The blood circulates in a closed system of branching conduits. Haemodynamics refers to the studies of blood flow and related forces in moving the blood through the circulatory system. It discusses the physical principles of blood flow t through the blood vessels with reference to the interrelationships among pressure, flow, and resistance.

1. PHYSICAL PROPERTIES OF BLOOD FLOW
DR. SARAN A K
JUNIOR RESIDENT
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2. CONTENTS
• Introduction
• Applicability of Physical Principles
• Flow, Pressure and Resistance
• Types of Flow
• Measurement of Flow
• General Principles governing Blood Flow
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3. Hemodynamics refers to the study of blood flow and related forces in
moving the blood through the circulatory system.
Interrelationships of pressure, resistance, and
blood flow
Figure 14-3 Guyton and Hall Medical Physiology
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4. • Blood vessels are not rigid tubes
• The blood is not a perfect fluid
Applicability of Physical Principles to Flow in Blood Vessels
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Diagram of Circulation in adult
Fig 30-1 Ganong’s Review of Medical Physiology 22th Ed
Distribution of Blood in parts of Circulatory System
Figure 14-1 Guyton and Hall Medical Physiology 14th Ed

6. 1.Large elastic arteries (Windkessel vessels)
2.Large muscular arteries (Distribution vessels)
3.Arterioles (Resistance vessels)
4.Meta-arterioles
5.Capillaries (Exchange vessels)
6.Venules (Postcapillary Resistance vessels)
7.Veins (Capacitance vessels)
Microcirculation
Fig 30-13 Ganong’s Review of Medical Physiology 22th Edition
Types of Blood Vessels
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7. Architectural properties of blood vessels across the whole vasculature
Figure 1.3 Mathematical modelling of Pulmonary Arterial SM Cell Subtypes
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8. • Most Non-Newtonian effects originate from the
red blood cells due to their high concentration
and distinguished mechanical properties.
• Although blood is a non-Newtonian fluid and it
follows Newtonian nature when the shear rate
is above 100 s-1
Blood – A Non Newtonian Fluid with Newtonian Nature
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9. • Therefore, the behavior of the circulation deviates, sometimes
markedly, from that predicted by these principles.
• But serves as an aid to understanding what goes on in the body.
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10. FLOW
PRESSURE RESISTANCE
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11. • Blood flow rate is the quantity of blood that passes a given point in
the circulation in a given period of time. mL/min or L/min.
• 5000 mL/min =CO
Interrelationships of pressure, resistance, and
blood flow
Figure 14-3 Guyton and Hall Medical Physiology
Flow Rate
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12. • Analogous to the relationship
between current, EMF and
resistance in an electrical
circuit expressed in Ohms
Law. (I= ΔV/R)
• Flow through any portion = Effective Perfusion Pressure
Resistance
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13. Resistance
• Resistance is the difference in mean pressure required to drive one unit of
flow in a steady state.
• The unit of resistance is dyne.s/cm5 or R units ( 1 Runit = 1mmHg/ml/sec)
𝑅 = ∆𝑃/𝐹
• Total Peripheral Resistance =
90 mm Hg – 0 mm Hg
90 mL/s
= 1 R Unit
• TPR varies between 4 R and 0.2 R in-between maximum vasoconstriction and
vasodilation
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14. Resistance to Blood Flow in Series and Parallel Circuits
Vascular Resistance in Series and Parallel
Figure 14-9 Guyton & Hall Textbook of Physiology 14th Ed
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16. Blood flow in vascular system is of two
types
1. Laminar : characterized by fluid particles
following smooth paths in layers, with
each layer moving smoothly past the
adjacent layers with little or no mixing.
2. Turbulent is fluid motion characterized
by chaotic changes in pressure and flow
velocity.
Types of Flow
Laminar and Turbulent Flow
Figure 17-3 Berne & Levy Physiology 7th Ed
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17. Laminar Blood Flow
• Parabolic Profile
• The laminar flow being
streamlined is noiseless and
within physiological limits
shows a linear relationship
with pressure.
Viscosity
Figure 17-5 Boron & Boulpaep Medical Physiology 2nd Ed
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18. Turbulent Blood Flow
• Above Critical Velocity, flow becomes
turbulent.
• Irregular varying path, continuously
mixing within the vessel and colliding with
the blood vessel (Eddy Current) causing
energy loss.
• Noisy and does not show a typical linear
relationship with pressure Laminar and Turbuent Flow
Figure 17-6 Boron & Boulpaep Medical Physiology 2nd
Ed
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19. where Re is the Reynolds number, ρ is the density of the fluid; D is the diameter of the tube (cm)
under consideration; V is the velocity of the flow (cm/s) ; and η is the viscosity of the fluid. (poise).
The higher the value of Re, the greater the probability of
turbulence. When Re is more than 3000, turbulence is almost always
present and not usually found less than a value of 2000.
Probability of Turbulence
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20. Conditions associated with Turbulence
• Normally, critical velocity is exceeded in
ascending aorta at the peak of systolic
ejection
• In case of constriction, the velocity of blood
flow through the constriction increases
producing turbulence and hence noisy flow.
• Occurs in anemia because the viscosity of
the blood is lower (reason for systolic
murmurs)
Effect of constriction on profile of
velocities
Figure 30-8 Ganong Review of Medical
Physiology 22nd Ed
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21. Blood flow can be measured by
• Invasive methods
• Non Invasive
• Electromagnetic Flow Meter
• Doppler Flow meter
• Adaptation of Fick's Principle and Indicator Dilution Method
• Plethysmography
Measurement of Blood Flow
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22. • A voltage is generated in a conductor
moving through a magnetic field and
that the magnitude of voltage is
proportional to the speed of
movement.
• Since blood is a conductor, when a
magnet is placed around a vessel, and
the voltage, which is proportional to
the blood flow, is measured
1. Electromagnetic Flow Meter
Electromagnetic flowmeter and Probe
Fig 14-4 Guyton & Hall Textbook of Physiology 14th Ed
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23. 2. Doppler Flowmeter ( Ultrasound Flow Meter)
Ultrasound Flow Meter
Fig 14-4 Guyton & Hall Textbook of Physiology 14th Ed
• Ultrasound flow meters are based on
the principle of Doppler Effect.
• Ultrasonic waves are sent into a vessel
diagonally from one crystal, and the
waves reflected are picked up
• The frequency of the reflected waves is
higher by an amount that is proportional
to the rate of flow.
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24. 3. Plethysmography
Plethysmography
Figure 30-6 Ganong Review of Medical Physiology 22nd
Ed
• A simple but crude step method
• Changes in the volume of the forearm, sealed
in a watertight chamber (plethysmograph)
are recorded reflecting changes in the
amount of blood and interstitial fluid.
• Venous occlusion plethysmography
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25. 4. Adaptation of Fick's Principle and Indicator Dilution Method
• Kety N2O method is used to measure Cerebral blood flow
• Clearance of PAH is a method used to measure Renal blood flow
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26. General Principles governing Blood Flow
1. Flow- Velocity- Area
2. Flow- Pressure- Resistance
3. Flow- Pressure Gradient
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27. • Velocity is the distance that a particle of
fluid in unit time, and it is expressed in
units of distance per unit time.
• In a rigid tube, velocity (v) and flow (F)
are related to one another by the cross-
sectional area (A) of the tube:
𝑉 = 𝐹/𝐴
Flow- Velocity- Area
Velocity
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28. For example in the aorta,
• V=F/A where F is the cardiac output
• CO = 5000 mL/min = 83.3 mL/sec
• Cross sectional area of aorta = 4.5 cm2
• V= 18.5 cm/sec
Diagram of changes in Pr. and Vel. in systemic circulation
Figure 30-13 Ganong Review of Medical Physiology 22nd Ed
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29. 𝐹 = 𝑉 𝑋 𝐴
• Because conservation of mass, (Q
constant) the velocity of the fluid
varies inversely with the cross-
sectional area.
𝐹 = 𝑉1𝐴1 = 𝑉2𝐴2
• If the vessel becomes narrow, velocity
will be more.
Flow Continuity Equation
Linear Velocity and Area
Figure 17-1 Berne & Levy Physiology 7th Ed
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30. • Daniel Bernoulli combined the law of conservation of energy and flow continuity
equation and put forth Bernoulli's principle. It states that when a constant
amount of fluid flows through a tube, sum of its potential energy and kinetic
energy remains constant.
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31. • The time taken by the particles in blood in
reaching from one point to another.
• Estimated by injecting a substance into one
segment - noting the time lapse when it can
be detected at another point.
• Arm to retina circulation time
• Arm to tongue circulation time
Circulation Time
Circulation Time Estimation
Figure 30-13 Ganong Review of Medical Physiology 22nd Ed
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32. F = Flow
ΔP = Pressure Difference between the two ends of the tube
Η = Viscosity
r = Radius
L = Length of the tube
Poiseuille Hagen Formula
Flow-Pressure-Resistance
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33. • Flow is equal to the pressure difference divided by resistance F= ΔP/R,
• Since in an intact body the length of the blood vessels does not change, the
major factors that determine the resistance to flood flow are
1.The viscosity of blood
2.The radius of vessels
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34. • A laminae in the fluid slip on one at
different speeds thereby causing a velocity
gradient in a direction perpendicular to the
wall of the tube. rate of shear
• Coefficient of viscosity α Rate of Shear
• Unit of viscosity is Poise
A. Viscosity
Laminar Blood Flow showing different velocities
Figure 4.4.14 Textbook of Medical Physiology Indu Khurana
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35. Factors Affecting Viscosity
1. Shear Rate / Velocity Gradient
.
Inverse Relationship
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Viscosity of Blood and Shear Rate
Figure 14-11 Levick’s Cardiovascular Physiology

36. • At high shear rate, the RBCs occupies
the central axis of the tube, leaving the
cell-free zone of plasma at the
periphery. This process is called plasma
skimming. Reduces friction
Features of streamlined flow high flow and slow
Figure 4.4.14 Textbook of Medical Physiology Indu Khurana
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37. 2. Haematocrit : Direct Relation
• In vitro, the viscosity of blood increases
proportionally with increase in hematocrit.
• In vivo,
• In large vessels, increase in hematocrit
causes appreciable increase in the
viscosity.
• In small vessels, change in viscosity per
unit change in hematocrit is much less.
Effect of changes in hematocrit on viscosity
Figure 30-8 Ganong Review of Medical Physiology 22nd
Ed
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38. Viscosity and Hematocrit
Figure 18-10 Boron & Boulpaep Medical Physiology 2nd
Ed
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39. • The apparent viscosity of blood
flowing through capillary tubes
become lower as the tube diameter
becomes smaller.
• 25% less PCV in capillaries,
• A satisfactory flow rate exists even in
the blood vessels whose diameters
are small.
3. Diameter of Vessels : Fahraeus–Lindquist effect
Viscosity of Blood and tube diameter
Figure 14-10 Levick’s Cardiovascular Physiology
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40. Due to plasma skimming, in
large and moderate-sized vessels,
width of the cell-free zone at the
periphery is constant, occupies a
bigger proportion in narrow
tubes. Relative viscosity less in
small vessels
Flow of blood in vessels
Figure 18-10 Boron & Boulpaep Medical Physiology 2nd
Ed
4. Temperature
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41. Effect of shear rate on Relative Viscosity of Blood , Relationship between PCV and
Viscosity
Figure 4.4.8, 4.4.9 Textbook of Medical Physiology Indu Khurana
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42. B. Radius
• The rate of flow is proportional to the fourth
power of the radius (r4) of the blood vessels
• Control of blood flow to organs or tissues is thus
regulated primarily by altering the radius of the
arterioles
Demonstration of effect of Vessel diameter on blood flow
Fig 14-8 Guyton & Hall Textbook of Physiology 14th Ed
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43. • Control of blood flow during
alteration in arterial blood pressure,
i.e. autoregulation
• Control of blood flow following
occlusive ischemia, i.e. reactive
hyperemia
• Control of blood flow during exercise
• Conversion of pulsatile flow from
heart to a steady flow, in
association with elastic vessels
Borelli’s Experiment
Figure 4.4.21 Textbook of Medical Physiology Indu
Khurana
Arterioles – Stopcocks of Circulation
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44. Flow- Pressure Gradient
Normal Blood Pressures in various portions of the Systemic Vascular Tree
Fig 14-4 Guyton & Hall Textbook of Physiology 14th Ed
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45. • Poiseuille’s law states the relationship
between pressure and flow through a
rigid tube is linear. The relationship
between flow of blood and pressure in
the blood vessels is however not
linear.
Relationship of pressure to flow A Rigid Tube B
Distensible Blood Vessel C Myogenic contractile
elements D Autoregulation
Figure 4.4.21 Textbook of Medical Physiology Indu Khurana
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46. Critical Closing Pressure
• Since the flow–pressure relationship in a
rigid tube is linear, the flow will cease only if
the pressure is zero
• However, in a blood vessel, the flow ceases
when the blood pressure is 20 mmHg or even
more
• The pressure value at which the vessel
collapses, its lumen closes and flow ceases is
called Critical Closing Pressure (CCP).
Relation to pressure to flow in thin walled blood
vessel
Figure 30-11 Ganong Review of Medical Physiology 22nd
Ed
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47. Equilibrium of factors at Critical Closing Pressure
• The vasomotor tone of the blood vessels
tries to constrict the vessels. This tone is
increased by sympathetic stimulation and
decreased by sympathetic inhibition
• Balance of forces between the intramural
distending pressure (P) and the tension (T)
in the vessel wall keeps vessel open.
Relationship of pressure and flow in A Rigid Tube B Blood
Vessel C On Sympathetic Stimulation depicting Critical
Closing Pressure
Figure 4.4.21 Textbook of Medical Physiology Indu Khurana
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48. Law of Laplace
This law states that tension in the wall of a
cylinder (T) is equal to the product of the
transmural pressure (P) and the radius (r)
divided by the wall thickness (w).
Cylinder P = T/R Sphere P= 2T/R
Relation between distending pressure (P) and Wall
Tension (T)
Figure 30-12 Ganong Review of Medical Physiology 22nd
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49. Physiological applications of Law of Laplace
1. Vascular System
• Smaller the radius of the vessel, lesser is the tension on the walls required to
balance the distending pressure
• This can explain the relative invulnerability to rupture of capillaries in spite of
being thin walled.
Category Tension at Normal Pressure
Aorta 170,000 dynes/cm
Inferior Vena Cava 21,000 dynes/ cm
Capillaries 16 dynes/cm
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Pressure, Radius, Wall Tension and Wall Thickness at
Various categories of Blood Vessels at Heart Level
Figure 8.18 Levick’s Cardiovascular Physiology

51. 2. In Heart
• When the radius of a cardiac chamber is increased, a greater tension must
be developed in the myocardium to produce any given pressure;
consequently, a dilated heart must do more work than a nondilated heart.
3. In Lung
• During expiration, the alveoli tend to collapse because of the pull of surface
tension if the tension were not reduced by the surface-tension-lowering
agent, surfactant
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52. 1. Ganong. Review of Medical Physiology 22nd Edition McGraw Hill
2. Guyton & Hall. Textbook of Medical Physiology 11th Edition Saunders
Elsevier
3. Walter F Boron. Medical Physiology 2nd Edition Saunders Elsevier
4. Berne & Levy Physiology 7th Edition Elsevier
5. Textbook of Medical Physiology 3rd Edition Indu Khurana Elsevier
6. Levicks Cardiovascular Physiology 6th Edition CRC Press
7. Ganong. Review of Medical Physiology 26th Edition McGraw Hill
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REFERENCES

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