1. Structure of Blood Vessels
Composed of three layers (tunics)
Tunica intima – composed of simple squamous epithelium
Tunica media – sheets of smooth muscle
Contraction – vasoconstriction
Relaxation – vasodilation
Tunica externa – composed of connective tissue
Lumen
Central blood-filled space of a vessel

2. Structure of Arteries, Veins, and Capillaries

3. Structure of Arteries, Veins, and Capillaries
Figure 19.1a

4. Arteries – carry blood away from the heart
Types of Blood Vessels
Arteries – carry blood away from the heart
Capillaries – smallest blood vessels
The site of exchange of molecules between blood and tissue fluid
Veins – carry blood toward the heart

5. Elastic arteries – the largest arteries
Types of Arteries
Elastic arteries – the largest arteries
Diameters range from 2.5 cm to 1 cm
Includes the aorta and its major branches
Sometimes called conducting arteries
High elastin content dampens surge of blood pressure
Figure 19.2a

6. Muscular (distributing) arteries
Types of Arteries
Muscular (distributing) arteries
Lie distal to elastic arteries
Diameters range from 1 cm to 0.3 mm
Includes most named arteries
Tunica media is thick
Unique features
Internal and external elastic laminae
Figure 19.2b

7. Types of Arteries Arterioles Smallest arteries
Diameters range from 0.3 mm to 10 µm
Larger arterioles possess all three tunics
Diameter of arterioles controlled by
Local factors in the tissues
Sympathetic nervous system
Figure 19.2c

8. Smallest blood vessels
Capillaries
Smallest blood vessels
Diameter from 8–10 µm
Red blood cells pass through single file
Site-specific functions of capillaries
Lungs – oxygen enters blood, carbon dioxide leaves
Small intestines – receive digested nutrients
Endocrine glands – pick up hormones
Kidneys – removal of nitrogenous wastes

9. RBCs in a Capillary
Figure 19.3

10. Network of capillaries running through tissues Precapillary sphincters
Capillary Beds
Network of capillaries running through tissues
Precapillary sphincters
Regulate the flow of blood to tissues
Tendons and ligaments – poorly vascularized
Epithelia and cartilage – avascular
Receive nutrients from nearby CT

11. Capillary Beds
Figure 19.4a

12. Capillary Beds
Figure 19.4b

13. Capillary Permeabillity
Endothelial cells – held together by tight junctions and desmosomes
Intercellular clefts – gaps of unjoined membrane
Small molecules can enter and exit
Two types of capillary
Continuous – most common
Fenestrated – have pores

14. Structure of Capillaries – Cross Section
Figure 19.5a

15. Structure of Capillaries – Cross Section
Figure 19.5b

16. Routes of Capillary Permeability
Four routes into and out of capillaries
Direct diffusion
Through intercellular clefts
Through cytoplasmic vesicles
Through fenestrations

17. Low Permeability Capillaries
Blood-brain barrier
Capillaries have complete tight junctions
No intercellular clefts are present
Vital molecules pass through
Highly selective transport mechanisms
Not a barrier against
Oxygen, carbon dioxide, and some anesthetics

18. Wide, leaky capillaries found in some organs
Sinusoids
Wide, leaky capillaries found in some organs
Usually fenestrated
Intercellular clefts are wide open
Occur in bone marrow and spleen
Sinusoids have a large diameter and twisted course

19. Sinusoids
Figure 19.5c

20. Conduct blood from capillaries toward the heart
Veins
Conduct blood from capillaries toward the heart
Blood pressure is much lower than in arteries
Smallest veins – called venules
Diameters from 8 – 100 µm
Smallest venules – called postcapillary venules
Venules join to form veins
Tunica externa is the thickest tunic in veins

21. Mechanisms to Counteract Low Venous Pressure
Valves in some veins
Particularly in limbs
Skeletal muscle pump
Muscles press against thin-walled veins
Figure 19.6

22. Vessels interconnect to form vascular anastomoses
Organs receive blood from more than one arterial source
Neighboring arteries form arterial anastomoses
Provide collateral channels
Veins anastomose more frequently than arteries

23. Tunica externa of large vessels have Vasa vasorum vessels of vessels
Tiny arteries, capillaries, and veins
Vasa vasorum vessels of vessels
Nourish outer region of large vessels
Inner half of large vessels receive nutrients from luminal blood

24. Pulmonary Circulation
Pulmonary trunk leaves the right ventricle
Divides into right and left pulmonary arteries
Superior and inferior pulmonary veins
Carry oxygenated blood into the left atrium