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what are the three layers of vessel walls?
- tunica intima: endothelial lining and elastic connective tissue
- tunica media: smooth muscle with collagen and elastic fibers
- tunica externa: sheath of connective tissue that protects vessel, anchor to other tissues
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what are the three types of arteries?
- elastic: first arteries leaving the heart (pulmonary trunk, aorta)
- muscular arteries: external carotid arteries (deliver blood to head and neck) deliver blood to body organs, active in vascoconstriction
- arterioles: smallest; leads to capillary beds, control flow by vasodilation and vasoconstriction
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what are the functions of capillaries?
exchange of gases, nutrients, wastes, hormones between blood and interstitual fluid
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why are capillaries ideal for diffusion between plasma and IF?
- thin walls provide short diffusion distance
- small diameter slows flow to increase diffusion rate
- enormous number of capillaries provide huge surface area for increased diffusion
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what are the three types of capillaries?
- continuous
- fenestrated
- sinusoid
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what are continuous capillaries?
- least permable, most common
- tight junctions connect endothelial cells
- intercellular clefts allowpassage of fluids and small solutes
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fenestrated capillaries?
- large fenestrations (pores) increase permeability
- occurs in areas of active absorption or filtration
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sinusoid capillaries?
- fewer tight junctions; usually fenestrated; larger intercellular clefts; large lumens
- blood flow sluggish: allow modification
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what is precapillary sphincter?
a band of smooth muscle that regulates entrance to capillary beds
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what is a vascular shunt?
connects the terminal arteriole with teh postcapillary venule in a capillary bed
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what are capillary beds regulated by?
local chemical conditions and vasomotor nerves
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where do true capillaries branch from and return to?
metarteriole, thoroughfare channel (pathway is called a vascular shunt)
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what are alternate routes for blood flow?
- formed by anastromaosis, a joining of blood vessels
- arteriovenous anastomosis: bypasses capillary bed, connective arteriole to venule
- arterial anastomosis: occurs where arteries fuse before branching into arterioles (ensure delivery of blood to key areas)
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what are the three types of veins?
- venules: smallest, some lack tunica media; very porous, allow fluids and WBC into tissues
- medium sized venules: tunica media has several smooth muscle layers
- large veins: thin tunica media and thick collagenous tunica externa; thinner walls than arteries b/c of low pressure; contain up to 65% of blood supply (called capacitance vessels aka blood reservoirs)
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what adaptations ensure return of blood to heart despite low pressure?
- large diameter lumens offer little resistance
- venous valves present backflow of blood
- venous sinuses: flattened veins with extrememly thin walls
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how to maintain adequate blood flow?
- normally, blood flow equals cardiac output (CO); increased CO leads to increased flow through capillaries
- decreased CO leads to reduced flow
- capillary flow influenced by pressure and resistance
- liquids exert hydrostatic presure in all directions
- circulatory pressure, high in aorta vs. low in venae cavae
- blood pressure
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what is blood pressure
force per unite area exerted on wall of blood vessel by blood
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what is peripheral resistance?
- any fore that opposes flow/movemnt of blood
- measure amount of friction blood encournters with vessel walls, generally in peripheral (systemic) circulation
- highest pressure gradient exists in arterioles due to high peripheral resistance
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what are three important sources of resistance?
- vascular resistance: largest component
- viscosity: due ot interactions between moles in liquid
- turbulence: eddies and swirts in flow of blood
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what is vascular resistance?
- caused by friction between blood and vessel walls
- amount of friction due to length and diameter of vessel.
- arteriolar diameter is primary source of vascular resistance
- the smaller the diameter, the greater the distance
- varies inversely with fourth power of vessel radius
- abrupt changes in diameter or fatty plagues from atheroclerosis increase resistance.
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