1. Chapter 19
BLOOD VESSELS
Part A

2. Blood Vessels A closed system of vessels that carry blood
It begins and ends at the heart
The three major types of vessels are:
arteries
capillaries
veins
Arteries carry blood away from the heart
Veins carry blood toward the heart
Capillaries:
Contact tissue cells
Directly serve cellular needs

3. Generalized Structure of Blood Vessels
Arteries and veins are composed of three tunics:
tunica interna (intima)
tunica media
tunica externa
Lumen:
central blood-containing space
surrounded by tunics
Capillaries are composed of:
Endothelium
Sparse basal lamina

4. Generalized Structure of Blood Vessels
Figure 19.1b

5. Tunics Tunica interna (tunica intima): Tunica media:
Endothelial layer lining the lumen of all vessels
Subendothelial connective tissue basement (vessels > 1 mm)
Tunica media:
A layer of smooth muscle and sheets of elastin
Regulated by sympathetic nervous system
Controls vasoconstriction or vasodilation of vessels

6. Tunics cont’d Tunica externa (tunica adventitia) Outermost layer
Composed mostly of collagen fibers that:
protect & reinforce vessels
Anchor vessels to surrounding structures
Infiltrated with:
Nerve fibers
Lymphatic vessels
Network of elastin fibers (in larger veins)
Larger vessels contain a system of tiny vessels
These vessels, called vasa vasorum, nourish them

7. Blood Vessel Anatomy
Table 19.1

8. Classification of Arteries:
Based on size & function, arteries are divided into:
Elastic arteries
Muscular arteries
Arterioles

9. Elastic Arteries Thick-walled arteries near the heart
Largest in diameter ( cm) (aorta & its major branches)
Have large lumen and low-resistance
Referred to as conducting arteries (conduction of blood)
Contain the higest elastin (of all vessels)
Elastin is found in all three tunics (most in tunica media)
Contain substantial smooth muscles (inactive in vasoconstriction)
Functionally, just elastic tubes
Serve as pressure reservoirs (expand & recoil with blood ejection)
Withstand & smooth out large blood pressure fluctuations

10. Muscular Arteries Also called distributing arteries
Are distal to elastic arteries
Deliver blood to specific body organs
Diameter ranges from 0.3 mm – 1 cm
Have the thickest tunica media
Compared to elastic arteries, their tunica media has:
More smooth muscle
Less elastic tissue
Elastin concentric sheets intersperse smooth muscle layers
Active in vasoconstriction and less distensible

11. Arterioles The smallest arteries Diameter range 10 um – 0.3 mm
Larger ones:
Have 3 tunics
Tunica media is chiefly smooth muscle with few scattered elastic fibers
Smaller ones:
Lead into capillaries
Almost a single layer of smooth muscle spiraling around endothelium
Control flow into capillary beds via vasodilation and constriction

12. Capillaries are the smallest blood vessels
Their walls consist of thin tunica intima (1-cell thick)
Allow only a single RBC to pass at a time
Cells called pericytes (smooth muscle-like cells) are found along the outer surface of some capillaries
Pericytes:
Sabilize capillary walls
Control permeability
Average capillary length is 1mm
Average lumen diameter is 8-10 um

13. There are three structural types of capillaries:
Continuous
Fenestrated
Sinusoids

14. Continuous Capillaries:
More common capilaries
Abundant in skin and muscle
Called continuous because their endothelial cells:
Provide a relatively uninterrupted lining due to:
Adjacent cells joined laterally by tight junctions
Junctions are usually incomplete leaving gaps
Membrane gaps are called intercellular clefts
Clefts allow limited passage of fluids and small solutes

15. Continuous Capillaries: cont’d
Cytoplasm of endothelial cells contains numerous pino-cytotic vesicles
Vesicles believed to transport fluids across capillary wall
Continuous capillaries of the brain:
Are unique
Their tight junctions are:
Complete, extending around the entire perimeter of endothelial cells
Constitute the structural basis of the blood-brain barrier

16. Vascular Components
Figure 19.2a, b

17. Continuous Capillaries
Figure 19.3a

18. Fenestrated Capillaries
Characterized by:
An endothelium riddled with pores (fenestrations)
Fenestrations are usually covered by a diaphragm
Greater permeability than continuous capillaries
Found where active capillary absorption or filtration occurs:
Small intestines
Endocrine glands
kidneys

19. Fenestrated Capillaries
Figure 19.3b

20. Sinusoids
Highly modified, leaky, fenestrated capillaries with large irregular lumens
Found only in the:
Liver
Bone marrow
Spleen
Adrenal medulla
Allow passage of large molecules and blood cells between the blood & surrounding tissues
Sluggish blood flow allowing for modification in various ways

21. Sinusoids
Figure 19.3c

22. Capillary Beds An interwoven networks of capillaries
Connect between an arteriole & a venule
Serve blood flow from arterioles to a venules, which is called the microcirculation
Fed by a terminal arteriole (arerial side of the circulation)
Drained by a postcapillary venule (venous side of the circulation)

23. Capillary Beds (cont’d)
In most body regions, a capillary bed consists of two types of vessels:
1. A vascular shunt made up of:
A short vessel (metarteriole - thoroughfare channel)
A metarteriole is intrmediate between:
An arteriole & a capillary
A thoroughfare is intrmediate between:
A veniole & a capillary
Connects an arteriole directly with a postcapillary venule

24. Capillary Beds (cont’d)
2. True capillaries:
Range between10 to 100 per capillary bed
Their number depends on the organ or tissue served
Usually, they branch off the metarteriole (proximal end of the shunt) and
Return to the thoroughfare channel (distal end of the shunt)
Occasionally the metarteriole branches empty directly into the venule

25. Capillary Beds
Figure 19.4a

26. Blood Flow Through Capillary Beds
Precapillary sphincter
Cuff of smooth muscle that surrounds each true capillary
Regulates blood flow into the capillary
Blood flow is regulated by:
Vasomotor nerves
Local chemical conditions

27. Capillary Beds
Figure 19.4b