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
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
what are the functions of capillaries?
exchange of gases, nutrients, wastes, hormones between blood and interstitual fluid
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
what are the three types of capillaries?
what are continuous capillaries?
- least permable, most common
- tight junctions connect endothelial cells
- intercellular clefts allowpassage of fluids and small solutes
- large fenestrations (pores) increase permeability
- occurs in areas of active absorption or filtration
- fewer tight junctions; usually fenestrated; larger intercellular clefts; large lumens
- blood flow sluggish: allow modification
what is precapillary sphincter?
a band of smooth muscle that regulates entrance to capillary beds
what is a vascular shunt?
connects the terminal arteriole with teh postcapillary venule in a capillary bed
what are capillary beds regulated by?
local chemical conditions and vasomotor nerves
where do true capillaries branch from and return to?
metarteriole, thoroughfare channel (pathway is called a vascular shunt)
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)
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)
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
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
what is blood pressure
force per unite area exerted on wall of blood vessel by blood
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
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
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.