BIO2305
Vascular Physiology
Physiology of Systemic Circulation
Determined by
Dynamics of blood flow
Anatomy of circulatory system
Regulatory mechanisms that control heart and blood vessels
Blood volume
Most in the veins (2/3rd)
Smaller volumes in arteries and capillaries
Dynamics of Blood Circulation
Interrelationships between:
Pressure
Flow
Resistance
Control mechanisms that regulate blood pressure
Blood flow through vessels
Blood Flow
Recall: Cardiac output - the total volume of blood pumped by the ventricle per minute
The actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given
period:
Is measured in ml per min.
Is equivalent to cardiac output (CO), considering the entire vascular system
Is relatively constant when at rest
Varies widely through individual organs, according to immediate needs
Circulatory Changes During Exercise
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Blood Pressure (BP)
Blood Pressure – The measure of force exerted by
blood against the vessel walls.
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
Blood moves through vessels because of blood
pressure, gravity, and skeletal pump
The differences in BP within the vascular system
provide the driving force that keeps blood moving from
higher to lower pressure areas
Blood Flow & Blood Pressure
Blood flow (F) is directly proportional to the
difference in blood pressure (P) between two
points in the circulation
F = Flow
α = directly proportional to
Δ = change in
P = Pressure
F α ΔP (“Flow is directly proportional to
change in pressure”)
If ΔP ↑, then F ↑
If ΔP ↓, then F ↓
Blood Flow: Pressure Changes
Flows down a pressure gradient
Varies with force of ventricular contraction
Highest at the left ventricle (driving P), decreases over
distance
Capillary beds virtually diminish pressure
Compliance (distensibility) on venous side maintains low pressure
Greatest drop in pressure occurs in arterioles
Decreases 90% from aorta to vena cava
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Blood Flow
Flow rate (F) through a vessel (volume of blood passing through per unit of time) is directly
proportional to the pressure gradient (ΔP) and inversely proportional to vascular resistance (R)
F = flow rate of blood through a vessel
ΔP = pressure gradient
R = resistance of blood vessels
(“Flow is directly proportional to Change in Pressure
and inversely proportional to Resistance.”)
Meaning:
If ΔP↑ and/or R↓, then F↑
if ΔP↓ and/or R↑, then F↓
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 (R) – opposition to flow
Measure of the amount of friction blood encounters as it passes through vessels
Referred to as peripheral resistance (PR)
Blood flow is inversely proportional to resistance (R)
As one goes up, the other goes down
R is more important than P in influencing local blood pressure
(“Flow is inversely proportional to Resistance”)
Resistance Factors
Resistance factors:
Viscosity of blood– thickness or “stickiness” of the blood
Hematocrit
[Plasma Proteins]
Length of blood vessel– the longer the vessel, the greater the resistance encountered
Radius of blood vessel – the wider the vessel, the lower the resistance
Slight change in radius (vasoconstriction or vasodilation) produces significant change in blood
flow
Resistance is inversely proportional to radius4
As one goes up, the other goes down
(“Resistance is inversely proportional to
the radius of the vessel to the 4th power”)
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Blood Vessel Diameter
Changes in vessel radius significantly alter
peripheral resistance
Resistance varies inversely with the fourth
power of vessel radius (one-half the
diameter)
Poiseuille’s Law:
R= L
r4
L = length of the vessel
 = viscosity of blood (Greek letter “eta”)
r = radius of the vessel
Vessel length and blood viscosity do not
vary significantly and are considered
CONSTANT = 1
Therefore: R = 1
r4
If the radius is doubled, the new resistance
is 1/16th the original resistance
If the radius is halved, the new resistance
is 16 times the original resistance
Note: α means “proportional to”
Blood Flow, Vessel Diameter and Velocity
As diameter of vessels increases, the total cross-sectional area increases and velocity of blood flow
decreases
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Blood Flow & Cross-Sectional Area
At the capillary bed:
Vessel diameter decreases, but number of vessels increase
Therefore, total cross-sectional area increases
Therefore, velocity slows, giving capillaries time to unload O2 and nutrients
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 veins
Veins
formed when venules merge
return blood to heart
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Structure of Blood Vessels
Arteries have thicker walls and narrower lumens than those that of veins
Arterial walls must withstand high pressures
Thick layer of smooth muscle in tunica media controls flow and pressure
Arteries and arterioles have more elastic and collagen fibers
Remain open and can spring back in shape
Pressure in the arteries fluctuates due to cardiac systole and diastole
Veins have larger lumens. Vein walls are thinner than arteries and have valves
Thin walls provide compliance - blood volume reservoir
Blood pressure is much lower in veins than in arteries
Valves prevent backflow of blood
Capillaries have very small diameters (many are only large enough for only one RBC to pass through
at a time)
Composed only of basal lamina and endothelium
Thin walls allow for gas, nutrient, waste exchange between blood and tissue cells.
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Role of Arteries
Elastic or conducting arteries
Largest diameters, pressure high and fluctuates
Pressure Reservoir
Elastic recoil propels blood after systole
Muscular or medium arteries
Smooth muscle allows vessels to regulate blood supply by constricting or dilating
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Role of Arterioles
Transport blood from small arteries to capillaries
Controls the amount of resistance
Greatest drop in pressure occurs in arterioles, which regulate blood flow through tissues
No large fluctuations in capillaries and veins
Blood Pressure
Force exerted by blood against a vessel wall
Depends on:
Volume of blood forced into the vessel
Compliance (distensibility) 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 Measurement
Critical closing pressure:
Pressure at which a blood
vessel collapses and blood
flow stops
Laplace’s Law:
Force acting on blood
vessel wall is proportional to
diameter of the vessel times
blood pressure
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Measurement of BP
Blood pressure cuff is inflated above systolic pressure,
occluding the artery
As cuff pressure is lowered, the blood will flow only when
systolic pressure is above cuff pressure, producing the
sounds of Korotkoff
Named after Dr. Nikolai Korotkoff, a Russian
physician who described them in 1905
Korotkoff sounds will be heard until cuff pressure equals
diastolic pressure, causing the sounds to disappear
Measurement of Blood Pressure
Different phases in measurement of blood pressure are identified on the basis of the quality of the
Korotkoff sounds
Average arterial BP is 120/80 mm Hg
Average pulmonary BP is 22/8 mm Hg
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Pulse Pressure
Pulse Pressure - Difference between systolic and diastolic pressures
Increases when stroke volume increases or vascular compliance decreases
Pulse pressure can be used to take a pulse to determine heart rate and rhythmicity
Example:
120 mmHg (SP) – 80 mmHg (DP) = 40 mmHg (PP)
Arterial Blood Pressure
Blood pressure in elastic arteries near the heart is pulsatile (BP rises and falls)
Pulse pressure – the 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
Effect of Gravity on Blood Pressure
Effect of Gravity: In a standing position, hydrostatic pressure caused by gravity increases blood
pressure below the heart and decreases pressure above the heart
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Role of Veins
Veins have much lower blood pressure and thinner walls than arteries
To return blood to the heart, veins have special adaptations
Large-diameter lumens, which offer little resistance to flow
Valves (resembling semilunar heart valves), which prevent backflow of blood
Venous Blood Pressure
Venous BP is steady and changes little during the cardiac cycle
The pressure gradient in the venous system is only about 20 mm Hg
Veins have thinner walls, thus higher compliance.
Vascular compliance:
Tendency for blood vessel volume to increase as blood pressure increases
More easily the vessel wall stretches, the greater its compliance
Venous system has a large compliance and acts as a blood reservoir
Capacitance vessels – 2/3 blood volume is in veins
Venous Return
Venous pressure is driving force for return of blood to the heart.
EDV, SV, and CO are controlled by factors which affect venous return
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 squeeze local veins
Muscular “pump” – contraction of skeletal muscles push
blood toward the heart
Valves prevent backflow during venous return
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Capillary Network
Blood flows from arterioles through metarterioles, then through capillary network
Venules drain network
Smooth muscle in arterioles, metarterioles, precapillary sphincters regulates blood flow
Organization of a Capillary Bed
True capillaries – exchange vessels
Oxygen and nutrients cross to cells
Carbon dioxide and metabolic waste products cross into blood
Atriovenous anastomosis – vascular shunt, directly connects an arteriole to a venule
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Capillaries
Capillary wall consists mostly of endothelial
cells
Types classified by diameter/permeability
Continuous capillary –
Least permeable
Do not have pores
Only small molecules
(water, ions) diffuse through
tight junctions
Seen in skin, muscle where
small ions and molecules
must enter/exit vessels
Fenestrated capillary –
Large fenestrations (pores)
Small molecules and limited
proteins diffuse
Seen in kidney, small
intestine where large
molecules must enter/exit
vessels
Sinusoidal capillary –
Most permeable
Discontinuous basement
Allow proteins and cells as
necessary
Seen in liver, bone marrow,
spleen, where whole cells,
proteins must enter/exit
vessels
Capillary Exchange and Interstitial Fluid Volume Regulation
Blood pressure, capillary permeability, and osmosis affect movement of fluid from capillaries
A net movement of fluid occurs from blood plasma into tissues – bulk flow
Fluid gained by tissues is removed by lymphatic system
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Diffusion at Capillary Beds
Distribution of ECF between plasma
and interstitial compartments
Is in state of dynamic
equilibrium
Balance between tissue fluid
and blood plasma
Hydrostatic pressure:
Exerted against the inner
capillary wall
Generated primarily by gravity
and blood pressure
Promotes formation of tissue
fluid
Source of Net Filtration
Pressure (NFP)
Colloid osmotic pressure:
Exerted against the outer
capillary wall
Generated by [plasma
proteins]
Promotes fluid reabsorption
into circulatory system
Lymphatic System
Extensive network of one-way vessels
Provides accessory route by which fluid can be returned from interstitial spaces to the blood
Initial lymphatics
Small, blind-ended terminal lymph vessels
Permeate almost every tissue of the body
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
Lymph
Interstitial fluid that enters a lymphatic vessel
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Lymphatic System
7,200 L blood pumped per day (5 L/min x 60 min x 24 hrs)
20 L of plasma leave arteries
17 L of plasma return to veins
3 L carried away as lymph
Lymphatic System Functions:
Return of excess filtered fluid (3 L/day) back to heart
Defense against disease
Lymph nodes contain phagocytes which destroy bacteria filtered from interstitial fluid
Transport of absorbed fat from GI tract to liver (chylomicrons)
Return of filtered protein
Regulation of Blood Flow
Intrinsic – local autoregulation (‘”self-regulation”)
Autoregulation – In most tissues, blood flow is proportional to metabolic needs of tissues. If
tissue metabolic processes increase, blood flow to tissue increases.
Extrinsic – nervous and endocrine (non-local forces)
Nervous System – CNS sends (or ceases) action potentials to smooth muscle in walls of blood
vessels, stimulating vasoconstriction, or allowing vasodilation.
Sympathetic neuron axon terminals at smooth muscle in blood vessels release NE,
which stimulates vasoconstriction, decreasing blood flow. If release of NE from axon
terminal decreases, blood vessels dilate, increasing blood flow.
Sympathetic neuron axon terminals at cardiac muscle in the heart release NE, which
increases cardiac output, increasing BP.
Nervous system is immediately responsible for routing blood flow and maintaining blood
pressure.
Hormonal Control – Sympathetic action potentials to medulla of adrenal gland stimulate
release of epinephrine and norepinephrine which reinforce systemic vasoconstriction, but cause
vasodilation at skeletal muscle.
Norepinephrine is primarily released from sympathetic neuron axon terminals directly
onto end target organs. Epinephrine is primarily released from the adrenal medulla (80%
Epi, 20% NE).
Epi from the adrenal medulla travels through the blood and is able to bind with α1receptors on blood vessels, reinforcing vasoconstriction. However, α1-receptors have a
lower affinity for Epi and do not respond as strongly to it as they do to NE.
Epinephrine also binds to β2-receptors, found on vascular smooth muscle of heart, liver,
and skeletal muscle arterioles. These receptors are not innervated and therefore
respond primarily to circulating epinephrine. Activation of vascular β2-receptors in the
heart, liver, and skeletal muscle by epinephrine causes vasodilation, increasing blood
flow to these tissues [p. 524].
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Intrinsic Regulation of Blood Flow (Autoregulation)
Blood flow can increase 7-8 times as a result of vasodilation of metarterioles and precapillary
sphincters
Vasodilator substances are produced as metabolism increases
Intrinsic receptors sense chemical changes in environment
Decreased O2
Increased CO2
Decreased pH (Lactic acid)
Increased adenosine
Increased K+
Intrinsic Regulation of Blood Flow (Autoregulation)
Myogenic Control Mechanism:
Contraction that originates within the vascular smooth muscle fiber as a result of stretch
An increase in systemic arterial pressure causes vessels to contract
A decrease in systemic arterial pressure causes vessels to dilate
If ↑ Pressure, then ↑ Stretch. Response: Vasoconstriction
If ↓ Pressure, then ↓ Stretch. Response: Vasodilation
Intrinsic Regulation of Blood Flow (Autoregulation)
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Extrinsic Regulation of Blood Flow
Sympathoadrenal
Increase cardiac output
Increase TPR: α-adrenergic stimulation - vasoconstriction of arteries in skin and viscera
Parasympathetic
Parasympathetic innervation limited, less important than sympathetic nervous system in control
of TPR
Parasympathetic endings in arterioles promote vasodilation to the digestive tract, external
genitalia, and salivary glands
Blood Pressure (BP) Regulation
Pressure of arterial blood is regulated by blood volume, TPR, and cardiac
rate.
MAP = CO  TPR
Arteriole resistance is greatest because they have the smallest diameter
Capillary BP is reduced because of the total cross-sectional area
3 most important variables are HR, SV, and TPR
Increase in each of these will result in an increase in BP
BP can be regulated by:
Kidney and sympathoadrenal system
Short-Term Regulation of Blood Pressure
Baroreceptor reflexes
Change peripheral resistance, heart rate, and stroke volume in
response to changes in blood pressure
Chemoreceptor reflexes
Sensory receptors sensitive to O2, CO2, and pH levels of blood
Central nervous system ischemic response
Results from high CO2 or low pH levels in medulla and increases
peripheral resistance
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Baroreceptor Reflex Control
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Chemoreceptor Reflex Control
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Baroreceptor Effects
Long-Term Regulation of Blood Pressure
Renin-angiotensin-aldosterone mechanism
Vasopressin (ADH) mechanism
Atrial natriuretic mechanism
Fluid shift mechanism
Stress-relaxation response
Renin-Angiotensin-Aldosterone System (RAAS)
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Vasopressin (ADH) Mechanism
Atrial Natriuretic Peptide (ANP)
Produced by the atria of the heart
Stretch of atria stimulates production of ANP
Antagonistic to Aldosterone and
Angiotensin II
Promotes Na+ and H2O excretion in the
urine by the kidney
Promotes vasodilation, lowering TPR
“Atrial” = atria of the heart
“natri-” = sodium
“-uretic” = urination
“Peptide” = small protein
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Atrial Natriuretic Peptide (ANP)
Cerebral Circulation
Cerebral blood flow (CBF) is not normally influenced by sympathetic nerve activity
Cerebral blood flow regulated almost exclusively by intrinsic mechanisms:
Myogenic Mechanism:
Dilate in response to decreased pressure
Cerebral Perfusion Pressure (CPP) is the blood pressure within the brain. CPP is
maintained within very narrow limits
Too low = cerebral ischemia
Too high = intracranial hematoma, cerebral edema
Metabolic Mechanism:
Areas of brain with high metabolic activity receive most blood
Cerebral arteries dilate due to increased [K+] [CO2] [H+] and [Adenosine] in CSF
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