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Lect 17, Chapter 21
Peripheral Circulation and Regulation
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Peripheral Circulatory System
• Systemic vessels
– Transport blood through most all body parts
from left ventricle and back to right atrium
• Pulmonary vessels
– Transport blood from right ventricle through
lungs and back to left atrium
• Blood vessels and heart regulated to ensure
blood pressure is high enough for blood
flow to meet metabolic needs of tissues
• Skeletal muscle pump
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Blood Vessel Structure
• Arteries
– Elastic, muscular, arterioles
• Capillaries
– Blood flows from arterioles to capillaries
– Most of exchange between blood and interstitial
spaces occurs across the walls
– Blood flows from capillaries to venous system
• Veins
– Venules, small veins, medium or large veins
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Capillaries
• Capillary wall consists
mostly of endothelial cells
• Types classified by
diameter/permeability
– Continuous
• Do not have fenestrae
many tissues
– Fenestrated
• Have pores (20-100nm)
found in endocrines
– Sinusoidal
• Large diameter with large
fenestrae
• In the liver, bone marrow,
spleen
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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
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Structure of Arteries and Veins
• Three layers except for
capillaries and venules
• Tunica intima
– Endothelium
• Tunica media
– Vasoconstriction
– Vasodilation
• Tunica adventitia
– Merges with connective
tissue surrounding blood
vessels
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Structure of Arteries
• Elastic or conducting arteries
– Largest diameters, pressure high and fluctuates
• Muscular or medium arteries
– Smooth muscle allows vessels to regulate blood
supply by constricting or dilating
• Arterioles
– Transport blood from small arteries to capillaries
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Structure of Veins
• Venules and small veins
– Tubes of endothelium on delicate basement
membrane
• Medium and large veins
• Valves
– Allow blood to flow toward heart but not in
opposite direction
• Atriovenous anastomoses
– Allow blood to flow from arterioles to small
veins without passing through capillaries
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Blood Vessel Comparison
21-10
Aging of the Arteries
• Arteriosclerosis
– General term for
degeneration changes
in arteries making them
less elastic
• Atherosclerosis
– Deposition of plaque
on walls
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Pulmonary Circulation
• Moves blood to and from the lungs
• Pulmonary trunk
– Arises from right ventricle
• Pulmonary arteries
– Branches of pulmonary trunk which project to
lungs
• Pulmonary veins
– Exit each lung and enter left atrium
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Fig. 21-8 Cardiovascular physiology
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Systemic Circulation: Arteries
• Aorta
– From which all arteries are derived either
directly or indirectly
– Parts
• Ascending, descending, thoracic, abdominal
• Coronary arteries
– Supply the heart
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Dynamics of Blood Circulation
• Interrelationships between
–
–
–
–
Pressure
Flow
Resistance
Control mechanisms that regulate blood
pressure
– Blood flow through vessels
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2.
Physics and Physiology of Circulation
Human's circulatory system is a closed
circulatory system. Thus the blood pressure and
the flow resistance become dependent on each
other.
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What is meant by 760 mmHg? 1 atmosphere is 760
mm of mercury (760 torr) and is the barometric
pressure at the sea level and at 0˚C, and decreases in
response to the altitude. At Detroit the barometric
pressure is about 740 torr.
1 A.P. = 14.7 lb/in2 = 1,033 gm/cm 2
1 lb/in2 = 70.31 gm/cm2
Tire pressure of 30 lb/in2 = 2.04 A.P.
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a.
Pressure and blood flow
Pressure is the driving force of the blood flow.
When blood vessels are connected, the blood flows from
the higher pressure site to the lower pressure site and the
rate of flow is proportional to the pressure difference.
FLOW RATE = PRESSUREDIFFERENCE/RESISTANCE
The overall pressure difference is between the ascending
aorta and the entrance to the right atrium - the circulatory
pressure (about 100 mmHg or torr). (Fig. 21.30 and 31 of
Seeley) Note the overall cross-sectional areas.
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POISEUILLE’S LAW
FLOW RATE = PRESSURE DIFFERENCE/Resistance
=( PRESSURE DIFFERENCE) / (8VL/  R4 )
Where V is the viscosity of the blood, L the length of the blood vessel and R
the radius of the blood vessel..
How can you obtain a faster flow rate?
Apply a large entrance pressure -- P
Keep the diameter large -- R
Use low viscosity fluid -- V
Use a short tube -- L
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Critical closing pressure and Laplace's law.
Blood vessels collapse below a critical closing
pressure.
At this point, the blood flow stops.
Thus, if the blood pressure is reduced by shock or loss
of blood, it may lead to a collapsed blood vessel.
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Laplace's law states:
F=D*P
where, F is the force applied to the vessel, D diameter of
the vessel and P the blood pressure.
If the blood pressure remains the same, but a disease
caused to enlarge the diameter of the blood vessel, the
force to the blood vessel will increase and could even
burst the blood vessel, if its wall is not strong enough!!
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Laminar and Turbulent Flow
• Laminar flow
– Streamlined
– Outermost layer
moving slowest and
center moving fastest
• Turbulent flow
– Interrupted
– Rate of flow exceeds
critical velocity
– Fluid passes a
constriction, sharp
turn, rough surface
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Blood Pressure
• Measure of force exerted by blood against
the wall
• Blood moves through vessels because of
blood pressure
• Measured by listening for Korotkoff sounds
produced by turbulent flow in arteries as
pressure released from blood pressure cuff
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Blood Pressure Measurement
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Physiology of Systemic Circulation
• Determined by
– Anatomy of circulatory system
– Dynamics of blood flow
– Regulatory mechanisms that control heart and
blood vessels
• Blood volume
– Most in the veins
– Smaller volumes in arteries and capillaries
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b.
Resistance
Resistance to circulation is dominated at the site of the
arterial system, inclusive of capillaries. The peripheral
resistance, and resistance of the venous system is low.
Resistance is dependent on the viscosity of blood, the
diameter and length of small blood vessels.
21-30
i.
Vascular resistance
It is not difficult to imagine that vascular resistance
becomes large when the blood attempts to pass
through thin capillary blood vessels.
It is the resistance between the blood matters and the
wall of the capillaries.
The size of capillaries may also be regulated with the
contraction and relaxation of muscle tissues, arterioles
and constriction of precapillary sphincter muscles.
21-31
ii.
Viscosity
We have already seen that the viscosity of blood is
about five times higher than that of water.
Under pathological conditions blood viscosity may
change due to the change of erythrocytes or plasma
protein contents.
21-32
iii.
Turbulence
When a solution passes through a smooth tubing, the flow will be
fastest at the center of the tube and, due to the adhesion to the wall,
the slowest near at the wall forming multiple layers of liquid with
different velocities - LAMINAR flow.
Turbulence of flow may be created when the LAMINAR flow is
disturbed by adding a restrictive substance in the flow, bending or
changing he diameter of the vessels.
When there is a sudden increase in the diameter of the vessel,
turbulence will be created causing the flow rate to decrease and
decrease the pressure.
Turbulent blood flow becomes the source of sound, especially in the
heart and can be heard with the stethoscope leading observation of
Korotkoff sounds.
21-33
c.
Circulatory pressures
The change of pressure in the circulatory system is
shown in Fig. 21-9, 10
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21-35
i.
Arterial blood pressure
Note that the systolic and diastolic pressures (pulse
pressure) of the ventricle are distinctive in the arteries.
The pressure drops gradually as the blood passes
through arterioles, then drops more in the pressure
prior to the capillary sphincter, and in the veins.
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ii.
Capillary pressures and capillary dynamics
Up to 40 mmHg of pressure difference exist between
the entrance and exit of a capillary and is an important
difference.
The exchanges of blood substances across the
capillary epithelia are via (a) passing through the
cellular gaps, (b) solubilizing through the membranes
(c) fenestrae, and (d) through vesicles. Fig. 21-12
Also note that in the brain, the matters cross through
the capillary epithelia only through mediated transport.
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At least, several driving forces must be considered:
(a)
Blood pressure difference between the lumen of
capillary and interstitial fluid,
(b)
The concentration gradient
(c)
Osmotic pressure.
Recall that the osmotic pressure drives transport of water
from the region of a low concentration of solute to that of a
higher concentration across a semi-permeable membrane.
Thus, at the entrance to the capillary, high blood pressure
pushes water out from the capillary, but a higher osmotic
pressure in the capillary due to its high content of plasma
proteins, brings the water back into the capillary. The
balance of the two is to push water out of the capillary.
(Fig. 21-13)
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Fluid Exchange Across
Capillary Walls
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21-42
The blood pressure decreases at the other side of the
capillary, but the osmostic pressure (proteins inside the
capillaries vs. proteins in the tissue fluid) remains
about the same. The balance will be that to bring
water back into the capillary.
Water and solubilized substances, such as oxygen,
glucose, nutrients, may get into the interstitial fluids
along with their concentration gradients at the start of
the capillary.
90% of water (the rest goes to the lymphatic circulatory
system) and the waste products will come back to the
capillary at the other end.
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Vein Characteristics and
Effect of Gravity on Blood Pressure
Vein Characteristics
• Venous return to heart
increases due to
increase in blood
volume, venous tone,
and arteriole dilation
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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iii.
Venous pressure
The venous pressure is low and it gets lower as the
blood vessels become larger.
It is only 2 torr at the right atrium, or about 16 torr in
the vein.
Blood flow through the venous of skeletal muscles may
be maintained through muscular compression and the
valves.
The calves are the second heart.
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Control of Blood Flow by Tissues
• Local control
– In most tissues, blood flow is proportional to
metabolic needs of tissues
• Nervous System
– Responsible for routing blood flow and
maintaining blood pressure
• Hormonal Control
– Sympathetic action potentials stimulate
epinephrine and norepinephrine
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3.
Cardiovascular regulation (Fig. 21-14)
The homeostatic mechanism to maintain the steady flow of blood relies
on (a) cardiac output, (b) peripheral resistance, and (c) blood pressure.
They are under the control of neuronal and endocrine systems.
Throughout the body, most of the cells are relatively close to the
capillaries. Within 134 um.
This is important, since after leaving capillaries, nutrients and oxygen
must reach the target sites mostly by diffusion.
In addition to the neuronal and endocrine factors, the regulation of
cardiovascular function may be maintained through "local factors"
Local factors imply the shift of blood flows within the capillary bed to
direct the flow to the needed site - auto-regulation.
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a.
Autoregulation of blood flow
Precapillary sphincter muscle responds to the
tissue oxygen or carbon dioxide level and
regulates the direction of blood flow.
Release of histamine, bacterial toxin and
prostaglandins cause a relaxation of precapillary
sphincters and vasodilation occurs at the injury
site - vasodilators vs. vasoconstrictors. (Fig.
21.33 of Seeley)
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Local Control of Blood Flow
by Tissues
• Blood flow can increase 7-8 times as a result of vasodilation of
metarterioles and precapillary sphincters in response to
increased rate of metabolism
– Vasodilator substances produced as metabolism increases
– Vasomotion is periodic contraction and relaxation of precapillary
sphincters
21-52
b.
Neuronal control of blood pressure and blood flow FIG. 21.34 of
Seeley.
The nervous system regulates the cardiac output and peripheral resistance.
We have already learnt that the cardioacceleratory center in the medulla
oblongata to increase cardiac output through sympathetic innervation and the
cardioinhibitory center to reduce cardiac output through parasympathetic
innervation.
The regulation of peripheral resistance at the region of the arterioles is primarily
dependent on the vasomotor center of the medulla oblongata. Through the
sympathetic motor neurons, it provides tonic stimulation demonstrating
vasomotor tone that partially constricts the blood vessels. This peripheral
resistance results in a lower cardiac output.
Another region of the vasomotor center is initiatory to the above and results in
vasodilation, thus an increased cardiac output.
The cardiovascular center of the medulla oblongata responds to the signals
from the baroreceptors and chemoreceptors in response to the blood pressure
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change, pH and dissolved gases.
Nervous Regulation of
Blood Vessels
21-54
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 oxygen, carbon dioxide,
and pH levels of blood
• Central nervous system ischemic response
– Results from high carbon dioxide or low pH levels in
medulla and increases peripheral resistance
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Baroreceptor Reflex Control
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21-57
Chemoreceptor Reflex Control
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21-60
Long-Term Regulation
of Blood Pressure
•
•
•
•
•
Renin-angiotensin-aldosterone mechanism
Vasopressin (ADH) mechanism
Atrial natriuretic mechanism
Fluid shift mechanism
Stress-relaxation response
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Renin-Angiotensin-Aldosterone
Mechanism
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21-63
Vasopressin (ADH) Mechanism
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iii. Erythropoietin
Loss of blood pressure or the oxygen content,
releases erythropoietin from the kidneys.
Increased erythrocytes.
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iv.
Atrial natriuretic peptide
Released when blood pressure is increased.
When the atrial wall is stretched this hormone is
made.
Reduces blood volume and pressure by (a) loss of
Na+ and water in the kidneys, (b) increasing water
losses at the kidneys by blocking ADH and
aldosterone, (c) blocking the release of epinephrine
and norepinephrine, (d) peripheral vasodilation.
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Shock
• Inadequate blood flow throughout body
• Three stages
– Compensated: Blood pressure decreases only a moderate
amount and mechanisms able to reestablish normal blood
pressure and flow
– Progressive: Compensatory mechanisms inadequate and
positive feedback cycle develops; cycle proceeds to next
stage or medical treatment reestablishes adequate blood
flow to tissues
– Irreversible: Leads to death, regardless of medical
treatment
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Fig. 21-35 Fetal circulation
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