1. Chapter 21: The Cardiovascular System: Blood Vessels
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Chapter 21: The Cardiovascular System: Blood Vessels

2. Vessel Structure - General
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All vessels same basic structure
3 wall layers (or tunics)
Tunica adventitia (externa) - elastic and laminar fibers
Tunica media
thickest layer
elastic fibers and smooth muscle fibers
Tunica interna (intima)
endothelium – non-stick layer
basement membrane
internal elastic lamina
Lumen - opening

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Structure/function relationship changes as move through cardiovascular tree
Tunic thickness and composition variable throughout cardiovascular tree

4. Vessel Structure – Elastic Arteries
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Elastic (conducting) arteries
Near heart
Thick walls
More elastic fiber, less smooth muscle
Lose elasticity with aging

5. Vessel Structure - Elastic Arteries
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Aorta and elastic arteries
Can vasoconstrict or vasodilate
Large arteries expand, absorb pressure wave then release it with elastic recoil - Windkessel effect
Help to push blood along during diastole
With aging have less expansion and recoil

6. Vessel Structure – Muscular Arteries
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Muscular (distributing) arteries
Deliver blood to organs
More smooth muscle
Less elastic fibers

7. Vessel Structure - Arterioles
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Arterioles
Distribution of blood in organs
Composition varies depending on position - more muscle, less elasticity nearer heart
Regulate flow from arteries to capillaries
Flow = ΔP/R
vary resistance by changing vessels size
Site of blood pressure regulation

8. Vessel Structure - Capillaries
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Microcirculation connects arteries and veins
Found in nearly every tissue in body
Higher the metabolic rate, more capillaries in tissue
Muscle many caps (>600/mm2)
Cartilage none

9. Vessel Structure - Capillaries
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Allow exchange of nutrients and wastes between blood and tissue
Capillary structure - simple
Basal lamina - connective tissue
Endothelial cells
Structure/function

10. Flow Regulation Regulation by vessels with smooth muscle Metarterioles
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Regulation by vessels with smooth muscle
Metarterioles
connect arterioles to venules through capillary bed
allows flow through capillary bed w/out flow through caps

11. Flow Regulation True capillaries Pre-capillary sphincter
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True capillaries
Pre-capillary sphincter
ring of smooth muscle
open/close to control flow
regulated by chemicals
Intermittent vasomotion – caps open for flow 5-10X min

12. Types of Capillaries 3 types of capillaries Continuous capillaries
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3 types of capillaries
Continuous capillaries
continuous endothelial cells except for cleft between cells
tight junctions between endothelial cells prevent most things from leaving caps
most capillaries in body

13. Types of Capillaries Fenestrated capillaries
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Fenestrated capillaries
fenestrations (slits) allow for filtration of small substances
glomerular capillaries in kidney

14. Types of Capillaries Sinusoid capillaries
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Sinusoid capillaries
wider gaps between endothelial cells allowing RBC’s to exit the caps
found in liver

15. Vessel Structure - Veins
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Venules
Collect blood from caps carry it to veins
Structure changes with position
Become more vessel-like (walls) as move from capillaries

16. Vessel Structure – Veins
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Veins
Interna thicker than arteries
Media thinner, less muscle
Externa thick
Valves
Pressure low
High compliance - change volume easily with small change in pressure
Varicose veins

17. Vessel Structure - Histology
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Very different morphology under light microscopy
Tunica media thickness differentiates artery from vein

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Artery Vein
Vein Artery

19. Vessel Structure/Function
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At rest
60% of blood located in veins and venules
Serve as reservoirs for blood, “storing” it until needed
Particularly veins of abdominal organs, skin
ANS regulates volume distribution
Vasoconstrict
Vasodilate
Open areas of circulation to be supplied with blood
veins at rest
caps during exercise
Can “shift” volumes to other areas as needed

20. Vessel Structure/Function
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 0.75 L/min
Rest
CO = 5 L/min

21. Vessel Structure/Function
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CO = 25 L/min
Heavy
Exercise
 20 L/min
 0.75 L/min
Rest
CO = 5 L/min

22. Physiology of Circulation
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Flow = ΔP/R
or CO = MAP/R
MAP - mean arterial pressure
Higher pressure to lower pressure with resistance (R) factor
Blood pressure
Pressure of blood on vessel wall
Measurement of pressure of a volume in a space
Systole/diastole - 120/80
BP falls progressively from aorta to O mm Hg at RA

23. Regulation of Blood Flow
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Resistance - opposition to blood flow from blood and vessel wall friction
Factors that affect resistance (R)
Viscosity - V R
thickness of blood
dehydration, polycythemia
R proportional to vessel length
garden hose vs. straw
obesity
Vessel diameter
changes in diameter affect flow
vessel wall drag – blood cells dragging against the wall
laminar flow – layers of flow
R inverse proportional to radius4
decrease in r by 1/2 R 16X
only important in vessels that can change their size actively

24. Regulation of Pressure, Resistance
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Systemic vascular resistance (Total Peripheral Resistance - TPR)
All vascular resistance offered by systemic vessels
Which vessels change size?
Resistance highest in arterioles
Largest pressure drop occurs in arterioles
Relationship of radius to resistance in arterioles important due to smooth muscle in walls

25. Systemic Blood Pressure
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Arterial Blood Pressure
Pulsatile in arteries due to pumping of heart
Systolic/diastolic
Pulse pressure = systolic (minus) diastolic
- Windkessel effect on pulse pressure?
- Decreases pulse pressure
- What is the effect of hardening of the arteries on pulse pressure?
- Increases pulse pressure

26. Systemic Blood Pressure
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Capillary Blood Pressure
Relatively low blood pressure
Low pressure good for caps because:
caps are fragile - hi pressure tears them up
caps are very permeable - hi pressure forces a lot of fluid out

27. Systemic Blood Pressure
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Venous return
Volume of blood flowing back to heart from systemic veins
Depends on pressure difference (ΔP) from beginning of venules (16 mmHg) to heart (0 mmHg)
Any change in RA pressure changes venous return

28. Help for venous return Skeletal muscle pump Respiratory pump
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Help for venous return
Skeletal muscle pump
muscles squeeze veins
force blood back to heart
valves prevent back flow
Respiratory pump
inhaling pulls air into lungs
helps to pull blood back into thorax

29. Velocity of Blood Flow
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Velocity of blood flow - inversely related to total cross sectional area (CSA) of vessels
Aorta
Total CSA 3-5 cm2
Velocity 40 cm/sec
Capillaries
Total CSA cm2
Velocity 0.1 cm/sec
Vena Cava
Total CSA 14 cm2 in vena cava
Velocity 5-20 cm/sec

30. Vessel Structure - Function
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Capillary Function
Site of exchange between blood and tissues
Delivery of nutrients and removal of wastes
Slow flow allows time for exchange
Mechanisms of nutrient exchange
Diffusion - O2, CO2, glucose, AA's, hormones diffuse down [ ] gradients
If lipid soluble, can travel through cell
If water soluble, between cells

31. Capillary Fluid Exchange
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Fluid movement
Fluid filtered and reabsorbed across capillary wall
Starling’s law of the capillaries
Forces driving the movement of fluid
Hydrostatic pressure capillary (HPc)
Hydrostatic pressure interstitial fluid (HPif)
Osmotic pressure capillary(OPc)
Osmotic pressure interstitial fluid (OPif)
Net filtration pressure (NFP) is a sum of all

32. Capillary Fluid Exchange
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On average 85% of fluid filtered at arteriole end is reabsorbed at venular end

33. Maintaining Blood Pressure - Short Term Mechanisms - CNS
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Neural Control - Cardiac centers in medulla
Vasomotor center
medullary area dedicated to control of blood vessels
sends sympathetic output to blood vessels
Vasoconstricts or vasodilates as needed
tone - normal amount of vasoconstriction or vasodilation
can vary tone which varies delivery of blood
receives input from different sources
baroreceptors
chemoreceptors

34. Maintaining Blood Pressure – Short term mechanisms – CNS reflexes
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Baroreceptor initiated reflex
Located at carotid sinus and aortic arch
Monitor changes in blood pressure
Regulate activity of Sympathetic Nervous System (vascular tone)

35. Maintaining Blood Pressure – Short term mechanisms – CNS reflexes
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Chemoreceptor initiated reflexes
Carotid bodies, aortic bodies
Monitors changes in chemicals (O2, CO2, [H+])
 CO2,  H+,  O2 (stresses) result in  sympathetic activity and  BP

36. Maintaining Blood Pressure – Short term mechanisms – CNS reflexes
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Influence of Higher Brain Centers (areas above medulla) - Cortex and Hypothalamus
Not involved in minute to minute regulation
Influence vasomotor center depending on conditions
public speaking
temperature regulation

37. Maintaining Blood Pressure - Short Term Mechanisms - Hormones
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Renin - Angiotensin - Aldosterone
Renin
enzyme from kidney
results in formation of Angiotensin II (AII)
AII
vasoconstrictor
stimulates ADH, thirst
stimulates aldosterone - Na+ reabsorption
Why/how would these things affect blood pressure?

38. Maintaining Blood Pressure - Short Term Mechanisms - Hormones
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Adrenal medulla - Epi and Norepi
 CO (HR,  SV)
Constrict abdominal, cutaneous arterioles/venules
Dilate cardiac, skeletal muscle beds
Why/how would this affect blood pressure?

39. Maintaining Blood Pressure - Short Term Mechanisms - Hormones
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Antidiuretic Hormone (ADH)
Osmoreceptors in hypothalamus
Retains fluid (inhibited by alcohol)
Vasoconstriction at high levels
Why/how would this affect blood pressure?

40. Maintaining Blood Pressure - Short Term Mechanisms - Hormones
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ANP (atrial natriuretic peptide)
Released from atrial cells in response to BP
Vasodilator, Na+ and water loss, opposes Aldosterone
Why/how would this affect blood pressure?

41. Maintaining Blood Pressure - Long Term Regulation
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Renal mechanism
Volume in a space
Regulate space in the short term – we just talked about it!
nervous control
hormones
Regulate volume in the long term
The kidneys!
 BP,  urine flow to  BP
 BP,  urine flow to  BP

42. Control of Blood Flow
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Autoregulation (local control) - local automatic adjustment of blood flow to match tissue needs
Physical changes
Warming -  vasodilation
Cooling -  vasoconstriction
Chemical changes - metabolic products
Vasodilators
Vasoconstrictors
Myogenic control
smooth muscle controls resistance
 stretch  contraction,  stretch  relaxation

43. Blood Flow in Special Areas
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Skeletal Muscle
Wide variability in amount of flow
Sympathetic regulation from brain in response to level of activity
α receptors - vasoconstrict
β receptors - vasodilate
Metabolic regulation in tissue
low O2  vasodilate to increase flow
hi O2  vasoconstrict to decrease flow
Brain
Very little variability in flow
Stores few nutrients so flow must be maintained!
Metabolic regulation

44. Blood Flow in Special Areas
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Skin
Supplies nutrients, aids in temperature regulation, provides a blood reservoir
Metabolic and sympathetic regulation
Lungs
Low pressure (25/10), low resistance
Flow regulated by O2 availability in the lungs
hi O2  vasodilate to increase flow – opposite to muscle
low O2  vasoconstrict to decrease flow – opposite to muscle
Heart
Variable flow depending on activity

45. Regulation of Blood Pressure
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Regulation of Blood Pressure
CO = MAP/R
MAP = CO x R