Histology - capillaries and veins PDF

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Module 4...

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Part 2: Capillaries and veins Arteries more circular, venules more irregular shaped Capillaries  Smallest narrowest vessels in body o Diameter ~ rbc  Network of capillaries are the sites of exchange between the blood and the surrounding interstitial fluid  Wall of capillary is a single endothelial layer with a supporting basement membrane  Capillaries in different regions of the body are distinguishable by features of their respective endothelium o Continuous (type I) capillaries o Fenestrated (type 2/windowed) capillaries o Discontinuous capillaries and sinusoids

Continuous capillaries  Continuous sheet of endothelial cells  Found in the muscles, lungs + CNS  Adjacent endothelial cells are sealed together by a belt-like junction → right/occluding junction or zonula occludens

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Some cases - there is pericytes surrounding capillaries which are contractile as well as providing structural support for the capillaries

Fenestrated capillaries  The plasma membrane of the endothelial cells have fenestrations → membrane spanning pores (80-100nm diameter) → often with a thin membrane across it  However basement membrane is continuous, even though there are gaps between endothelial cells  Found in the GIT and the glomeruli of nephrons

Sinusoid capillaries  ‘Leaky’  Found in liver, spleen and marrow  They have a wider diameter than other capillaries (~ 30µm)  In liver: adjacent hepatocytes send processes that wedge between adjacent endothelial cells creating gaps → large enough for blood cells to pass through → forms discontinuous capillaries The venous system: Venules  Based on location and structure there are o Post-capillary venules (small) → have an endothelium surrounded by pericytes  Sites of extravasation of plasma and WBC during inflammation o Upstream are larger diameter muscular venules which possess thin media (but not pericytes) and adventicia  Thin adventicia Medium Veins

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Diameter ~ 10mm - deep veins

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Medium veins often possess valves → particularly those draining blood from the lower limbs o Material in centre of vein in image above o Valves prevent backflow of blood eg. in legs where blood goes up These veins have clearly discernible tunics o Thin tinima with no clear internal elastic lamina o Media - thinner than that of the respective artery. Made up of several layers of SMC interspersed with collagen and elastin fibres o The adventitia is generally thicker than the media

The venous system: Large veins

Diameters > 10 mm this group includes the vena cavae - capacitance vessels They don’t possess valves The intima often includes a layer of SMC at the intima-media boundary The media is relatively thinner than that of matched arteries The adventitia is the thickest layer and contains a layer of longitudinally arranged smooth muscle Key points  The circulatory system made of arteries, veins and capillaries  Basic structure of most vessels is a three layered structure - intima, media and adventitia  Layers or tunics differ markedly in their histology which is linked to the respective roles they play in the vessel  The histological differences between these vessels serve their different functions o Arteries are muscular/elastic vessels which serve to conduct blood through the body as well as regulate its flow o Veins are larger diameter vessels with walls thinner than matched arteries which serve their functions of capacitance and returning blood to the heart o Capillaries are extremely narrow and thin-walled vessels which are sites of exchange between the blood and the interstitial fluid  Within each group of vessels, categories exist based on histological differences and location that represent differences in function     ...