table 21.1 distinguishing features of blood vessels

blood vessels

elastic arteries

muscular arteries

arterioles

capillaries

postcapillary venules

muscular venules

veins

elastic arteries size

largest arteries in the body

elastic arteries tunica intima

well-defined internal elastic membrane

elastic arteries tunica media

thick and dominated by elastic fibers

well-defined external elastic membrane

elastic arteries tunica externa

thinner than tunica media

elastic arteries function

conduct blood from heart to muscular arteries

muscular arteries size

medium-sized arteries

muscular arteries tunica intima

well-defined internal elastic membrane like elastic arteries

muscular arteries tunica media

thick and dominated by smooth muscle

thin external elastic membrane

muscular arteries tunica externa

thicker than tunica media

muscular arteries function

distribute blood to arterioles

arterioles size

microscopic 15-300 micrometers in diameter

arterioles tunica intima

thin with a fenestrated internal elastic membrane that disappears distally

arterioles tunica media

one or two layers of circularly oriented smooth muscle

distalmost smooth muscle fiber forms a precapillary sphincter

arterioles tunica externa

loose collagenous connective tissue and sympathetic nerves

arterioles function

deliver blood to capillaries and help regulate blood flow from arteries to capillaries

capillaries size

microscopic

smallest blood vessels

5-10 micrometers in diameter

capillaries tunica intima

endothelium and basement membrane

capillaries tunica media

none

capillaries tunica externa

none

capillaries function

permit exchange of nutrients and wastes between blood and interstitial fluid

distribute blood to postcapillary venules

postcapillary venules size

microcospic

10-50 micrometers in diameter

postcapillary venules tunica intima

endothelium and basement membrane

postcapillary venules tunica media

none

postcapillary venules tunica externa

sparse

postcapillary venules function

pass blood into muscular venules

permit exchange of nutrients and wastes between blood and interstitial fluid and function in white blood cell emigration

muscular venules size

microscopic

50-200 micrometers in diameter

muscular venules tunica intima

endothelium and basement membrane m

muscular venules tunica media

one or two layers of circularly oriented smooth muscle

muscular venules tunica externa

sparse

muscular venules function

pass blood into vein

act as reservoirs for accumulating large volumes of blood along with postcapillary venules

veins size

range from 0.5 mm to 3 cm in diameter

veins tunica intima

endothelium and basement membrane

no internal elastic membrane

contain valves

lumen much larger than in accompanying artery

veins tunica media

much thinner than in arteries

no external elastic membrane

veins tunica externa

thickest of three layers

veins function

return blood to heart

facilitated by valves in limb veins