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Chapter 23
Blood Vessels
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Blood Vessel Tunics• Walls of blood vessels have three layers, or tunics
1. Tunica externa (adventitia) – anchor BV to an organ • Larger blood vessels require own blood supply
– vasa vasorum in tunica externa
2. Tunica media – smooth muscle• Sympathetic input vasoconstriction • Parasympathetic input vasodilation
3. Tunica intima (interna) – endothelium (simple squamous ET lining)• Continuous with endocardium
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Blood Vessel Tunics
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Microscopic Comparison of Arteries and Veins
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Types of Arteries• Three types of arteries:
1. Elastic arteries• Largest arteries, close to heart• Thick tunica media w/ elastic fibers
2. Muscular arteries• Medium diameter• Proportionally thicker tunica media
3. Arterioles –smallest arteries• Thin tunica media (< 6 layers)• Connect to capillaries
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Capillaries
• Capillaries– Diameter slightly larger than
erythrocyte• Tunica intima only– Allows for rapid diffusion
• Form capillary beds– Blood flow regulated by
precapillary sphincters– Thoroughfare channel
bypasses bed• Site of metabolic exchange
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Types of Capillaries1. Continuous – most common– continuous and complete endothelium (no physical holes)
2. Fenestrated – endothelial cells possess small “holes” – allow fluid exchange between blood and interstitial fluid
• eg. kidneys
3. Sinusoid – large gaps between endothelial cells – promotes transport of large molecules and cells to and from
blood• eg. Liver and spleen
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Veins
• Drain capillaries – return blood to heart
• Pressure much lower than in arteries– Walls much thinner– Very little muscle in tunica media
• At rest, veins hold about 60% of body’s blood– function as blood reservoirs
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Venules
• Smallest veins• Postcapillary venules -
smallest– Diapedesis occurs here
• Venules merge to form veins
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Veins• Skeletal muscle pump moves
blood toward heart– Contraction of muscles
• Blood pressure too low to overcome gravity – valves prevent backflow and
pooling of blood in limbs– formed from tunica intima
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Venous Return from the Abdomen
• Special system of circulation - hepatic portal system
• Drains blood from GI organs and shunts blood to liver– Allows for filtering
of ingested substances
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Fetal-Placental Circulation
Fetal circulation bypasses the developing lungs, kidneys, and digestive tract
ttp://www.ethal.org.my/opencms/opencms/ethal/Images/MedGeneralImages/Placenta.gif
All nutritional, respiratory, and excretory needs are
met by the placenta
The exchange occurs via capillaries- mom’s and baby’s blood don’t mix.
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• Fetal system has structures that are modified or cease to exist after birth
• Fetal circulatory pathway:– Oxygenated blood from placenta umbilical vein
ductus venosus (bypasses liver) inferior vena cava right atrium foramen ovale (shunt to LA) left ventricle aorta
– Some blood from RA RV pulmonary trunk ductus arteriosus (shunt to aorta)
– Aorta body umbilical arteries (now deoxygenated) placenta nutrient and gas exchange
Fetal-Placental Circulation
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• Postnatal changes– Umbilical vessels constrict
and cease function– Ductus venosus becomes
ligamentum venosum– Foramen ovale becomes
fossa ovalis • Failure to close at birth =
patent foramen ovale– Ductus arteriosus becomes
ligamentum arteriosum• Failure to close at birth =
patent ductus arteriosus
Fetal-Placental Circulation