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Lymphatic vessels

Lymphatic vessels (vasa lymphatica) are vessels that conduct lymph from the tissues into the venous bed. Lymphatic vessels are found in almost all organs and tissues. The exception is the epithelial layer of the skin and mucous membranes, cartilage, sclera, vitreous and lens of the eye, brain, placenta and parenchyma of the spleen.

The beginning of the formation of the lymphatic system in a human embryo belongs to the 6th week of development, when paired jugular lymphatic bags can already be distinguished. By the beginning of the 7th week, these bags are connected to the anterior cardinal veins. A little later, all the other lymphatic bags appear. The growth of lymphatic vessels from the primary sacs is accomplished by the growth of endothelial processes. Valves of the lymphatic vessels are laid on the 2-5th month of uterine life in the form of flat annular thickening of the endothelium.

Among the lymphatic vessels there are: lymphatic capillaries; small intraorgan lymph vessels; extraorgan (so-called abduction) lymphatic vessels; lymphatic vessels, connecting lymph nodes; large trunks - lumbar (trunci lumbales dext. et sin.), intestinal (tr. intestinalis), subclavian (trr. subclavii dext. et sin.), bronchomediastinal (trr. bron-chomediastinales dext. et sin.), jugular (trr jugulares dext. et sin.), which are formed from the lymphatic vessels of the respective areas, and two lymphatic ducts — the thoracic (ductus thoracicus) and the right duct (ductus lymphaticus dext.). Both these ducts flow to the left and right, respectively, at the confluence of the internal jugular and subclavian veins.

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The combination of lymphatic capillaries is like a source of the lymphatic system. Lymphatic capillaries receive metabolic products from the tissues. The capillary wall consists of endothelial cells with a weakly pronounced basal membrane. The diameter of the lymphatic capillary exceeds the diameter of the blood capillary. The body secrete superficial and deep networks of lymphatic capillaries, which are interconnected. The passage of lymphatic capillaries into subsequent lymphatic vessels is determined by the presence of valves. Along with significant fluctuations in the caliber of the lymphatic vessels, the presence of constrictions in the locations of valves is characteristic. Small intraorgan lymphatic vessels with a caliber of 30-40 microns do not have a muscular layer. In lymphatic vessels with a caliber of 0.2 mm and above, the wall consists of three layers: the inner (tunica intima), middle muscular (tunica media) and outer connective tissue (tunica adventitia). Valves of the lymphatic vessels are folds of the inner lining. The number of valves in the lymphatic vessels and the distance between them vary. The distance between the valves in small lymphatic vessels is 2-3 mm, and in large - 12-15 mm. Valves provide current lymph in one direction. In pathologically dilated lymphatic vessels, valve insufficiency appears, in which retrograde lymph flow is possible.

The number of lymphatic capillaries flowing into separate small collecting lymphatic vessels ranges from 2 to 9. Intraorganic lymphatic vessels form broad-leafy plexuses with various forms of loops in the organs. Often they accompany blood vessels, forming between themselves transverse and oblique anastomoses. Several groups of the diverting lymphatic vessels come out of the organ or part of the body, which, merging, are sent to the regional lymph nodes. The diverting lymphatic vessels of the small intestine, passing through its mesentery, are called milky (vasa chylifera), as they carry the milky juice (chylus).

The lymph flow in the lymphatic vessels is determined by the contractile ability of their wall, the mechanical influence of passive and active movements, and the energy of lymph formation. The pressure in the diverting lymphatic vessels varies due to the different functional state of the organ.

Lymphatic vessels regenerate well. After 3-20 weeks, the cut vessels are fully restored. Lymphatic vessels, as well as blood vessels, have their own vessels (vasa vasorum) that feed their walls. Innervation of the lymphatic vessels is carried out by the nerve plexuses present in the vessel wall; in the adventitia and middle layer of the wall, free nerve endings were found.

Pathology of lymphatic vessels - see. Thoracic duct, Lymphangioma, Lymphangitis, Lymphangiectasia, Cholangioma.