1. Overview
Structure:
Arteries
Veins
Capillaries
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2. Blood Vessels
• Transport blood
• Arteries: carry blood away from the heart
• Usually ↑ O2
• Pulmonary circulation exception
• Veins: carry blood toward the heart
• Usually ↓ O2
• Capillaries: contact tissue directly
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3. Venous system
Large veins
Arterial system
Heart
Elastic arteries
Muscular arteries
Small veins
Arteriovenous
anastomosis
Sinusoid
Arterioles
Postcapillary
venule
sphincter
Capillaries
(exchange vessels)
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Figure 18.2

4. BLOOD VESSEL STRUCTURE
Tunics (extera, media, intima)
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5. Structure of Blood Vessel Walls
• Arteries and veins
1. Tunica intima
2. Tunica media
3. Tunica externa
•
Lumen
• Central space, contains blood
• Capillaries
•
Endothelium with basal layer
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6. Tunica intima
• Endothelium
• Subendothelial layer
Internal elastic lamina
Tunica media
(smooth muscle and
elastic fibers)
External elastic lamina
Valve
Tunica externa
(collagen fibers)
Lumen
Artery
(b)
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Capillary
network
Lumen
Vein
Basement membrane
Endothelial cells
Capillary
Figure 18.1b

7. Tunics
• Tunica intima
• Endothelium
• lines the lumen
• Subentothelial layer
• In larger vessels (>1 mm)
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Endothelium
Subendothelial layer

8. Tunics
• Tunica media
• Smooth muscle
• Controls:
• Vasoconstriction
• Vasodilation
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Smooth muscle

9. Tunics
• Tunica externa
• Outer most layer
• Collagen fibers reinforce
• Large vessels have vasa
vasorum
• “vessels to the vessels”
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Tunica externa

10. TYPES OF ARTERIES
Elastic, Muscular and Arterioles
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11. Elastic (Conducting) Arteries
• Largest thick-walled arteries
• Aorta / major branches
• elastin in all tunics
• Blood directly from heart
• Resist high pressure
• Large volume
• Large lumen = low-resistance
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12. Muscular (Distributing) Arteries
• Distal to elastic arteries
• Deliver blood to organs
• Have thick tunica media
• smooth muscle
• Vasoconstriction
• Vasodilation
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13. Arterioles
• Smallest arteries
• Lead to capillary beds
• Control of flow w/
• vasodilation and vasoconstriction
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14. Copyright © 2011 Pearson Education, Inc.
Table 18.1 (1 of 2)

15. Capillaries
• Smallest blood vessels
• thin tunica intima
• one cell thick
• Only one RBC wide
• Pericytes support and control permeability
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16. CAPILLARIES
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17. Capillaries
• Location• all tissues except for:
• cartilage, epithelia, cornea and lens of eye
• Functions• Exchange:
• gases, nutrients, wastes, hormones, etc.
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18. Venous system
Large veins
(capacitance
vessels)
Arterial system
Heart
Elastic arteries
Muscular arteries
Small veins
(capacitance
vessels)
Arteriovenous
anastomosis
Sinusoid
Arterioles
Postcapillary
venule
Capillaries
(exchange vessels)
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Figure 18.2

19. Capillaries
•
Three structural types
1. Continuous capillaries
2. Fenestrated capillaries
3. Sinusoidal capillaries
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20. Continuous Capillaries
• Location:
• skin & muscles
• Cells connected with tight junctions
• Clefts allow passage
• Blood-brain barrier
• ↑ Tight junctions
• Limits passage
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21. Pericyte
Red blood
cell in lumen
Intercellular
cleft
Endothelial
cell
Basement
membrane
Tight junction
Pinocytotic
Endothelial
vesicles
nucleus
(a) Continuous capillary. Least permeable, and
most common (e.g., skin, muscle).
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Figure 18.3a

22. Fenestrated Capillaries
• Endothelial cells contain pores “fenestrations”
• Function:
• Absorption / Filtration
• Location:
• small intestines, endocrine glands, and kidneys
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23. Pinocytotic
vesicles
Red blood
cell in lumen
Fenestrations
(pores)
Endothelial
nucleus
Basement membrane
Tight junction
Intercellular
cleft
Endothelial
cell
(b) Fenestrated capillary. Large fenestrations
(pores) increase permeability. Occurs in special
locations (e.g., kidney, small intestine).
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Figure 18.3b

24. Sinusoidal Capillaries
• Larger intercellular clefts
• Function:
• Passage of large molecules and blood cells
• Location:
• liver, bone marrow, spleen
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25. Endothelial
cell
Red blood
cell in lumen
Large
intercellular
cleft
Tight junction
Nucleus of
Incomplete
endothelial
basement
cell
membrane
(c) Sinusoidal capillary. Most permeable. Occurs in
special locations (e.g., liver, bone marrow, spleen).
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Figure 18.3c

26. Capillary Beds
• Networks connecting arterioles and venules
• Sphincters control flow into capillaries
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27. Venules & Anastomosis
• Venules (smallest vein)
• Form from capillary beds
• “Postcapillary venules”
• Anastomosis
• Alternative connections between arteries and
veins
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28. Veins
• Return blood flow to heart
• Converging venules
• larger lumens w/ thin walls
• ↓ blood pressure
• Blood reservoirs
• contain ~65% of blood supply
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29. Veins
Adaptations to ensure blood return
•
1. Low resistance
Large lumen
•
1. Valves
Prevent backflow
•
•
in limbs
Venous sinuses: flattened veins
•
•
Ex. Coronary sinus
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30. Vein
Artery
(a)
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Figure 18.1a

31. Differences Between Arteries and Veins
Arteries
Veins
Delivery
Blood pumped into single
systemic artery—the aorta
Blood returns via
superior and interior
venae cavae and the
coronary sinus
Location
Deep, and protected by tissues
Both deep and
superficial
Pathways
Fairly distinct
Numerous
interconnections
Supply/drainage
Predictable supply
Usually similar to
arteries, except dural
sinuses and hepatic
portal circulation
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