Chapter 10
The Blood Vessels and Blood Pressure
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Outline
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Vessel types and flow
Arteries
Blood pressure
Arterioles
Capillaries
Lymphatic system
Veins
Outline
• Vessel types and flow
– Vessel anatomy
– Flow, flow rate, pressure gradients
– Blood distribution, Poiselle’s law
Human Coronary Artery. LM X6.
Human artery showing atherosclerosis. LM X3.
Human artery cross-section, elastic tissue stain. X80.
Credit: © Biodisc/Visuals Unlimited
213940
Red and white blood cells within an arteriole.
Arteries branch into arterioles within organs
and deliver blood to the capillaries. SEM X6130.
Artery and Vein. A distributing artery (right)
and a medium-sized vein (left) surrounded
by connective tissue. Arteries and veins
are part of the extensive network of vessels
that make up the vascular system. SEM
X305.
Credit: © Dr. Richard Kessel & Dr. Randy Kardon/Tissues & Organs/Visuals Unlimited
900019
Blood Flow
• Blood is constantly reconditioned so composition
remains relatively constant
• Reconditioning organs receive more blood than
needed for metabolic needs
– Digestive organs, kidneys, skin
– Adjust extra blood to achieve homeostasis
• Blood flow to other organs can be adjusted
according to metabolic needs
• Brain can least tolerate disrupted supply
Distribution of Cardiac
Output at Rest
Blood Flow
• Flow rate through a vessel (volume of blood passing
through per unit of time) is directly proportional to
the pressure gradient and inversely proportional to
vascular resistance
F = ΔP
R
F = flow rate of blood through a vessel
ΔP = pressure gradient
R = resistance of blood vessels
Blood Flow
• Pressure gradient is pressure difference between beginning
and end of a vessel
– Blood flows from area of higher pressure to area of lower
pressure
• Resistance is measure of opposition of blood flow through a
vessel
– Depends on three things
• Blood viscosity, vessel length, vessel radium
– Major determinant of resistance to flow is vessel’s radius
– Slight change in radius produces significant change in
blood flow
R is proportional to 1
r4
Relationship of
Resistance and Flow
to Vessel Radius
Vascular Tree
• Closed system of vessels
• Consists of
– Arteries
• Carry blood away from heart to tissues
– Arterioles
• Smaller branches of arteries
– Capillaries
• Smaller branches of arterioles
• Smallest of vessels across which all exchanges are made
with surrounding cells
– Venules
• Formed when capillaries rejoin
• Return blood to heart
– Veins
• Formed when venules merge
• Return blood to heart
Basic Organization of
the Cardiovascular
System
Outline
• Arteries
– “pressure reservoir”
– anatomy
Arteries
• Specialized to
– Serve as rapid-transit passageways for blood
from heart to organs
• Due to large radius, arteries offer little resistance to
blood flow
– Act as pressure reservoir to provide driving force
for blood when heart is relaxing
• Arterial connective tissue contains
– Collagen fibers
» Provide tensile strength
– Elastin fibers
» Provide elasticity to arterial walls
Arteries as a Pressure Reservoir
Outline
• Blood pressure
– Systolic/diastolic
– Measurements
• Pressure and pulse
– Mean arterial pressure
– Pressure abnormalities
Blood Pressure
• Force exerted by blood against a vessel wall
– Depends on
• Volume of blood contained within vessel
• Compliance of vessel walls
• Systolic pressure
– Peak pressure exerted by ejected blood against
vessel walls during cardiac systole
– Averages 120 mm Hg
• Diastolic pressure
– Minimum pressure in arteries when blood is
draining off into vessels downstream
– Averages 80 mm Hg
Blood Pressure
• Can be measured indirectly using
sphygmomanometer
• Korotkoff sounds
– Sounds heard when determining blood pressure
– Sounds are distinct from heart sounds associated
with valve closure
Blood
Pressure
Pulse Pressure
• Pressure difference between systolic and diastolic
pressure
• Example
– If blood pressure is 120/80, pulse pressure is 40
mm Hg (120mm Hg – 80mm Hg)
• Pulse that can be felt in artery lying close to surface
of skin is due to pulse pressure
Mean Arterial Pressure
• Average pressure driving blood forward into tissues
throughout cardiac cycle
• Formula for approximating mean arterial pressure
Mean arterial pressure = diastolic pressure + ⅓ pulse pressure
At 120/80, mean arterial pressure = 80 mm Hg +
⅓ (40 mm Hg) = 93 mm Hg
Outline
• Arterioles
– Resistance vessels
– Vasoconstriction and vasodilation
– Regulation of arteiolar diameter
• Local factors
• Endothelial cells, nitric oxide
Arterioles
• Major resistance vessels
• Radius supplying individual organs can be adjusted
independently to
– Distribute cardiac output among systemic organs,
depending on body’s momentary needs
– Help regulate arterial blood pressure
Arterioles
• Mechanisms involved in adjusting arteriolar
resistance
– Vasoconstriction
• Refers to narrowing of a vessel
– Vasodilation
• Refers to enlargement in circumference and radius of
vessel
• Results from relaxation of smooth muscle layer
• Leads to decreased resistance and increased flow
through that vessel
Arteriolar Vasoconstriction and Vasodilation
Arterioles
• Only blood supply to brain remains constant
• Changes within other organs alter radius of vessels
and adjust blood flow to organ
• Local chemical influences on arteriolar radius
– Local metabolic changes
– Histamine release
• Local physical influences on arteriolar radius
– Local application of heat or cold
– Chemical response to shear stress
– Myogenic response to stretch
Magnitude and Distribution
Of the Cardiac Output at Rest
and During Moderate Exercise
Arterioles
• Specific local chemical factors that produce
relaxation of arteriolar smooth muscle
– Decreased O2
– Increased CO2
– Increased acid
– Increased K+
– Increased osmolarity
– Adenosine release
– Prostaglandin release
Arterioles
• Local vasoactive mediators
– Endothelial cells
• Release chemical mediators that play key role in locally
regulating arteriolar caliber
• Release locally acting chemical messengers in
response to chemical changes in their environment
• Among best studied local vasoactive mediators is nitric
oxide (NO)
Functions of
Endothelial Cells
Functions of
Nitric Oxide (NO)
Arterioles
• Extrinsic control
– Accomplished primarily by sympathetic nerve
influence
– Accomplished to lesser extent by hormonal
influence over arteriolar smooth muscle
Arterioles
• Cardiovascular control center
– In medulla of brain stem
– Integrating center for blood pressure regulation
• Other brain regions also influence blood distribution
– Hypothalamus
• Controls blood flow to skin to adjust heat loss to environment
• Hormones that influence arteriolar radius
– Adrenal medullary hormones
• Epinephrine and norepinephrine
– Generally reinforce sympathetic nervous system in most organs
• Vasopressin and angiotensin II
– Important in controlling fluid balance
Outline
• Capillaries
– Exchange and anatomy
– Pressures driving diffusion
Capillaries
• Thin-walled, small-radius, extensively branched
• Sites of exchange between blood and surrounding
tissue cells
– Maximized surface area and minimized diffusion
distance
– Velocity of blood flow through capillaries is
relatively slow
• Provides adequate exchange time
– Two types of passive exchanges
• Diffusion
• Bulk flow
• Factors affecting diffusion
– Ficks law of diffusion
– Slow blood velocity
– Permeability
– Metarterioles and precapillary sphincters
– Exchanges
• Blood to interstitial fluid to cell
– Passive
– Active
– Bulk Flow
• Ultrafiltration
• reabsorption
Table 3-1, p. 62
Fig. 10-16, p. 355
Capillary blood pressure
Plasma colloid osmotic pressure
Interstitial fluid hydrostatic pressure
Interstitial fluid colloid osmotic pressure
Fig. 10-22, p. 359
Fig. 10-9, p. 345
Arteriole
Smooth muscles
Precapillary
sphincter
Metarteriole
Myogenic tone
Stopcock
Local metabolic
changes
Capillary
Venule
Fig. 10-19, p. 357
Capillaries
• Narrow, water-filled gaps (pores) lie at junctions
between cells
• Permit passage of water-soluble substances
• Lipid soluble substances readily pass through
endothelial cells by dissolving in lipid bilayer barrier
• Size of pores varies from organ to organ
Capillaries
• Under resting conditions many capillaries are not
open
• Capillaries surrounded by precapillary sphincters
• Contraction of sphincters reduces blood flowing into
capillaries in an organ
• Relaxation of sphincters has opposite effect
• Metarteriole
– Runs between an arteriole and a venule
Outline
• Lymphatic system
– Drainage of tissue
– Immune function
– Edema
Lymphatic
System
Lymphatic System
• Extensive network of one-way vessels
• Provides accessory route by which fluid can be returned from
interstitial to the blood
• Initial lymphatics
– Small, blind-ended terminal lymph vessels
– Permeate almost every tissue of the body
• Lymph
– Interstitial fluid that enters a lymphatic vessel
• Lymph vessels
– Formed from convergence of initial lymphatics
– Eventually empty into venous system near where blood
enters right atrium
– One way valves spaced at intervals direct flow of lymph
toward venous outlet in chest
Lymphatic System
• Functions
– Return of excess filtered fluid
– Defense against disease
• Lymph nodes have phagocytes which destroy bacteria
filtered from interstitial fluid
– Transport of absorbed fat
– Return of filtered protein
To venous
system
Interstitial
fluid
Arteriole
Tissue
cells
Venule
Blood capillary
Initial
lymphatic
Fig. 10-24, p. 362
Fluid pressure on the outside of the vessel
pushes the endothelial cell’s free edge inward,
permitting entrance of interstitial fluid
(now lymph).
Overlapping
endothelial cell
Interstitial
fluid
Lymph
Fluid pressure on the inside of the vessel
forces the overlapping edges together so
that lymph cannot escape.
Fig. 10-24, p. 362
Edema
• Swelling of tissues
• Occurs when too much interstitial fluid accumulates
• Causes of edema
– Reduced concentration of plasma proteins
– Increased permeability of capillary wall
– Increased venous pressure
– Blockage of lymph vessels
Outline
• Veins
– Capacitance vessels
– Regulation of pressure and flow
– Venous return
Veins
• Venous system transports blood back to heart
• Capillaries drain into venules
• Venules converge to form small veins that exit
organs
• Smaller veins merge to form larger vessels
• Veins
– Large radius offers little resistance to blood flow
– Also serve as blood reservoir
Pulmonary
vessels
9%
Systemic arteries
13%
Systemic arterioles
2%
Heart
7%
Systemic capillaries
5%
Systemic veins
64%
Capacitance!
Fig. 10-27, p. 364
Veins
• Factors which enhance venous return
– Driving pressure from cardiac contraction
– Sympathetically induced venous vasoconstriction
– Skeletal muscle activity
– Effect of venous valves
– Respiratory activity
– Effect of cardiac suction
Skeletal muscle pump
Fig. 10-29, p. 366
Effects of Gravity
Fig. 10-30, p. 367
Standing
Walking
Heart
Thigh
150 cm
Calf
34 cm
Foot
100
mm Hg
Foot vein supporting
column of blood 1.5 m
(150 cm) in height
Venous
pressure
in foot
27
mm Hg
Foot vein supporting
column of blood
34 cm in height
Fig. 10-31, p. 367
Vein
Open venous valve
permits flow of
blood toward heart
Contracted
skeletal muscle
Closed venous valve
prevents backflow
of blood
Stepped art
Fig. 10-32, p. 368
Respiratory activity and venous return
5 mm Hg less than
atmospheric pressure
Atmospheric pressure
5 mm Hg less than
atmospheric pressure
Atmospheric pressure
Cardiac suction also
Fig. 10-33, p. 368
Factors that Influence Venous Return
summary
Blood pressure
Mean Arterial Pressure
• Blood pressure that is monitored and regulated in
the body
– Regulation is important:
• Provides adequate driving pressure (brain and organs)
• Prevents extra work
• Primary determinants
– Cardiac output
– Total peripheral resistance
Mean arterial pressure = cardiac output x total
peripheral resistance
Determinants of Mean Arterial Pressure
Mean Arterial Pressure
• Constantly monitored by baroreceptors (pressure
sensors) within circulatory system
– Short-term control adjustments
• Occur within seconds
• Adjustments made by alterations in cardiac output and
total peripheral resistance
• Mediated by means of autonomic nervous system
influences on heart, veins, and arterioles
– Long-term control adjustments
• Require minutes to days
• Involve adjusting total blood volume by restoring
normal salt and water balance through mechanisms
that regulate urine output and thirst
Fig. 10-11, p. 347
Carotid sinus baroreceptor
Common carotid arteries
(Blood to the brain)
Neural signals to
cardiovascular
control center
in medulla
Aortic arch baroreceptor
Aorta
(Blood to rest of body)
Fig. 10-35, p. 371
Parasympathetic
stimulation
Sympathetic
stimulation
Heart
Heart
rate
Cardiac
output
Heart
Contractile
strength
of heart
Arterioles
Veins
Cardiac
output
Heart
rate
Vasoconstriction
Vasoconstriction
Venous
return
Blood
pressure
Blood
pressure
Stroke
volume
Total peripheral
resistance
Stroke
volume
Cardiac
output
Blood
pressure
Blood
pressure
Fig. 10-37, p. 372
Baroreceptor Reflexes to Restore Blood
Pressure to Normal
Normal
Arterial
pressure
(mm Hg)
Increased
Decreased
120
80
Firing rate in
afferent neuron
arising from carotid
sinus baroreceptor
Time
Fig. 10-36, p. 371
Blood Pressure
• Additional reflexes and responses that influence blood
pressure
– Left atrial receptors and hypothalamic osmoreceptors
affect long-term regulation of blood pressure by controlling
plasma volume
– Chemoreceptors in carotid and aortic arteries are sensitive
to low O2 or high acid levels in blood – reflexly increase
respiratory activity
– Associated with certain behaviors and emotions mediated
through cerebral-hypothalamic pathway
– Exercise modifies cardiac responses
– Hypothalamus controls skin arterioles for temperature
regulation
– Vasoactive substances released from endothelial cells play
role
Blood Pressure Abnormalities
• Hypertension
– Blood pressure above 140/90 mm Hg
– Two broad classes
• Primary hypertension
• Secondary hypertension
• Hypotension
– Blood pressure below 100/60 mm Hg
Hypertension
• Most common of blood pressure abnormalities
• Primary hypertension
– Catchall category for blood pressure elevated by variety of
unknown causes rather than by a single disease entity
– Potential causes being investigated
•
•
•
•
•
•
•
•
Defects in salt management by the kidneys
Excessive salt intake
Diets low in K+ and Ca2+
Plasma membrane abnormalities such as defective Na+-K+
pumps
Variation in gene that encodes for angiotensinogen
Endogenous digitalis-like substances
Abnormalities in NO, endothelin, or other locally acting
vasoactive chemicals
Excess vasopressin
Hypertension
• Secondary hypertension
– Accounts for about 10% of hypertension cases
– Occurs secondary to another known primary
problem
– Examples of secondary hypertension
• Renal hypertension
• Endocrine hypertension
• Neurogenic hypertension
Hypertension
• Complication of hypertension
– Congestive heart failure
– Stroke
– Heart attack
– Spontaneous hemorrhage
– Renal failure
– Retinal damage
Hypotension
• Low blood pressure
• Occurs when
– There is too little blood to fill the vessels
– Heart is too weak to drive the blood
• Orthostatic (postural) hypotension
– Transient hypotensive condition resulting from
insufficient compensatory responses to
gravitational shifts in blood when person moves
from horizontal to vertical position
Hypotension
• Circulatory shock
– Occurs when blood pressure falls so low that
adequate blood flow to the tissues can no longer
be maintained
– Four main types
•
•
•
•
Hypovolemic (“low volume”) shock
Cardiogenic (“heart produced”) shock
Vasogenic (“vessel produced”) shock
Neurogenic (“nerve produced”) shock
Circulatory shock
( mean arterial pressure)
Cardiac output
Cardiac output
Total peripheral resistance
Widespread
vasodilation
Loss of blood volume
Loss of fluids
derived from
plasma
Severe
hemorrhage
Excessive
vomiting,
diarrhea,
urinary losses,
etc.
Hypovolemic
shock
Weakened
heart
Cardiogenic
shock
Vasodilator
substances
released from
bacteria
Histamine
released
in severe
allergic
reaction
Septic
shock
Anaphylactic
shock
Cardiogenic
shock
Loss of
vascular tone
Sympathetic
nerve activity
Neurogenic
shock
Fig. 10-39, p. 377
Fig. 10-40, p. 378