1. Marieb Chapter 19 Part A: Blood Vessels

2. Precapillary sphincter
Venous system
Arterial system
Large veins
(capacitance
vessels)
Heart
Elastic arteries
(conducting
vessels)
Large
lymphatic
vessels
Lymph
node
Muscular arteries
(distributing
vessels)
Lymphatic
system
Small veins
(capacitance
vessels)
Arteriovenous
anastomosis
Lymphatic
capillary
Sinusoid
Arterioles
(resistance vessels)
Postcapillary
venule
Terminal arteriole
Metarteriole
Thoroughfare
channel
Capillaries
(exchange vessels)
Precapillary sphincter
Figure 19.2

3. • Subendothelial layer Internal elastic lamina
Tunica intima
• Endothelium
Valve
• Subendothelial layer
Internal elastic lamina
Tunica media
(smooth muscle and
elastic fibers)
External elastic lamina
Tunica externa
(collagen fibers)
Lumen
Vein
Lumen
Artery
Capillary
network
Basement membrane
Endothelial cells
(b)
Capillary
Figure 19.1b

4. Table 19.1 (1 of 2)

5. Table 19.1 (2 of 2)

6. Elastic (Conducting) Arteries
Large thick-walled arteries with elastin in all three tunics
Aorta and its major branches
Large lumen offers low-resistance
Act as pressure reservoirs—expand and recoil as blood is ejected from the heart
The recoil keeps blood flowing when the entire heart is in diastole.
What would happen if recoil couldn’t occur?

7. Muscular (Distributing) Arteries and Arterioles
Distal to elastic arteries; deliver blood to body organs
Have thick tunica media with more smooth muscle
Arterioles control flow into capillary beds via vasodilation and vasoconstriction

8. Capillaries
Microscopic blood vessels
Walls of thin tunica intima, one cell thick
Size allows only a single RBC to pass at a time (blood flow slows and RBCs need to flex to get through)
Diffusion of nutrients and wastes

9. Capillary Beds
• Networks of microscopic capillaries link arterioles and venules
Two types of vessels
Vascular shunt (metarteriole — thoroughfare channel):
Directly connects a terminal arteriole and a postcapillary venule
Always remain open so tissue receives oxygen and nutrients
2. True capillaries
10 to 100 vessels per bed
Branch off the metarteriole or terminal arteriole
Precapillary sphincters can shut off flow

10. Precapillary sphincter
Venous system
Arterial system
Large veins
(capacitance
vessels)
Heart
Elastic arteries
(conducting
vessels)
Large
lymphatic
vessels
Lymph
node
Muscular arteries
(distributing
vessels)
Lymphatic
system
Small veins
(capacitance
vessels)
Arteriovenous
anastomosis
Lymphatic
capillary
Sinusoid
Arterioles
(resistance vessels)
Postcapillary
venule
Terminal arteriole
Metarteriole
Thoroughfare
channel
Capillaries
(exchange vessels)
Precapillary sphincter
Figure 19.2

11. Blood Flow Through Capillary Beds
Precapillary sphincters regulate blood flow into true capillaries
Primarily regulated by local chemical conditions
( , and )

12. (a) Sphincters open—blood flows through true capillaries.
Vascular shunt
Precapillary
sphincters
Metarteriole
Thoroughfare channel
True capillaries
Terminal arteriole
Postcapillary venule
(a) Sphincters open—blood flows through true capillaries.
Terminal arteriole
Postcapillary venule
(b) Sphincters closed—blood flows through metarteriole
thoroughfare channel and bypasses true capillaries.
Figure 19.4

13. Veins Formed when venules converge
Have thinner walls, larger lumens than arteries
Blood pressure is much lower than in arteries
Called capacitance vessels (blood reservoirs); contain up to ______ of the blood supply
Veins stretch (like )
In what body regions do they stretch and why?

14. Vein
Artery
(a)
Figure 19.1a

15. Where is the blood, at rest?
Pulmonary blood
vessels 12%
Systemic arteries
and arterioles 15%
Heart 8%
Capillaries 5%
Systemic veins
and venules 60%
Figure 19.5

16. Physiology of Circulation: Definition of Terms
Blood flow
Volume of blood flowing through a vessel, an organ, or the entire circulation in a given period
Measured as ml/min or L/min
Equivalent to cardiac output (CO) for entire vascular system
Relatively constant when at rest
Varies widely through individual organs, based on needs

17. Physiology of Circulation: Definition of Terms
Blood pressure (BP)
Force per unit area exerted on the wall of a blood vessel by the blood
Expressed in mm Hg (systolic/diastolic)
Can calculate an average value (MAP)
Measured as systemic arterial BP in large arteries near the heart
A pressure gradient keeps blood moving from higher to lower pressure areas

18. Physiology of Circulation: Definition of Terms
Resistance (peripheral resistance)
Opposition to flow
Measure of the amount of friction the blood encounters
Mostly in the peripheral systemic circulation
Three important sources of resistance:
Blood viscosity
Total blood vessel length
Blood vessel diameter

19. Resistance Factors that remain relatively constant: Blood viscosity
The “stickiness” of the blood due to formed elements and plasma proteins
remember the karo syrup?
Blood vessel length
The longer the vessel, the greater the resistance encountered
Need more pressure to send water through a longer hose!

20. Changing vessel diameter significantly alters peripheral resistance
Varies inversely with the fourth power of vessel radius
E.g., if the radius is doubled, the resistance is 1/16 as much (not 1/2 as you would expect)

21. Resistance
Small-diameter arterioles are the site of change in peripheral resistance
Abrupt changes in diameter or fatty plaques from atherosclerosis dramatically increase resistance
Disrupt laminar flow and cause turbulence

22. Relationship Between Blood Flow and Resistance
Blood flow is inversely proportional to peripheral resistance (R)
If R increases, blood flow decreases
R is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter

23. Systemic Blood Pressure
The pumping action of the heart generates blood flow
Pressure results when the flow is opposed by resistance
Systemic pressure is highest in the aorta and then declines throughout the pathway
The steepest drop occurs in arterioles

24. Systolic pressure
Mean pressure
Diastolic
pressure
Figure 19.6

25. Arterial Blood Pressure
Systolic pressure: pressure exerted during ventricular contraction
Diastolic pressure: lowest arterial pressure when the heart is completely at rest

26. Arterial Blood Pressure
Mean arterial pressure (MAP): average pressure that propels the blood to the tissues
MAP = diastolic pressure + 1/3 (Systolic - diastolic)
MAP declines with increasing distance from the heart

27. Let’s Calculate Some MAP Values
BP is 115/85
BP is 145/95
BP is 105/60

28. Capillary Blood Pressure
Ranges from only 15 to 35 mm Hg
Low capillary pressure is desirable
High BP would rupture fragile, thin-walled capillaries
Most are very permeable, so low pressure forces filtrate into interstitial spaces

29. Maintaining Blood Pressure
Requires the heart, blood vessels, and kidneys to work together
Supervision by the brain

30. Maintaining Blood Pressure
The main factors influencing blood pressure:
Cardiac output (CO)
Peripheral resistance (TPR)
Blood volume (BV)

31. Maintaining Blood Pressure
Blood pressure = CO x PR (and CO depends on blood volume)
MAP = CO X TPR = (SV X HR) X TPR
Blood pressure varies directly with CO, TPR, and blood volume
Changes in one variable are quickly compensated for by changes in the other variables

32. Cardiac Output (CO)
Determined by venous return and neural and hormonal controls
Resting heart rate is maintained by the CIC via the parasympathetic vagus nerves
Stroke volume is controlled by preload (EDV)

33. Cardiac Output (CO)
During stress, the CAC increases heart rate and stroke volume via sympathetic stimulation
ESV decreases and MAP increases

34. BP activates cardiac centers in medulla
Exercise
BP activates cardiac centers in medulla
Activity of respiratory pump
(ventral body cavity pressure)
Sympathetic activity
Parasympathetic
activity
Activity of muscular pump
(skeletal muscles)
Epinephrine in blood
Sympathetic venoconstriction
Venous return
Contractility of cardiac muscle
EDV
ESV
Stroke volume (SV)
Heart rate (HR)
Initial stimulus
Physiological response
Result
Cardiac output (CO = SV x HR
Figure 19.8

35. Control of Blood Pressure
Short-term neural and hormonal controls
Oppose blood pressure changes by altering peripheral resistance (VMC action)
Also regulate HR and force of contraction
Long-term renal regulation
Counteracts fluctuations in blood pressure by altering blood volume (renin-angiotensin)

36. Short-Term Mechanisms: Neural Controls
Neural controls operate via reflex arcs that use:
Baroreceptors
Vasomotor centers and vasomotor fibers
Vascular smooth muscle

37. The Vasomotor Center
A cluster of sympathetic neurons in the medulla that send a message to change blood vessel diameter
Part of the cardiovascular center, along with the cardiac centers
Maintains vasomotor tone (moderate constriction of arterioles by symp NS)
Receives inputs from baroreceptors, chemoreceptors, and higher brain centers

38. Short-Term Mechanisms: Baroreceptor-Initiated Reflexes
Baroreceptors are located in the:
Carotid sinuses
Aortic arch
Walls of large arteries of the neck and thorax

39. Short-Term Mechanisms: Baroreceptor-Initiated Reflexes
Increased blood pressure stretches the baroreceptors so they fire faster
Inhibits the vasomotor center (VMC), causing arteriole dilation and venodilation
Inhibits the cardioacceleratory center (CAC)
Stimulates the cardioinhibitory center (CIC)

40. Figure 19.9 3 4a 2 4b 5 1 1 5 4b 2 4a 3 Impulses from baroreceptors
stimulate cardioinhibitory center
(and inhibit cardioacceleratory
center) and inhibit vasomotor
center. 4a
Sympathetic
impulses to heart
cause HR,
contractility, and
CO. 2
Baroreceptors
in carotid sinuses
and aortic arch
are stimulated. 4b
Rate of
vasomotor impulses
allows vasodilation,
causing R 5
CO and R
return blood
pressure to
homeostatic range. 1
Stimulus:
Blood pressure
(arterial blood
pressure rises above
normal range).
Homeostasis: Blood pressure in normal range 1
Stimulus:
Blood pressure
(arterial blood
pressure falls below
normal range). 5
CO and R
return blood pressure
to homeostatic range. 4b
Vasomotor
fibers stimulate
vasoconstriction,
causing R 2
Baroreceptors
in carotid sinuses
and aortic arch
are inhibited. 4a
Sympathetic
impulses to heart
cause HR,
contractility, and
CO. 3
Impulses from baroreceptors stimulate
cardioacceleratory center (and inhibit cardioinhibitory
center) and stimulate vasomotor center.
Figure 19.9

41. What happens when the blood pressure rises?3
Impulses from baroreceptors
stimulate cardioinhibitory center
(and inhibit cardioacceleratory
center) and inhibit vasomotor
center. 4a
Sympathetic
impulses to heart
cause HR,
contractility,
and CO. 2
Baroreceptors
in carotid
sinuses and
aortic arch are
stimulated. 4b
Rate of
vasomotor impulses
allows vasodilation,
causing R 1
Stimulus:
Blood pressure
(arterial blood
pressure rises
above normal
range). 5
CO and
R return
blood pressure
to homeostatic
range.
Homeostasis: Blood pressure in normal range
Figure 19.9 step 5

42. What happens when the blood pressure falls?
Homeostasis: Blood pressure in normal range 1
Stimulus:
Blood pressure
(arterial blood
pressure falls
below normal
range). 5
CO and R
return blood
pressure to
homeostatic
range. 4b
Vasomotor
fibers stimulate
vasoconstriction,
causing R 2
Baroreceptors
in carotid sinuses
and aortic arch
are inhibited. 4a
Sympathetic
impulses to heart
cause HR,
contractility, and
CO. 3
Impulses from baroreceptors
stimulate cardioacceleratory center
(and inhibit cardioinhibitory center)
and stimulate vasomotor center.
Figure 19.9 step 5

43. Short-Term Mechanisms: Baroreceptor-Initiated Reflexes
Baroreceptors taking part in the carotid sinus reflex protect the blood supply to the brain
Respond to MAP of mm Hg
Baroreceptors taking part in the aortic reflex help maintain adequate blood pressure in the systemic circuit
Respond to MAP > 100 mm Hg

44. What Happens to BP When You Get Out Of Bed?

45. Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes
Chemoreceptors are located in the
Carotid sinus
Aortic arch
Large arteries of the neck
Pretty much in the same places as the baroreceptors!

46. Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes
Chemoreceptors respond to rise in CO2, drop in pH or O2
Increase blood pressure via the vasomotor center and the cardioacceleratory center
Are MUCH more important in the regulation of respiratory rate and depth (Chapter 22)

47. Short-Term Mechanisms: Hormonal Controls
Adrenal medulla hormones norepinephrine (NE) and epinephrine cause generalized vasoconstriction and increase cardiac output
Angiotensin II, generated by kidney release of renin, causes vasoconstriction

48. Short-Term Mechanisms: Hormonal Controls
Atrial natriuretic peptide causes blood volume and blood pressure to decline, causes generalized vasodilation
Antidiuretic hormone (ADH)(vasopressin) causes intense vasoconstriction in cases of extremely low BP

49. Long-Term Mechanisms: Renal Regulation
Baroreceptors adapt ( ) to chronic high or low BP
Since they stop working, what does our body then do?
Let the kidneys spring into action to regulate arterial blood pressure
Direct renal mechanism via blood volume
Indirect renal (renin-angiotensin) mechanism

50. Indirect Mechanism Involves Renin-Angiotensin
The renin-angiotensin mechanism
 Arterial blood pressure  release of renin
Renin production of angiotensin II
Angiotensin II is a potent vasoconstrictor
Angiotensin II  aldosterone secretion
Aldosterone  renal reabsorption of Na+ and  urine formation
Angiotensin II stimulates ADH release

51. Figure 19.10 Arterial pressure Direct renal Indirect renal mechanism
mechanism (hormonal)
Baroreceptors
Sympathetic stimulation
promotes renin release
Kidney
Renin release
catalyzes cascade,
resulting in formation of
Angiotensin II
Filtration
ADH release
by posterior
pituitary
Aldosterone
secretion by
adrenal cortex
Water
reabsorption
by kidneys
Sodium
reabsorption
by kidneys
Blood volume
Vasoconstriction
( diameter of blood vessels)
Initial stimulus
Physiological response
Arterial pressure
Result
Figure 19.10

52. Activation of vasomotor and cardiac acceleration centers in brain stem
Activity of
muscular
pump and
respiratory
pump
Release
of ANP
Fluid loss from
hemorrhage,
excessive
sweating
Crisis stressors:
exercise, trauma,
body
temperature
Bloodborne
chemicals:
epinephrine,
NE, ADH,
angiotensin II;
ANP release
Dehydration,
high hematocrit
Body size
Conservation
of Na+ and
water by kidney
Blood volume
Blood pressure
Blood pH, O2,
CO2
Blood
volume
Baroreceptors
Chemoreceptors
Venous
return
Activation of vasomotor and cardiac
acceleration centers in brain stem
Stroke
volume
Diameter of
blood vessels
Blood
viscosity
Blood vessel
length
Heart
rate
Cardiac output
Peripheral resistance
Initial stimulus
Physiological response
Result
Mean systemic arterial blood pressure
Figure 19.11

53. Control of Hypertension

54. Control of Hypertension