Hemodynamics or Why Blood Flows Through Little Vessels Jil

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Hemodynamics or Why Blood Flows Through Little
Vessels Jil Tardiff Dept of Physiology
Biophysics Dept of Medicine (Cardiology)
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Outline
Laminar vs Turbulent Flow Flow vs
Velocity Relation of Pressure, Flow and
Resistance Determinants of Resistance Regulation
of Blood Flow Role of Large Vessel Elasticity in
Maintaining Continuous Flow Determinants of Blood
Pressure Why do atherosclerotic blockages reduce
blood flow? How does blood pressure change as it
moves through a resistance vessel?
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Laminar vs Turbulent Flow
Berne and Levy, Physiology 3rd Ed. p. 447
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Difference Between Flow and Velocity
Flow is a measure of volume per unit time
Velocity is a measure of distance per second
along the axis of movement
r 4
Velocity Flow/Cross sectional area
r 2
r 1
Flow
velocity
100 ml/sec
100 ml/s
radius (cm) 1 2 4 area (cm2)
(?r2) 3.14 12.56 50.24 flow
(cm3/sec) 100 100 100 fluid velocity
(cm/sec) 32 8 2
Note This assumes constant flow and ignores
resistance
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Pressure is a form of Potential Energy

• Blood flows from high to low pressure
• Heart increases blood pressure
• Flow through resistance blood vessels decreases
  blood pressure

Distinction between Blood Pressure in people
and pressure in blood vessels pressure is
different in different parts of the vascular tree
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Relationship Between Velocity and Pressure
Pressure is a form of potential energy.
Differences in pressure are the driving force
for fluid movement. Kinetic energy is
proportional to (velocity)2 If we ignore
turbulence and friction, total energy (Potential
Kinetic) of the fluid is conserved and so as
velocity increases, pressure decreases
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Relationship Between Pressure, Flow and Resistance
?P QR
Change in Pressure Flow x Resistance
?V
or V IR
Similar to Ohms Law I for electricity
R
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Resistance to Fluid Flow
The preceding discussion ignored resistance to
flow in order to focus on some basic concepts.
Resistance is important in the Circulatory
System. As fluid passes through a resistance
pressure drops. A resistance dissipates energy,
so as the fluid works its way through
the resistance it must give up energy. It gives
up potential energy in the form of a drop in
pressure.
P1 gt P2
?P
QR
Pressure
distance
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Origin of Resistance in Laminar Flow
resistance arises due to 1) interactions
between the moving fluid and the stationary tube
wall 2) interactions between molecules in the
fluid (viscosity)
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Determinants of Resistance in Laminar Flow
Poiseuilles Law

8 ? l
r
R
Q
? r4
l
length viscosity radius

• ? 3.14159 as always
• l tube length
• fluid viscosity
• r tube radius

role of circumference vs cross-sectional area
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Physiological Basis of Exertional Angina Why
Atherosclerotic Narrowing of Coronary Arteries
Reduces Blood Flow Implications of Poiseuilles
Law
If ?P is constant, flow is very sensitive to tube
radius
r4
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Think about this in terms of hypertension
(vascular tone!)
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Path of Blood Flow in the Circulatory System
Systemic
Pulmonary
Heart (left ventricle) Heart (left atrium)
aorta arteries arterioles capillaries venules vein
s vena cava
pulmonary veins capillaries pulmonary arteries
Heart (right atrium) Heart (right ventricle)
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Mean Blood Pressure in the Systemic Circulation
100
75
Mean Blood Pressure (mm Hg)
50
25
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Blood Vessel Diameter and Blood Velocity
West, Physiological Basis of Medical Practice
11th Ed. p. 120
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Distinction between cross sectional area of
individual vessel vs total cross sectional area
in a given region of the circulatory system
Total cross sectional area
Average vessel cross sectional area
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A Brief Digression on the Cardiac Pump Cycle
Each pump cycle is subdivided into two times 1)
Diastole filling, no forward pumping (2/3) 2)
Systole forward pumping (1/3)
Blood Pressure (mm Hg) systolic /
diastolic normal BP ??? 120/80 mmHg Hypertension
gt 140/90 mm Hg
Arterial Blood Pressure
pressure (mm Hg)
Berne and Levy, Physiology 3rd Ed. p. 457
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Converting Intermittent Pumping to Continuous Flow
The heart is the pump that keeps the fluid
circulating. The heart is a pulsatile,
intermittent pump. During each pump cycle blood
flows out of the heart for only 1/3 of the
time. THE PROBLEM To maintain continuous flow
during diastole.
THE SOLUTION Large elastic arteries distend
during systole to absorb ejected volume pulse
relax during diastole maintaining arterial
pressure and flow to the periphery
volume ejected large elastic arteries
distend aortic valve closes blood flows into
periphery under pressure created by elastic
recoil of arteries while the heart fills during
diastole
Berne and Levy, Physiology 3rd Ed. p. 457
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What Can the Body Regulate to Alter Blood Flow
and Specific Tissue Perfusion?
?P Mean Arterial Pressure Mean Venous
Pressure ?P, not subject to significant short
term regulation
R Resistance
8, ?, l, ? are not subject to significant
regulation by body r4 can be regulated especially
in arterioles, resistance vessels
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Arterioles are Heavily Innervated Radius
Controlled by Autonomic Nervous System and Local
Factors
VASODILATION In some arterial beds parasympathet
ic stimulation gt acetylcholine release
gt muscarinic receptors gt ??? gt arteriolar
vasodilation
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Nitric Oxide and Vasodilation
Vanhoutte. (1998) Nature 396213-216
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Local Factors in the Control of Arteriolar
Resistance
endothelial derived relaxing factor (EDRF)
nitric oxide (NO)
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Arterioles are Heavily Innervated Radius
Controlled by Autonomic Nervous System and Local
Factors
VASOCONSTRICTION In most arterial beds
sympathetic stimulation gt NE release gt arteriole
vasoconstriction fight or flight reflex
Redirects blood flow from internal organs to
skeletal muscle. stimulation of ? adrenergic
receptors gt ? Ca2i in vascular smooth
muscle cells gt constriction
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?-Adrenergic Receptors Stimulate Vasoconstriction
Ca2-calmodulin dependent protein kinases,
CaM-kinase
Katzung, Basic and Clinical Pharmacology, 2001,
p. 123
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Phospholipase C Mediated Signaling
http//www.answers.com/topic/phosphatidylinositol-
4-5-bisphosphate
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Other Local Factors in the Control of Arteriolar
Resistance
hypoxia
arteriolar vasodilation
increased tissue perfusion
HORMONES
endothelin bradykinin
angiotensin II vasopressin, ADH
atrial naturetic peptide adenosine
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Autonomic Nervous System Regulates Distribution
of Blood Volumes in Different Parts of the
Vascular System
West, Physiological Basis of Medical Practice
11th Ed. p. 121
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Vaso-Vagal Episodes Neural Control
Lying down gt stand up quickly gt briefly feel
lightheaded
Failure of the venoconstrictor system to respond
in a timely fashion. To prevent blood pooling in
large veins must constrict veins on standing or
the rise in hydrostatic pressure will cause
veno-dilation and thus blood pooling in the large
veins of the legs and abdomen. This pooling
reduces venous return to the heart. This in turn
reduces forward cardiac output and reduces
arterial blood pressure and perfusion of the
brain. Thus, the feeling of lightheadedness.
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Determinants of Arterial Blood Pressure and Flow
1) Heart Cardiac Output 2) Vascular
Resistance 3) Vascular Volume (Capacitance) 4)
Blood Volume
For an exercise, think about how a patient may
develop hypertension (elevated blood pressure)
and how we might treat it..
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Factor 1 Heart Cardiac Output
Blood Pressure (Blood Flow)(Total Peripheral
Resistance) BP Q TPR
Determinants of Blood Flow (Cardiac Output)
cardiac output (heart rate) x (stroke volume)
Determinants of Stroke Volume
venous return and venous blood pressure
(preload) duration of diastole (heart
rate) ventricular wall relaxation during
diastole arterial blood pressure (afterload)
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Factor 2 Determinants of Vascular Resistance
Arterial blood pressure systole vs
diastole Perfusion pressure largely determined by
arterial blood pressure Major site of pressure
drop is in arterioles
West, Physiological Basis of Medical Practice
11th Ed. p. 120
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Total Peripheral Resistance and Fractional Drop
in Pressure
Total Peripheral Resistance Rartery
Rarteriole Rcapillary Rvenule Rvein
Resistance in series
Fractional Resistance in Arterioles
Rarterioles/TPR
?P mean arterial pressure mean venous pressure
Fractional Drop in Pressure in the arterioles
?P(Rarterioles/TPR)
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Factor 3 Vascular Volume - Capacitance
CNS control arterial volume by regulating vessel
diameter venous volume by regulating vessel
diameter ratio of arterial to venous
volume Examples vaso-vagal episodes shock
peripheral vasodilation drops pressure
Factor 4 Determinants of Blood Volume Renal
Physiology
Cardiac Physiology in Lectures Starting on Wed.
Oct. 29