.163(2): 209–13.doi:10.1083/jcb.200308082.PMC2173536.PMID14581448.
- ^abShayan R, Achen MG, Stacker SA (September 2006)."Lymphatic vessels in cancer metastasis: bridging the gaps".Carcinogenesis.27(9): 1729–38.doi:10.1093/carcin/bgl031.PMID16597644.
- ^abBaluk P, Fuxe J, Hashizume H, Romano T, Lashnits E, Butz S, et al. (October 2007)."Functionally specialized junctions between endothelial cells of lymphatic vessels".The Journal of Experimental Medicine.204(10): 2349–62.doi:10.1084/jem.20062596.PMC2118470.PMID17846148.
- ^Weitman E, Cuzzone D, Mehrara BJ (September 2013)."Tissue engineering and regeneration of lymphatic structures".Future Oncology.9(9): 1365–74.doi:10.2217/fon.13.110.PMC4095806.PMID23980683.
- ^abRosse C, Gaddum-Rosse P (1997). "The Cardiovascular System (Chapter 8)".Hollinshead's Textbook of Anatomy(Fifth ed.). Philadelphia: Lippincott-Raven. pp. 72–73.ISBN0-397-51256-2.
- ^Venugopal AM, Stewart RH, Rajagopalan S, Laine GA, Quick CM (2004)."Optimal Lymphatic Vessel Structure".The 26th Annual International Conference of the IEEE Engineering in Medicine and Biology Society.26th Annual International Conference of the IEEE. Vol. 2. Engineering in Medicine and Biology Society. pp. 3700–3703.doi:10.1109/IEMBS.2004.1404039.ISBN0-7803-8439-3.
- ^Alitalo K (November 2011). "The lymphatic vasculature in disease".Nature Medicine.17(11): 1371–80.doi:10.1038/nm.2545.PMID22064427.S2CID5899689.
- ^Krebs R, Jeltsch M (2013)."The lymphangiogenic growth factors VEGF-C and VEGF-D. Part 2: The role of VEGF-C and VEGF-D in lymphatic system diseases".Lymphologie in Forschung und Praxis.17(2): 96–104.
Further reading
edit- Nosek TM."Role of Lymphatic Vessels".Essentials of Human Physiology.Archived fromthe originalon 2016-03-24.
External links
edit- Lymphatic+Vesselsat the U.S. National Library of MedicineMedical Subject Headings(MeSH)
- Efferent lymph vessel- BioWeb at University of Wisconsin System