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Capillaries and veins all blood vessels except for

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capillaries, and veins. All blood vessels (except for the very smallest) have three distinct layers. (Layersare inner most to outer most)Arteries: carry blood away from the heart, under high pressure.oTunica Intima (Interna): Inner most tunic (nice and intimate with that blood, ooh yeah)Endothelium: continuous with endocardial lining of the heart. Made of flatsquamous cells that are fit closely together to form a slick surface to decreasefrictionBasement membrane (Subendothelial layer): supports endotheliumInternal elastic lamina: Separates tunica intima from tunica mediaoTunica Media: Made of smooth muscle cells (regulated by sympatheticvasomotor nervefibers of autonomic nervous system = this is what is controlled invasoconstriction/vasodilation) and elastin. Usually the thickest layer with the mostresponsibility.Smooth muscleExternal elastic laminaoTunica Externa (Adventitia): is composed of loosely woven collagen fibers to protect andreinforce the vessel.Veins: Carry blood towards the heart. Under lower pressure, have valves to prevent blood fromflowing backwards. Usually thinner with larger lumens compared to arteries. Appear collapsedand slit like. Also known as reservoirs because they can hold up to 65% of the body’s bloodsupply.oTunica Intima (Interna)EndotheliumBasement membraneoTunica media: Very little elastin and smooth muscle – very thin layer in veinsSmooth muscleoTunica Externa (Adventitia): Heaviest wall layer, with thick bundles of collagen fibers andelastic networks.NB: Varicose veins: twisted and dilated superficial veinsoCaused by leaky venous valves and allow backflow and poolingof bloodDeeper veins are not susceptible to this because of thesupport of surrounding muscles that help to ‘milk’ bloodup through the venous system.**Pericytes: contractile stem cells that wrap around the endothelial cells and can generate newvessels or scar tissue.Capillaries: Smallest blood vessels and have very thin walls, made of just tunica intima. Containshunts (short vessel that directly connects the arteriole and venule at opposite ends of the bed)and thoroughfares (blood can bypasses true capillaries and flow only through metarteriole-thoroughfare). Three types of capillaries:oContinuous: Least permeable and most commonFound in skin, muscles, lungs and CNS.Have pericytes.Pinocytotic vesicles carry fluid across the endothelial cell.Found in the blood brain barrier
oFenestrated: have large pores that have increased permeabilityFound in areas of active filtration (kidney) or absorption (small intestine) andareas of hormone secretion.Fenestrations tunnel through endothelial cells (like swiss cheese – yum)oSinusoid: most permeable and occur limited locationsFound in liver, bone marrow, spleen and adrenal medullaIncomplete basement membranesIrregularly shaped with larger lumens that other capillariesAllow large molecules and even cells to pass across their walls.There are several processes through which exchange or transport of fluids may occur (Covered in Cell

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Term
Fall
Professor
Dr.PeterHelli
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