Dr.HamedaabdulmahdiCollegeofMedicine
Dep.ofanatomy&histology2ndstage
LectureObjectives
Bytheendofthislecture,studentsareexpectedto:
1. Summarizethestructuresofmusculararteriesandveinsdiffer.
2. identifythethreelayersofthevein.
3. identifythedifferenttypesofcapillaries,andhowtheirstructureis
relatedtotheirfunction.
4. identifythethreelayersoftheartery.
5. Compare the histological features of conducting and distributing
arteries.
6. Comparethehistologicalfeaturesoflargearteriesandlargeveins
intunicaadventitia.
7. Summarizethefunctionalandhistologicalstructureofcapillarybed.
8. Outlinethehistologicalstructureofarterioleandvenules.
ElasticArteries
Elastic arteries are the aorta, the pulmonary artery, and their largest
branches; these large vessels are also called conducting arteries because their
major role is to carry blood to smaller arteries, the most prominent feature of
elastic arteries is the thick media in which elastic lamellae, each about 10 μm
thick,alternatewithlayersofsmoothmusclefibers.Theadultaortahasabout50
elasticlamellae(moreiftheindividualishypertensive).
 Tunicaintimais madeupofanepithelium,which isasinglelayerofflattened
epithelialcells,togetherwithasupportinglayerofelastinrichcollagen.Itiswell
developed, with many smooth muscle cells in the subendothelial connective
tissue, and often shows folds in cross section as a result of the loss of blood
pressureandcontractionofthevesselatdeath.Theinternalelasticlaminaisnot
easilydiscernedbecauseitissimilartotheelasticlaminaeofthenextlayer.
 Tunicamediaisbroadandelasticwithconcentricfenestratedsheetsofelastin,
andcollagenandonlyrelativelyfewsmoothmusclefibers.
 Tunicaadventitiahassmall'vasavasorum'asthelargearteriesneedtheirown
bloodsupply.
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Dr.HamedaabdulmahdiCollegeofMedicine
Dep.ofanatomy&histology2ndstage
MuscularArteries
Themusculararteriesdistributebloodtotheorgansandhelpregulateblood
pressure by contracting or relaxing, these arteries distribute blood to various
parts of the body, These included arteries such as the femoral and coronary
arteries, the walls of these arteries have lots ofsmooth muscle, which means
that they are able to contract or relax (dilate) to change the amount of blood
delivered,asneeded.
Comparing these arteries to the elastic arteries, the sheet of elastin is now
much reduced, and found at the border between thetunica intimaandtunica
mediain a layer called theinternal elastic layer(IEL) which can be seen very
clearly. Less well defined is theexternal elastic layer(EEL), between thetunica
mediaand tunicaadventitia. There is a well defined circular layer of smooth
muscleinthetunicamedia.
 Thetunica intima has an endothelium of flattened endothelial
cells.Thetunicamediaisprimarilyalayerofsmoothmuscle,withsomeelastin
ancollagen.musclelayer,andissandwichedbetweentheIELandEEL.
 TheTunicaAdventitiaisverybroad,andmostlycontainscollagenandelastin.
Arteriole.
Arterioles almost always branch toformanastomosingnetworksorbedsof
capillaries that surround the parenchymal cells of the organ. Smooth muscle
fibersactassphinctersclosingarteriolesandproducingperiodicbloodflowinto
capillaries . Acting as “resistance vessels,” muscle tone usually keeps arterioles
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Dr.HamedaabdulmahdiCollegeofMedicine
Dep.ofanatomy&histology2ndstage
partiallyclosedIncertaintissuesandorgansarteriolesdeviatefromthissimple
path
to
accommodate
various
specialized
functions,
for
example,
thermoregulation by the skin involves arterioles that can bypass capillary
networksandconnectdirectlytovenules.
The media and adventitia are thicker in these arteriovenous shunts (or
arteriovenous anastomoses) and richly innervated by sympathetic and
parasympathetic nerve fibers. The autonomic fibers control the degree of
vasoconstrictionattheshunts,regulatingbloodflowthroughthecapillarybeds.
Highcapillarybloodflowintheskinallowsmoreheatdissipationfromthebody,
while reducedcapillarybloodflowconserves heat—importantfunctionswhen
theenvironmentaltemperatureishotorcold,respectively
Theinternalelasticlaminalayerisstillpresent.TheT.M.(tunicamedialayer)
hasnomorethansixconcentricringsofsmoothmuscle,andthetunicaadventia
(T.A.)layerisapproximatelythesamesizeastheT.M.
Largerarterioleshavealumenlessthan100to300µmindiameter.Arterioles
aresmallarteriesthatdeliverbloodtocapillaries.Arteriolescontrolbloodflow
through capillary beds by contracting or dilating the size of the lumen, and
therefore thetunica medialayer contains concentric rings of smooth muscle to
do this. This compartment is important in determining your blood pressure as
the narrow diameter of these blood vessels resists blood flow, and the back
pressurehelpstostretchthewallsofthearteriesduringheartcontractions.
Thetunicaintimaisverythin,andmostlyconsistsofasinglelayerofsquamous
epithelium.
 Thetunicamediaconsistsalmostentirelyofasinglelayeruptosixlayersof
smoothmusclecells,andthereisnoEEL.
 TheTunica adventitiais about the same size as thetunica medialayer,
mergesinwithsurroundingtissue.
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Dr.HamedaabdulmahdiCollegeofMedicine
Dep.ofanatomy&histology2ndstage
ThisisanEMofaaverysmallarteriole.Thereisonlyonelayerofsmoothmuscle
(M),butthereisstillaninternalelasticlayer(IEL).
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