Hemodynamics
Lecture by
Dr.Mohammed Sharique Ahmed
Quadri
Assistant professor ,Physiology
‫بسم هللا الرحمن الرحيم‬
Hemodynamics
Hemodynamics refers to principle that
govern the blood flow in the
cardiovascular system .
PATTERN & PHYSICS OF BLOOD FLOW
• Blood is transported to all parts of body
through blood vessels.
• Blood vessel bring Oxygen and nutrition.
• They remove waste product.
We will study general principles regarding
blood flow, Physics of blood flow, Role of
blood vessels, then blood pressure
regulation.
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Distribution of Cardiac
Output at Rest
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
• Digestive organ, kidney, skin receive blood in
excess of their own needs, therefore, can
withstand better main blood flow is reduced.
• Brain suffers irreparable damage when blood
supply is not there for more than 4mins. If
oxygen is not supplied to brain, permanent
damage occurs.
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Blood Flow
• Flow rate through a vessel (volume of blood
passing through per unit of time):
– Directly proportional to the pressure gradient
– Inversely proportional to vascular resistance
F = ΔP
R
F = flow rate of blood through a vessel
ΔP = pressure gradient
R = resistance of blood vessels
PRESSURE GRADIENT
• Pressure Gradient is difference in pressure
between the beginning and end of vessel.
• Blood flows from area of high pressure to an area
of low pressure, down the pressure gradient.
• When heart contracts, it gives pressure to the
blood, which is main driving force for flow
through a vessel.
• Due to resistance in the vessel, the pressure
drops as blood flows.
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Relationship of flow to Pressure
Gradient
RESISTANCE
What is Resistance?
• It is measure of hindrance or opposition to blood
flow through a vessel, caused by friction between
the blood in the vessel wall.
• If resistance to flow increases, it is difficult for
blood to pass through a vessel, therefore, flow
rate decreases.
• When resistance increases, the pressure gradient
must increase to maintain the same flow rate.
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APPLIED
• If vessels offer more resistance to flow
e.g. increased peripheral resistance, which
occurs in high blood pressure then heart must
work harder to maintain adequate circulation.
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RESISTANCE
• Resistance to blood flow depends on three
factors:
1. Viscosity of blood
2. Vessel length
3. Radius of the vessel – this is most
important.
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RESISTANCE
•
•
•
•
1. Viscosity of blood
We write η for Viscosity.
Viscosity refers to the friction, which is
developed between the molecules of fluid as
they slide over each other during flow of fluid.
Greater the viscosity, Greater the resistance to
flow.
Blood viscosity is determined by number of
circulating RBC, blood viscosity is increased in
Polycythemia and decreased in Anaemia.
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RESISTANCE
2. Vessel length
• Greater the length of a vessel, more will be
the resistance.
How length of a vessel affects the resistance?
• When blood flows through a vessel, blood
rubs against the vessel wall, greater the vessel
surface area in contact with the blood ,
greater will be the resistance to the flow.
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RESISTANCE
3. Radius of the vessel
• It is the most important factor to determine the
resistance to flow.
• Fluid passes more readily through a large vessel.
• Slight change in radius of a vessel brings great
change to flow because Resistance is inversely
proportional to the fourth power of the Radius
[multiplying the radius by itself four times].
R α 1/r4
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RESISTANCE
3. Radius of the vessel
• Therefore, doubling the radius, reduces the
resistance to 1/16 its original value.
[r4 = 2×2×2×2 = 16 or R α 1/16]
and there is increased flow through a vessel
16 fold.
• On the other hand, when we decrease the
radius to the half, blood flow will be
decreased 16 times.
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Relationship of
Resistance and Flow
to Vessel Radius
APPLIED
• Clinically radius of arteriole can be regulated
and is the most important factor in controlling
resistance to blood flow throughout the
vascular system.
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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
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
Chapter 10 The Blood Vessels
and Blood Pressure
Human Physiology by Lauralee
Sherwood ©2010 Brooks/Cole,
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
Arterioles
• Major resistance vessels
WHY?
• Because their radius is small.
• As arteriolar resistance is high, it causes marked
drop in mean pressure as blood flows through
arteriole.
• Mean Arterial Blood Pressure [ABP] of 93mm Hg
in arteries falls to mean ABP of 37mm Hg as
blood leaves the arteriole and enters the
capillaries.
ARTERIOLES
• Arteriolar Resistance converts the pulsatile
systolic to diastolic pressure swings in the
arteries into the non-fluctuating pressure
present in the capillaries.
• Radius of the arteriole can be adjusted to
achieve 2 functions:
– Distribute cardiac output among systemic organs,
depending on body’s momentary needs
– Help regulate arterial blood pressure
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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
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
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
References
• Human physiology by Lauralee Sherwood,
seventh edition
• Text book physiology by Guyton &Hall,11th
edition
• Text book of physiology by Linda .s
contanzo,third edition