Chapter 23
Blood Vessels
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
Blood Vessel Tunics
Microscopic Comparison of
Arteries and Veins
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
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
Types of Capillaries
1. 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
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
Venules
• Smallest veins
• Postcapillary venules smallest
– Diapedesis occurs here
• Venules merge to form
veins
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
Varicose Veins
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
Fetal-Placental Circulation
Fetal circulation bypasses the developing lungs, kidneys,
and digestive tract
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.
ttp://www.ethal.org.my/opencms/opencms/ethal/Images/MedGeneralImages/Placenta.gif
Fetal-Placental Circulation
• 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
• 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