Notes on lymphatics C. Riedinger 2011

1

LYMPHATIC SYSTEM Consists of:

lymph - lymphocytes - lymphatic vessels - lymph nodes - lymphatic organs Function:

- maintains fluid balance (prevent oedema) - absorbs facts from small intestine - proliferation, development and storage of immune cells - defends against microorganisms and foreign substances - =part of circulatory and immune system!

Lymph:

- formed from interstitial fluid (derived from blood) - consistency similar to blood plasma (cellular components of blood cannot

leave the capillaries) - taken up by passive diffusion and transient negative pressures in their lumina,

generated by contractile activity of smooth muscles in largest lymphatic vessel walls (or by movement of other tissues)

- clear and colourless, but from small intestine: dense and milky due to chylomicrons

- lymph in lacteals (because lymph here is white!) of small intestine is called chyle

Lymphatic vessels:

- parallel circulatory system - originates in peripheral tissues as blind-ended endothelial tubes (simple

squamous with valves) - not present in cornea, cartilage, thymus, CNS, PNS, bone marrow, epidermis

and very few in endomysium of skeletal muscles - collect excess fluid form the interstitial spaces (10%) - smallest = lymph capillaries

o resemble capillaries but endothelial lining permeable to larger molecules, such as colloidal proteins and particulate material (cell debris, microorganisms)

o have funnel-shaped valves o area between two valves: lymphangion o intercellular junctions lack tight junctions o no intrinsic contractility o extrinsic forces for propulsion: respiratory movement, muscle

contraction, pulse pressure of adjacent arteries - terminal lymphatic vessels in mucosa of small intestine: lacteals - lymphatic capillaries join into larger vessels which pass to local lymph nodes - lymph passes through series of nodes until it reaches a major collecting duct

(few drain directly, such as thyroid, oesophagus, coronary and triangular ligaments of liver to thoracic duct)

- largest lymphatic vessels (200um) like small veins, but with more valves (semilunar)

- lymph drains into large veins in the root of the neck - almost all reach either thoracic or right lymphatic duct

Notes on lymphatics C. Riedinger 2011

2

Right lymphatic duct: - Drains right half of head and neck, right arm and right hemithorax

Thoracic duct:

- Similar to medium-sized vein, but more prominent smooth muscle in tunica media.

- Drains rest of body - Ascends in the posterior mediastinum between the descending thoracic aorta

and the azygos vein - Length 38-45cm - From L2 to root of the neck

capillaries afferent vessels lymph node efferent vessels lymph trunk

terminal vessels (TD, right lymphatic duct) great veins Lymph nodes:

- filter lymph - have convex cortex into which afferent vessels open - also have medulla and hilum (concave surface), through which efferent vessel

exits - are encapsulated centres of antigen presentation and lymphocyte activation,

differentiation and proliferation - generate antigen-primed B and T cells - filter particles (i.e. microbes) from the lymph by phagocytic macrophages - ~450 in total

o 60-70 head and neck o 100 thorax o 250 abdomen and pelvis

- mainly close to viscera, in mesenteries - lymph flows through channels in lymph nodes after entering from afferent

vessels - reticulin to support sinus macrophages - superficial lymph nodes: inguinal region, axilla, neck

o receive lymph from skin, mucous membranes, serous linings o superficial system drains to the deep system

- deep lymphatic vessels: preaortic, para-aortic, bronchomediastinal o drain organs o accompany arteries or veins

Lymphatic organs:

- clusters of lymphocytes and other cells (macrophages) in framework of short branching connective tissue fibres

- lymphocytes from bone marrow, but proliferate within lymphatic organs - tonsils, spleen, thymus gland

Spread of disease:

- easy for tumour to enter, slow flow does little damage to foreign cells - presence of metastases in lymph nodes indicates metastases to distant organs - lymphadenitis: inflammation, lymphangitis: infection of vessels (red line

under skin) lymphoma: cancer of lymphocytes