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Marieb Chapter 19 Part A: Blood Vessels
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Venous system
Large veins
(capacitance
vessels)
Small veins
(capacitance
vessels)
Postcapillary
venule
Thoroughfare
channel
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Arterial system
Heart
Large
lymphatic
vessels
Lymph
node
Lymphatic
system
Arteriovenous
anastomosis
Elastic arteries
(conducting
vessels)
Muscular arteries
(distributing
vessels)
Lymphatic
Sinusoid
capillary
Arterioles
(resistance vessels)
Terminal arteriole
Metarteriole
Precapillary sphincter
Capillaries
(exchange vessels)
Figure 19.2
Tunica intima
• Endothelium
• Subendothelial layer
Internal elastic lamina
Tunica media
(smooth muscle and
elastic fibers)
External elastic lamina
Tunica externa
(collagen fibers)
Lumen
Artery
(b)
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Capillary
network
Valve
Lumen
Vein
Basement membrane
Endothelial cells
Capillary
Figure 19.1b
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Table 19.1 (1 of 2)
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Table 19.1 (2 of 2)
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?
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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
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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
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Capillary Beds
•
Networks of microscopic capillaries link arterioles
and venules
•
Two types of vessels
1. 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
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Venous system
Large veins
(capacitance
vessels)
Small veins
(capacitance
vessels)
Postcapillary
venule
Thoroughfare
channel
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Arterial system
Heart
Large
lymphatic
vessels
Lymph
node
Lymphatic
system
Arteriovenous
anastomosis
Elastic arteries
(conducting
vessels)
Muscular arteries
(distributing
vessels)
Lymphatic
Sinusoid
capillary
Arterioles
(resistance vessels)
Terminal arteriole
Metarteriole
Precapillary sphincter
Capillaries
(exchange vessels)
Figure 19.2
Blood Flow Through Capillary Beds
• Precapillary sphincters regulate blood flow into
true capillaries
• Primarily regulated by local chemical conditions
(
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,
and
)
Precapillary
sphincters
Vascular shunt
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.
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Figure 19.4
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?
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Vein
Artery
(a)
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Figure 19.1a
Where is the blood, at rest?
Pulmonary blood
vessels 12%
Systemic arteries
and arterioles 15%
Heart 8%
Capillaries 5%
Systemic veins
and venules 60%
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Figure 19.5
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
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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
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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
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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!
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Resistance
• 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)
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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
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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
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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
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Systolic pressure
Mean pressure
Diastolic
pressure
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Figure 19.6
Arterial Blood Pressure
• Systolic pressure: pressure exerted during
ventricular contraction
• Diastolic pressure: lowest arterial pressure
when the heart is completely at rest
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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
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Let’s Calculate Some MAP Values
• BP is 115/85
• BP is 145/95
• BP is 105/60
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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
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Maintaining Blood Pressure
• Requires the heart, blood vessels, and
kidneys to work together
• Supervision by the brain
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Maintaining Blood Pressure
• The main factors influencing blood pressure:
• Cardiac output (CO)
• Peripheral resistance (TPR)
• Blood volume (BV)
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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
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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)
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Cardiac Output (CO)
• During stress, the CAC increases heart rate
and stroke volume via sympathetic stimulation
• ESV decreases and MAP increases
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Exercise
BP activates cardiac centers in medulla
Activity of respiratory pump
(ventral body cavity pressure)
Activity of muscular pump
(skeletal muscles)
Parasympathetic
activity
Sympathetic activity
Epinephrine in blood
Sympathetic venoconstriction
Venous return
Contractility of cardiac muscle
EDV
ESV
Stroke volume (SV)
Heart rate (HR)
Initial stimulus
Physiological response
Result
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Cardiac output (CO = SV x HR
Figure 19.8
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)
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Short-Term Mechanisms: Neural Controls
• Neural controls operate via reflex arcs that
use:
• Baroreceptors
• Vasomotor centers and vasomotor fibers
• Vascular smooth muscle
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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
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Short-Term Mechanisms: BaroreceptorInitiated Reflexes
• Baroreceptors are located in the:
• Carotid sinuses
• Aortic arch
• Walls of large arteries of the neck and thorax
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Short-Term Mechanisms: BaroreceptorInitiated 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)
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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).
Homeostasis: Blood pressure in normal range
5
CO and R
return blood
pressure to
homeostatic range.
1 Stimulus:
5
CO and R
return blood pressure
to homeostatic range.
Blood pressure
(arterial blood
pressure falls below
normal 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.
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3 Impulses from baroreceptors stimulate
cardioacceleratory center (and inhibit cardioinhibitory
center) and stimulate vasomotor center.
Figure 19.9
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.
1 Stimulus:
Blood pressure
(arterial blood
pressure rises
above normal
range).
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4b Rate of
vasomotor impulses
allows vasodilation,
causing R
Homeostasis: Blood pressure in normal range
5
CO and
R return
blood pressure
to homeostatic
range.
Figure 19.9 step 5
What happens when the blood pressure falls?
Homeostasis: Blood pressure in normal range
1 Stimulus:
5
CO and R
return blood
pressure to
homeostatic
range.
4b Vasomotor
Blood pressure
(arterial blood
pressure falls
below normal
range).
fibers stimulate
vasoconstriction,
causing R
2 Baroreceptors
4a Sympathetic
impulses to heart
cause HR,
contractility, and
CO.
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in carotid sinuses
and aortic arch
are inhibited.
3 Impulses from baroreceptors
stimulate cardioacceleratory center
(and inhibit cardioinhibitory center)
and stimulate vasomotor center.
Figure 19.9 step 5
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 40 - 180 mm Hg
• Baroreceptors taking part in the aortic reflex
help maintain adequate blood pressure in the
systemic circuit
• Respond to MAP > 100 mm Hg
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What Happens to BP When You Get Out Of Bed?
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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!
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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)
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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
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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
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Long-Term Mechanisms: Renal Regulation
•
Baroreceptors adapt (
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
) to
1. Direct renal mechanism via blood volume
2. Indirect renal (renin-angiotensin) mechanism
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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
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Arterial pressure
Indirect renal
mechanism (hormonal)
Direct renal
mechanism
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
Result
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Arterial pressure
Figure 19.10
Activity of
muscular
pump and
respiratory
pump
Release
of ANP
Fluid loss from Crisis stressors:
hemorrhage, exercise, trauma,
excessive
body
sweating
temperature
Conservation
of Na+ and
water by kidney
Blood volume
Blood pressure
Blood pH, O2,
CO2
Blood
volume
Baroreceptors
Chemoreceptors
Venous
return
Stroke
volume
Bloodborne
Dehydration,
chemicals:
high hematocrit
epinephrine,
NE, ADH,
angiotensin II;
ANP release
Body size
Activation of vasomotor and cardiac
acceleration centers in brain stem
Heart
rate
Cardiac output
Diameter of
blood vessels
Blood
viscosity
Blood vessel
length
Peripheral resistance
Initial stimulus
Physiological response
Result
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Mean systemic arterial blood pressure
Figure 19.11
Control of Hypertension
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Control of Hypertension
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