1. Chapter 9 Circulatory Systems Sections 9.8-9.16

2. 9.11 Circulatory Vessels: Arteries
Arteries provide rapid passage of blood from the heart to the tissues
Large radii offer little resistance to flow
Blood flows at high velocity
Arteries serve as pressure reservoirs
Arteries’ elasticity enables them to expand during ventricular systole
Elastic recoil is the driving force for continued flow of blood during diastole 2

3. 9.11 Circulatory Vessels: Arteries3

4. Elastin fibers
FIGURE Elastin fibers in an artery. Light micrograph of a portion of the aorta wall in cross section, showing numerous wavy elastin fibers, common to all arteries.
Figure 9-42 p430 4

5. 9.11 Circulatory Vessels: Arteries
Arterial blood pressure
Maximum pressure exerted on the arteries (systolic blood pressure) averages 120 mmHg in humans
Minimum pressure (diastolic blood pressure) averages 80 mmHg
Arterial blood pressure is expressed as a fraction (e.g. 120/80 mmHg)
Mean arterial pressure is the main driving force of blood flow
Mean arterial pressure = diastolic pressure /3 (systolic pressure - diastolic pressure) 5

6. Arterial pressure (mm Hg)v
120
Systolic pressure
Pulse
pressure
Arterial pressure (mm Hg)
Mean
pressure 93
FIGURE Arterial blood pressure. Values are for an average human. The systolic pressure is the peak pressure exerted in the arteries when blood is pumped into them during ventricular systole. The diastolic pressure is the lowest pressure exerted in the arteries when blood is draining off into the vessels downstream during ventricular diastole. The pulse pressure is the difference between systolic and diastolic pressure. The mean pressure is the average pressure throughout the cardiac cycle. 80
Diastolic pressure
Time (msec)
Figure 9-44 p432 6

7. 9.12 Circulatory Vessels: Arterioles
Arterioles are the major resistance vessels
Radii of arterioles are small enough to offer considerable resistance to flow
Large drop in blood pressure through the arterioles
Mean arterial pressure of 93 mmHg drops to 37 mmHg where blood enters the capillaries
Eliminates pulsatile pressure swings
Thick layer of smooth muscle is innervated by sympathetic nerve fibers
Vasoconstriction results from smooth muscle contraction —-> decreased radius, increased resistance
Vasodilation results from smooth muscle relaxation —-> increased radius, decreased resistance 7

8. 9.12 Circulatory Vessels: Arterioles
Distribution of blood flow
Amount of blood flow received by each organ is determined by the number and diameter of its arterioles
Local (intrinsic) controls are changes within a tissue that alter the radii of arterioles
Local metabolic changes produce relaxation of arteriolar smooth muscle to increase blood flow to the organ (active hyperemia)
Histamine release causes vasodilation as an inflammatory response
Exposure to heat or cold
Stretch produces myogenic vasoconstriction 8

9. 9.12 Circulatory Vessels: Arterioles
Extrinsic control includes neural and hormonal influences with the sympathetic nervous system dominating
Sympathetic stimulation redistributes blood flow to the heart and skeletal muscle at the expense of other organs
Vasoconstriction of most arterioles due to activation of α1-adrenergic receptors by NE or epinephrine
Lung and brain arterioles do not have α1-receptors and do not constrict
Epinephrine activates β2-adrenergic receptors in arterioles of skeletal muscle and heart to cause vasodilation
Mean arterial pressure is the product of cardiac output and total peripheral resistance 9

10. Total peripheral resistance
Arteriolar radius
Blood
viscosity
Number of
red blood cells
Extrinsic control
(important in
regulation of blood
pressure)
Response to shear stress
(compensates for
changes in longitudinal
Force to blood flow)
Local (intrinsic) control
(local changes acting on arteriolar smooth muscle in the vicinity)
Vasopressin
(hormone important
in fluid balance; exerts vasoconstrictor effect)
Myogenic responses to stretch (important in autoregulation)
Angiotensin II
(hormone important
in fluid balance; exerts
vasoconstrictor effect)
FIGURE Factors affecting total peripheral resistance. The primary determinant of total peripheral resistance is the adjustable arteriolar radius. Two major categories of factors influence arteriolar radius: (1) local (intrinsic) control, which is primarily important in matching blood flow through a tissue with the tissue’s metabolic needs and is mediated by local factors acting on the arteriolar smooth muscle, and (2) extrinsic control, which is important in the regulation of blood pressure and is mediated primarily by sympathetic influence on arteriolar smooth muscle.
Heat, cold application
(therapeutic use)
Epinephrine
(hormone that generally reinforce sympathetic nervous system)
Histamine release
(involved with injuries
and allergic responses)
Local metabolic changes
In O2 and other
Metabolites (important in matching blood flow with metabolic needs)
Sympathetic activity
(exerts generalized
vasoconstrictor effect)
Figure 9-48 p438
Major factors affecting arteriolar radius 10

11. 9.13 Circulatory Vessels: Capillaries
Capillaries maximize diffusion rates for exchange of materials
Diffusion distance is short
Capillary walls are thin
Capillaries are narrow
Capillaries branch extensively
Surface area is large
Permeability is high
Pores between capillary endothelial cells
Tight junctions in brain capillaries form protective blood-brain barrier
Large pores in liver capillaries allow passage of proteins 11

12. Endothelial cell nucleus Capillary lumen
FIGURE Capillary anatomy. (a) Electron micrograph of a cross section of a capillary. The capillary wall consists of a single layer of endothelial cells. The nucleus of one of these cells is shown. (b) Photograph of a capillary bed. The capillaries are so narrow that the red blood cells must pass through single file.
(a) Cross section of a capillary
Figure 9-49a p439 12

13. Capillary Red blood cell (b) Capillary bed
FIGURE Capillary anatomy. (a) Electron micrograph of a cross section of a capillary. The capillary wall consists of a single layer of endothelial cells. The nucleus of one of these cells is shown. (b) Photograph of a capillary bed. The capillaries are so narrow that the red blood cells must pass through single file.
(b) Capillary bed
Figure 9-49b p439 13

14. 9.13 Circulatory Vessels: Capillaries14

15. (b) Transport across a typical capillary wall
Interstitial fluid
Endothelial cell
Water-filled pore
Plasma proteins
generally cannot
cross the capillary
wall
Plasma
Plasma
proteins
Plasma
membrane
Lipid-soluble
substances
pass through
the
endothelial
cells
typical
Cytoplasm
O2, CO2
Exchangeable
proteins
Na+, K+, glucose,
amino acids
FIGURE Exchanges across a continuous capillary wall, the most common type of capillary. (a) Slitlike gaps between adjacent endothelial cells form pores within the capillary wall. (b) As depicted in this cross section of a capillary wall, small water-soluble substances are exchanged between the plasma and the interstitial fluid by passing through the water-filled pores, whereas lipid-soluble substances are exchanged across the capillary wall by passing through the endothelial cells. Proteins to be moved across are exchanged by vesicular transport. Plasma proteins generally cannot escape from the plasma across the capillary wall.
Exchangeable
proteins are
moved across
by vesicular
transport
Small
water-soluble
substances pass
through the pores
(b) Transport across a typical capillary wall
Figure 9-50 p439 15

16. 9.13 Circulatory Vessels: Capillaries
Diffusion follows concentration gradients between blood and the surrounding cells
Cells consume glucose and oxygen
Cells produce CO2 and other metabolic wastes
Precapillary sphincters controlling flow through individual capillaries are sensitive to local metabolic changes
Blood velocity is slowest due to the very large cross-sectional area of the capillaries 16

17. Precapillary sphincter Metarteriole (thoroughfare channel)
Smooth muscle
Precapillary sphincter
Metarteriole (thoroughfare channel)
Capillary
FIGURE Capillary bed. Capillaries branch either directly from an arteriole or from a metarteriole, a thoroughfare channel between an arteriole and venule. Capillaries rejoin at either a venule or a metarteriole. Smooth muscle cells form precapillary sphincters that encircle capillaries as they arise from a metarteriole. (a) When the precapillary sphincters are relaxed, blood flows through the entire capillary bed. (b) When the precapillary sphincters are contracted, blood flows only through the metarteriole, bypassing the capillary bed.
Venule
(a) Sphincters relaxed
Figure 9-52 p440 17

18. 9.14 Circulatory Vessels: Lymphatic System
The lymphatic system returns excess filtered fluid to the blood.
Network of one-way vessels
Interstitial fluid enters small blind-ended terminal vessels (initial lymphatics) to form lymph
Lymph vessels flow into the venous system near the heart
Passage of lymph through the lymph nodes aids in immune defense against disease
Lymph vessels absorb fat form the digestive tract 18

19. 9.14 Circulatory Vessels: Lymphatic System19

20. (a) Relationship between initial lymphatics and blood capillaries
To venous
system
Arteriole
Tissue
cells
Interstitial
fluid
Venule
FIGURE Initial lymphatics. (a) Blind-ended initial lymphatics pick up excess fluid filtered by blood capillaries and return it to the venous system in the chest. (b) Note that the overlapping edges of the endothelial cells create valvelike openings in the vessel wall.
Blood capillary
Initial
lymphatic
(a) Relationship between initial lymphatics and blood capillaries
Figure 9-58a p445 20

21. ANIMATION: Lymph vascular system
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22. 9.15 Circulatory Vessels: Venules and Veins
Veins serve as a blood reservoir and return blood to the heart.
Veins are large in diameter with little elasticity and low myogenic tone.
Easily distend to accommodate additional volumes of blood with no recoil (capacitance vessels)
Mammalian veins contain more than 60% of the total blood volume
Blood velocity accelerates on return from capillaries 22

23. 9.15 Circulatory Vessels: Venules and Veins23

24. 9.15 Circulatory Vessels: Venules and Veins
Factors enhancing venous return to the heart
Driving pressure of cardiac contraction
Sympathetic activity produces venous vasoconstriction
Repeated contraction of skeletal muscles compresses veins (skeletal muscle pump)
Venous valves prevent blood from flowing backward (away from the heart)
Respiratory activity reduces pressure near the heart (respiratory pump)
Cardiac suction due to low ventricular pressures during diastole 24

25. (b) Action of venous valves, permitting flow of blood toward
Open venous valve
permits flow of
blood toward heart
Vein
Contracted
skeletal muscle
Closed venous valve
prevents backflow
of blood
FIGURE Function of venous valves. (a) When a tube is squeezed in the middle, fluid is pushed in both directions. (b) Venous valves permit the flow of blood only toward the heart.
(b) Action of venous valves,
permitting flow of blood toward
heart and preventing backflow
of blood
Figure 9-61b p448 25

26. Mean arterial blood pressure1 1
Total
peripheral
resistance
Cardiac
output 2 2 15 15
Heart
rate
Stroke
volume
Arteriolar
radius
Blood
viscosity 3 6 17 16 4 5 20
Para-sympathetic
activity
Sympathetic
activity and
epinephrine 7 10
Cardiac- suction effect
Local
metabolic
control
Extrinsic
vaso-constrictor
control
Number of
red blood
cells
Venous
return
FIGURE Determinants of mean arterial blood pressure. Note that this figure is basically a composite of Figure 9-29, p. 417, “Control of cardiac output”; Figure 9-48, p. 438, “Factors affecting total peripheral resistance”; and Figure 9-60, p. 447, “Factors that facilitate venous return.” See the text for a discussion of the circled numbers. 11 9 8 18 19 21
Skeletal
muscle
activity
Sympathetic
activity and
epinephrine
Vasopressin
and
angiotensin II
Blood
volume
Respiratory
activity 22 12 13
Salt and
water
balance
Vasopressin, renin–angiotensin–
aldosterone system
(Chapters 14 and 15)
Passive bulk-flow fluid shifts between vascular and interstitial fluid compartments 14
Figure 9-67 p454 26

27. 9.16 Integrated Cardiovascular Function
Cardiovascular function is regulated in an integrated fashion
Two major goals:
Proper gas and heat transport
Maintaining arterial blood pressure
Cardiac output is homeostatically regulated when an animal is at rest to maintain consistent delivery of oxygen to key organs.
During activity, cardiac output is reset higher to boost blood flow to skeletal muscles and skin. 27

28. 9.16 Integrated Cardiovascular Function
Regulation of arterial blood pressure
Pressure must be high enough to overcome gravity, friction and other resistance factors
Pressure must be high enough for ultrafiltration in the kidneys
Pressure must not be so high that it creates extra work for the heart and increases the risk of vascular damage.
Chronic high blood pressure (hypertension) is due to excess resistance 28

29. 9.16 Integrated Cardiovascular Function29

30. (a) Baroreceptor reflex in response to an elevation in blood pressure
When blood pressure
becomes elevated
above normal
Carotid sinus
and aortic arch
receptor potential
Rate of firing
in afferent nerves
Cardiovascular
center
Heart rate
and
stroke volume
arteriolar and
venous vasodilation
Blood
pressure
decreased
toward
normal
Sympathetic cardiac nerve activity
and
sympathetic vasoconstrictor nerve activity
parasympathetic nerve activity
Cardiac output
and
total peripheral
resistance
(a) Baroreceptor reflex in response to an elevation in blood pressure
Carotid sinus
and aortic arch
receptor potential
When blood pressure
falls below normal
Rate of firing
in afferent nerves
Cardiovascular
center
FIGURE Baroreflexes to restore the blood pressure to normal.
Heart rate
and
stroke volume
arteriolar and
venous vasoconstriction
Blood
pressure
increased
toward
normal
Sympathetic cardiac nerve activity
and
sympathetic vasoconstrictor nerve activity
parasympathetic nerve activity
Cardiac output
and
total peripheral
resistance
(b) Baroreceptor reflex in response to a fall in blood pressure
Figure 9-65 p451 30

31. 9.16 Integrated Cardiovascular Function
Response to increased locomotor activity
Increased heart rate and stroke volume result in increased cardiac output
Blood flow to skeletal muscle, heart and skin increases
Anticipatory responses
Increased locomotor activity induces cardiac and vascular changes before that activity leads to disturbances in blood gases.
Perception of stress can trigger the fight-or-flight response 31