Download Blood Pressure

Classes of Blood Vessels
• Arteries
• Carry blood away from heart
• Arterioles
• Are smallest branches of arteries
• Capillaries
• Are smallest blood vessels
• Location of exchange between blood and interstitial fluid
• Venules
• Collect blood from capillaries
• Veins
• Return blood to heart
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Vascular Components
Oxygenated
blood from heart
Deoxygenated
blood to heart
Valve
Connective tissue
Smooth muscle
Endothelium
Lumen
Microcirculation
Arteriole
Venule
Network of capillaries
Artery
Vein
Basement
membrane
Endothelium
Capillary
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Generalized Structure of Blood Vessels
• Arteries and veins are composed
of three tunics
• tunica interna,
• tunica media
• tunica externa
• Capillaries are composed of
endothelium with sparse basal
lamina
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Structure of vessel walls
• The walls of blood vessels are too thick to allow diffusion
between blood stream and surrounding tissues or the tissues of the
blood vessels.
• The walls of large vessels contain small blood vessels that supply
both tunica media and externa – vasa vasorum
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Capillaries
• Capillaries are the smallest blood vessels
• Walls consisting of a thin tunica interna, one cell
thick
• Allow only a single RBC to pass at a time
• Pericytes on the outer surface stabilize their walls
• There are three structural types of capillaries:
continuous, fenestrated, and sinusoids
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Continuous Capillaries
• Continuous capillaries are abundant in the skin and muscles
• Endothelial cells provide an uninterrupted lining
• Adjacent cells are connected with tight junctions
• Intercellular clefts allow the passage of fluids
• Continuous capillaries of the brain:
• Have tight junctions completely around the endothelium
• Constitute the blood-brain barrier
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Fenestrated Capillaries
• Found wherever active capillary absorption or filtrate
formation occurs (e.g., small intestines, endocrine
glands, and kidneys)
• Characterized by:
• An endothelium riddled with pores (fenestrations)
• Greater permeability than other capillaries
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Sinusoids
• Highly modified, leaky, fenestrated capillaries with
large lumens
• Found in the liver, bone marrow, lymphoid tissue, and
in some endocrine organs
• Allow large molecules (proteins and blood cells) to
pass between the blood and surrounding tissues
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Capillary Beds
• Vascular shunts – Metarteriole--is a vessel that emerges from an
arteriole, passes through the capillary network and empties into a
venule.
• Proximal portions are surrounded by scattered smooth muscle
cells whose contraction and relaxation help regulate the amount
and force of the blood.
• Distal portion has no smooth muscle fibers and is called a
thoroughfare channel.
• True capillaries – 10 to 100 per capillary bed, capillaries branch off
the metarteriole and return to the thoroughfare channel at the distal
end of the bed
• At their site of origin, there is a ring of smooth muscle fibers called
a precapillary sphincter that controls the flow of blood entering a
true capillary
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Capillary Beds
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Figure 19.4a
Capillary Beds
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Figure 19.4b
Blood Flow
• The purpose of cardiovascular regulation is to maintain
adequate blood flow through the capillaries to the
tissues
• Actual volume of blood flowing through a vessel, an
organ, or the entire circulation in a given period:
• Is measured in ml/min.
• Is equivalent to cardiac output (CO), considering
the entire vascular system
• Is relatively constant when at rest
• Varies widely through individual organs
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Blood Flow Through Tissues
• Blood flow, or tissue perfusion, is involved in:
• Delivery of oxygen and nutrients to, and removal
of wastes from, tissue cells
• Gas exchange in the lungs
• Absorption of nutrients from the digestive tract
• Urine formation by the kidneys
• Blood flow is precisely the right amount to provide
proper tissue function
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Circulatory Shock
• Circulatory shock – any condition in which blood vessels
are inadequately filled and blood cannot circulate normally
• Results in inadequate blood flow to meet tissue needs
• Three types include:
• Hypovolemic shock – results from large-scale blood loss
or dehydration
• Vascular shock – normal blood volume but too much
accumulate in the limbs (long period of standing/sitting)
• Cardiogenic shock – the heart cannot sustain adequate
circulation
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Blood flow
• Capillary blood flow is determined by the interplay
between:
• Pressure
• Resistance
• The heart must generate sufficient pressure to
overcome resistance
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Physiology of Circulation: Blood Pressure (BP)
• Force per unit area exerted on the wall of a blood
vessel by its contained blood
• Expressed in millimeters of mercury (mm Hg)
• Measured in reference to systemic arterial BP in
large arteries near the heart
• The differences in BP within the vascular system
provide the driving force that keeps blood moving
from higher to lower pressure areas
• Absolute pressure is less important than pressure
gradient
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A model that relates blood flow to the pressure gradient
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A model that relates blood flow to the pressure gradient
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A model that relates blood flow to the pressure gradient
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Physiology of Circulation: Resistance
• Opposition to flow
• Measure of the amount of friction blood encounters
• Generally encountered in the peripheral systemic
circulation
• Because the resistance of the venous system is very low
(why?) usually the focus is on the resistance of the
arterial system:
• Peripheral resistance (PR) is the resistance of the
arterial system
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Physiology of Circulation: Resistance
• Three important sources of resistance
• Blood viscosity
• Total blood vessel length
• Blood vessel diameter
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Resistance – constant factors
• Blood viscosity
• The “stickiness” of the blood due to formed
elements and plasma proteins
• Blood vessel length
• The longer the vessel, the greater the resistance
encountered
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Resistance – frequently changed factors
• Changes in vessel diameter are frequent
significantly alter peripheral resistance
and
• Small-diameter arterioles are the major determinants of
peripheral resistance
• Frequent changes alter peripheral resistance
• Fatty plaques from atherosclerosis:
• Cause turbulent blood flow
• Dramatically increase resistance due to turbulence
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The effect of resistance on flow
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Systemic Blood Pressure
• The pumping action of the heart generates blood flow
through the vessels along a pressure gradient, always
moving from higher- to lower-pressure areas
• Pressure results when flow is opposed by resistance
• Systemic pressure:
• Is highest in the aorta
• Declines throughout the length of the pathway
• Is ~0 mm Hg in the right atrium
• The steepest change in blood pressure occurs between
arteries to arterioles
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Arterial Blood Pressure
• Arterial pressure is important because it maintains blood
flow through capillaries
• Blood pressure near the heart is pulsatile
• Systolic pressure: pressure exerted during ventricular
contraction
• Diastolic pressure: lowest level of arterial pressure
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Arterial Blood Pressure
• A pulse is rhythmic pressure oscillation that
accompanies every heartbeat
• Pulse pressure = difference between systolic and
diastolic pressure
• Mean arterial pressure (MAP): pressure that propels
the blood to the tissues
MAP = diastolic pressure + 1/3 pulse pressure
• Pulse pressure and MAP both decline with increasing
distance from the heart
• Efficiency of the circulation can be assessed by taking
pulse and blood pressure measurements
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Factors that Influence Mean Arterial Pressure
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Figure 15-10
How increases in cardiac output and total peripheral resistance
increase mean arterial pressure
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Alterations in Blood Pressure
• Hypotension – low BP in which systolic pressure is below
100 mm Hg
• Hypertension – condition of sustained elevated arterial
pressure of 140/90 or higher
• Transient elevations are normal and can be caused
by fever, physical exertion, and emotional upset
• Chronic elevation is a major cause of heart failure,
vascular disease, renal failure, and stroke
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Capillary Blood Pressure
• Ranges from 15 to 35 mm Hg
• Low capillary pressure is desirable
• High BP would rupture fragile, thin-walled capillaries
• Low BP is sufficient to force filtrate out into interstitial
space and distribute nutrients, gases, and hormones
between blood and tissues
• Capillary blood pressure is one of the driving forces for
transport through the capillary wall (filtration)
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Exchange of materials across the wall of a capillary
Endothelial cell
Pores
Plasma
Lumen
O2, CO2,
fatty acids,
Steroid
hormones
Na+, K+,
glucose
Capillary
Proteins
Water-filled
pore
Interstitial fluid
Diffusion through
cells (lipid-soluble
substances)
Diffusion through
pores (water-soluble
substances)
Restricted movement
of most plasma
proteins (cannot
cross capillary wall)
Transcytosis of
exchangeable
proteins across
cells
Cytoplasm
Plasma membrane
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Net Filtration Pressure (NFP)
• NFP—comprises all the forces acting on a capillary
bed
• NFP = (HPc—HPif)—(OPc—OPif)
• At the arterial end of a bed, hydrostatic forces
dominate
• At the venous end, osmotic forces dominate
• Excess fluid is returned to the blood via the lymphatic
system
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Hydrostatic Pressures
• Capillary hydrostatic pressure (HPc) (capillary blood
pressure)
• Tends to force fluids through the capillary walls
• Is greater at the arterial end (35 mm Hg) of a bed
than at the venule end (17 mm Hg)
• Interstitial fluid hydrostatic pressure (HPif)
• Usually assumed to be zero because of lymphatic
vessels
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Colloid Osmotic Pressures
• Capillary colloid osmotic pressure (oncotic pressure)
(OPc)
• Created by non-diffusible plasma proteins, which
draw water toward themselves
• ~26 mm Hg
• Interstitial fluid osmotic pressure (OPif)
• Low (~1 mm Hg) due to low protein content
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Arteriole
Venule
Interstitial fluid
Net HP—Net OP
(35—0)—(26—1)
Net
HP
35
mm
Capillary
Net
OP
25
mm
NFP (net filtration pressure)
is 10 mm Hg; fluid moves out
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Net HP—Net OP
(17—0)—(26—1)
Net
HP
17
mm
Net
OP
25
mm
NFP is ~8 mm Hg;
fluid moves in
HP = hydrostatic pressure
• Due to fluid pressing against a wall
• “Pushes”
• In capillary (HPc)
• Pushes fluid out of capillary
• 35 mm Hg at arterial end and
17 mm Hg at venous end of
capillary in this example
• In interstitial fluid (HPif)
• Pushes fluid into capillary
• 0 mm Hg in this example
OP = osmotic pressure
• Due to presence of nondiffusible
solutes (e.g., plasma proteins)
• “Sucks”
• In capillary (OPc)
• Pulls fluid into capillary
• 26 mm Hg in this example
• In interstitial fluid (OPif)
• Pulls fluid out of capillary
• 1 mm Hg in this example
CHP – 35-17 mmHg
IHP – 0 mmHg
BCOP – 25 mmHg
ICOP – 0 mmHg
Figure 19.17
Fluid Exchange at a Capillary
• Hydrostatic pressure and osmotic pressure regulate
bulk flow
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Figure 15-19a
Venous Blood Pressure
• Changes little during the cardiac cycle
• Small pressure gradient, about 15 mm Hg
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Factors Aiding Venous Return
• Venous BP alone is too low to promote adequate
blood return and is aided by the:
• Respiratory “pump” – pressure changes created
during breathing suck blood toward the heart by
squeezing local veins
• Muscular “pump” – contraction of skeletal muscles
“milk” blood toward the heart
• Valves prevent backflow during venous return
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Controls of Blood Pressure
• Short-term controls:
• Counteract moment-to-moment fluctuations in
blood pressure by altering peripheral resistance
• Long-term controls regulate blood volume
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Central Regulation
Central regulation involves
neuroendocrine mechanisms
that control the total systemic
circulation. This regulation
involves both the
cardiovascular centers
and the vasomotor centers.
Neural
mechanisms
Stimulation of
receptors sensitive
to changes in
systemic blood
pressure or
chemistry
Stimulation
of endocrine
response
Activation of
cardiovascular
centers
Short-term
elevation of blood
pressure by
sympathetic
stimulation of the
heart and
peripheral
vasoconstriction
Long-term increase
in blood volume
and blood pressure
Endocrine mechanisms
If autoregulation is ineffective
HOMEOSTASIS
RESTORED
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Short-Term Mechanisms to controls of Blood Pressure
• Autoregulation
• Causes immediate, localized homeostatic adjustments
• Neural mechanisms
• Respond quickly to changes at specific sites
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Autoregulation of blood flow within tissues
• Autoregulation – automatic adjustment of blood flow to each
tissue in proportion to its requirements at any given point in time
• Local vasodilators accelerate blood flow in response to:
• Decreased tissue O2 levels or increased CO2 levels
• Generation of lactic acid
• Rising H+ concentrations in interstitial fluid
• Local inflammation
• Elevated temperature
• Vasoconstrictors:
• Injured vessels constrict strongly (why?)
• Drop in tissue temperature (why?)
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Myogenic (myo =muscle; gen=origin) Controls
• Inadequate blood perfusion (tissue might die) or
excessively high arterial pressure (rupture of vessels)
may interfere with the function of the tissue
• Vascular smooth muscle can prevent these problems by
responding directly to passive stretch (increased
intravascular pressure)
• The muscle response is by resist to the stretch and
that results in vasoconstriction
• The opposite happens when there is a reduced stretch
• The myogenic mechanisms keeps the tissue perforation
relatively constant.
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The myogenic response to changes in perfusion pressure
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Short-Term Mechanisms: Neural Controls
• Neural controls of peripheral resistance:
• Alter blood distribution in response to demands
• Maintain MAP by altering blood vessel diameter
• Neural controls operate via reflex arcs involving:
• Vasomotor centers and vasomotor fibers
• Baroreceptors
• Vascular smooth muscle
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Short-Term Mechanisms: Vasomotor Center
• Vasomotor center – a cluster of sympathetic neurons in
the medulla that oversees changes in blood vessel
diameter
• Maintains blood vessel tone by innervating smooth
muscles of blood vessels, especially arterioles
• Vasomotor activity is modified by:
• Baroreceptors (pressure-sensitive)
• chemoreceptors (O2, CO2, and H+ sensitive)
• bloodborne chemicals
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Short-Term Mechanisms: Baroreceptor-Initiated Reflexes
Carotid bifurcation
Carotid sinus
Common carotid
artery
Arterial
baroreceptors
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Aortic
arch
Short-Term Mechanisms: Baroreceptor-Initiated Reflexes
• Increased blood pressure stimulates the cardioinhibitory
center to:
• Increase vessel diameter
• Decrease heart rate, cardiac output, peripheral
resistance, and blood pressure
• Declining
blood
pressure
cardioacceleratory center to:
stimulates
the
• Increase cardiac output and peripheral resistance
• Low blood pressure also stimulates the vasomotor center
to constrict blood vessels
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Blood Pressure
KEY
Stimulus
Sensory receptor
Medullary
cardiovascular
control center
Integrating center
Efferent path
Effector
Change in
blood
pressure
Parasympathetic
neurons
Sympathetic
neurons
Carotid and aortic
baroreceptors
SA node
Ventricles
Veins
Arterioles
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Figure 15-22 (10 of 10)
Response of arterial baroreceptors to changes in arterial pressure
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Cardioinhibitory
centers stimulated
Cardioacceleratory
centers inhibited
Responses to Increased
Baroreceptor Stimulation
Decreased
cardiac
output
Vasomotor centers
inhibited
Baroreceptors
stimulated
Vasodilation
occurs
HOMEOSTASIS
DISTURBED
HOMEOSTASIS
RESTORED
Rising blood
pressure
Blood pressure
declines
Start
HOMEOSTASIS
Normal range
of blood
pressure
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HOMEOSTASIS
Start
Normal range
of blood
pressure
HOMEOSTASIS
DISTURBED
HOMEOSTASIS
RESTORED
Falling blood
pressure
Blood pressure
rises
Vasoconstriction
occurs
Baroreceptors
inhibited
Vasomotor centers
stimulated
Responses to Decreased
Baroreceptor Stimulation
Cardioacceleratory
centers stimulated
Cardioinhibitory
centers inhibited
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Increased
cardiac
output
Short-Term Mechanisms: Chemoreceptor-Initiated
Reflexes
• Chemoreceptors are located in the
• Carotid sinus
• Aortic arch
• Large arteries of the neck
• Chemoreceptors respond to rise in CO2, drop in pH or
O2
• Increase blood pressure via the vasomotor center
and the cardioacceleratory center
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Respiratory centers in
the medulla oblongata
stimulated
Increasing CO2 levels,
decreasing pH
and O2 levels
Effects on
Cardiovascular Centers
Reflex Response
Chemoreceptors
stimulated
Respiratory Response
Respiratory rate
increases
Cardiovascular
Responses
Cardioacceleratory
centers stimulated
Cardioinhibitory
centers inhibited
Vasomotor centers
stimulated
Start
Vasoconstriction
occurs
HOMEOSTASIS
DISTURBED
HOMEOSTASIS
RESTORED
Elevated CO2 levels,
decreased pH and O2
levels in blood and CSF
Decreased CO2 levels,
increased pH and O2
levels in blood and CSF
HOMEOSTASIS
Normal pH, O2,
and CO2 levels in
blood and CSF
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Increased cardiac
output and blood
pressure
Chemicals that Increase Blood Pressure
• Adrenal medulla hormones – norepinephrine
epinephrine increase blood pressure
• Antidiuretic hormone (ADH) – causes
vasoconstriction in cases of extremely low BP
and
intense
• Angiotensin II – kidney release of renin generates
angiotensin II, which causes vasoconstriction
• Endothelium-derived
factors
–
endothelin
and
prostaglandin-derived growth factor (PDGF) are both
vasoconstrictors
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Chemicals that Decrease Blood Pressure
• Atrial natriuretic peptide (ANP) – causes blood
volume and pressure to decline
• Nitric oxide (NO) – is a brief but potent vasodilator
• Inflammatory chemicals – histamine, prostacyclin, and
kinins are potent vasodilators
• Alcohol – causes BP to drop by inhibiting ADH
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Long-Term Mechanisms: Renal Regulation
• Long-term mechanisms control BP by altering blood
volume
• Increased BP stimulates the kidneys to eliminate water, thus
reducing BP
• Decreased BP stimulates the kidneys to increase blood
volume and BP
• Kidneys act directly and indirectly to maintain long-term
blood pressure
• Direct renal mechanism alters blood volume (changes in
urine volume)
• Indirect renal mechanism involves the renin-angiotensin
mechanism
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Indirect Mechanism: 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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