Dr.MaryamMohammed2ndstageCollegeofMedicine/Dep.ofanatomy&
histology LectureObjectives
Bytheendofthislecture,studentsareexpectedto:
1. Summarizethestructuresofmusculararteriesandveinsdiffer
2. identifythethreelayersofthevein:
3. identifythedifferenttypesofcapillaries,andhowtheirstructureis
relatedtotheirfunction.
4. identifythethreelayersoftheartery
5. Compare the histological features of conducting and distributing
arteries
6. Summarizethefunctionalandhistologicalstructureofcapillarybed
CapillaryBeds
Capillaries permit and regulate metabolic exchange between blood and
surrounding tissues, these smallest blood vessels always function in groups
called capillary beds, whose size and overall shape conforms to that of the
structure , the richness of the capillary network is related to the metabolic
activity of the tissues, tissues with high metabolic rates, such as the kidney,
liver,andcardiacandskeletalmuscle,haveanabundantcapillarynetwork;the
oppositeistrueoftissueswithlowmetabolicrates,suchassmoothmuscleand
denseconnectivetissue.
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Dr.MaryamMohammed2ndstageCollegeofMedicine/Dep.ofanatomy&
histology Capillarybedsaresuppliedpreferentiallybyoneormoreterminalarteriole
branches called metarterioles, which are continuous with thoroughfare
channelsconnectedwiththepostcapillaryvenules.truecapillariesbranchfrom
the metarterioles, which are encircled by scattered smooth muscle cells, and
convergeintothethoroughfarechannels,whichlackmuscle,atthebeginningof
eachtruecapillary,musclefibersactasprecapillarysphinctersthatcontractor
relaxtocontroltheentryofblood,thesesphincterscontractandrelaxcyclically,
with 5 to 10 cycles per minute, causing blood to pass through capillaries in a
pulsatilemanner,whenthesphinctersareclosed,bloodflowsdirectlyfromthe
metarteriolesandthoroughfarechannelsintopostcapillaryvenules.
Capillariesaresmall,normallyaround3‐4µm,butsomecapillariescanbe30‐
40 µm in diameter. The largest capillaries are found in the liver.
Capillariesconnectarteriolestovenules.
Capillarieshaveasinglelayerofflattenedendothelialcells,asshownhere
inthediagram,therearenomuscularoradventitiallayers,thethinnessofthe
capillarieshelpsefficientexchangebetweenthelumenofthecapillaryandthe
surroundingtissue.
Therearethreetypesofcapillary:
o continuous
o fenestrated
o discontinuous
Continuouscapillaries often have pericytes associated with them. lie just
underneath the endothelium of blood capillaries, and are a source of new
fibroblasts. Continuous capillaries lack fenestrae and have a continuous basal
lamina.Theyarelocatedinnervoustissue,muscle,connectivetissue,exocrine
glands,andthelungs.
Sinusoids, possess many large fenestrae that lack diaphragms. Sinusoidal
capillaries are 30 to 40 _m in diameter, much larger than continuous and
fenestratedcapillaries.Sinusoidalcapillarieshaveadiscontinuousbasallamina
and lack pinocytotic vesicles. Gaps may be present at the cell junctions,
permitting leakage between endothelial cells ,they are located in the liver,
spleen,bonemarrow,lymphnodes,andadrenalcortex.
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Dr.MaryamMohammed2ndstageCollegeofMedicine/Dep.ofanatomy&
histology 
fenestratedcapillariesfound
inakidneyglomerulus,thesearefoundinsometissueswherethereis
extensivemolecularexchangewiththebloodsuchasthesmallintestine,
endocrineglandsandthekidney. The'fenestrations'areporesthatwill
allow larger molecules though, these capillaries are more permeable
thancontinuouscapillaries.
Venuleshaveadiameterof0.2to1mmandareinvolvedinexchangeof
metaboliteswithtissuesandindiapedesis(exitingofbloodcellsthrough
vesselwalls).
Veins
conductbloodawayfromtheorgansandtissuesandreturnittotheheart,veins
contain about 70% of the body’s total blood volume at any given time. Their
walls are composed of three layers: the tunica intima (inner), tunica media
(middle), andtunicaadventitia(outer),thethickestand most prominent. Vasa
vasorum are more numerous in veins than arteries. A distinct internal elastic
laminaisalsoabsentinveins.
Comparison with arteries. Veins have thinner walls and larger, more irregular
Luminathanthecompanionarteries.TheymayhavevalvesintheirLuminathat
preventretrogradeflowoftheblood.
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Dr.MaryamMohammed2ndstageCollegeofMedicine/Dep.ofanatomy&
histology Typesofveins
 Large veins include the vena cava and pulmonary veins. These veins
possess cardiac muscle in the tunica adventitia for a short distance as
theyentertheheart.Thislayeralsocontainsvasavasorumandnerves.
 Small and medium‐sized veins include the external jugular vein. These
veinshaveadiameterof1to9mm.
Venules have a diameter of 0.2 to 1 mm and are involved in exchange of
metaboliteswithtissuesandindiapedesis(exitingofbloodcellsthroughvessel
walls).
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