Transcript Ppts/hemodynamics dental 2009

Hemodynamics of the
Vasculature
OBJECTIVES:
• Distribution of blood volume, flow, pressure,
vessel resistance throughout the circulatory
system.
• Discuss Poiseuille's Law and the effects of
radius, length, viscosity and resistance on blood
flow.
• Limitations of applying classical hemodynamics
to blood.
HEMODYNAMICS
The Physical properties of blood, blood vessels
and the heart and their interactions
Consists of :
Pressure = Mean Arterial Pressure (MAP)
Flow = Cardiac Output (CO)
Resistance = Total peripheral resistance (TPR)
Flow = Pressure Difference
Resistance
(Ohm’s Law)
Effect of Pressure
Difference on
Blood Flow
Flow P
Flow is inversely
proportional to
vessel length (L)
Q= 10 ml/s
Q= 5 ml/s
Q 
1/L
Q= 10 ml/s
Q= 160 ml/s
Q  r4
Flow is
dependent
on 4th power
of the
radius (r4)
Effect of Radius on Flow
Q  r4
Flow is Inversely Proportional
to Viscosity
Q  ή
Poiseuille’s
Law
Poiseuille’s Law - Assumptions
• Flow is steady (constant)
– The pump (heart) is pulsatile
– Arterial vessels dampen changes, but not steady
• Flow is laminar
– Generally true except at bifurcations
• Fluid is Newtonian
– Newtonian fluid is homogeneous, fixed viscosity
– Is suspension, non-homogeneous
– Viscosity increases with increasing hematocrit
Poiseuille’s Law
Q = ΔP π
ήL 8
r4
Q = ΔP/R
R = ΔP/Q
R=8ήL
4
πr
Where:
R = Resistance
ή = Viscosity of Blood
L = length of blood vessel
R4 = radius of blood vessel
raised to the 4th power
Effect of the diameter of the blood vessel on the velocity of blood flow.
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Cardiovascular Dynamics
Simulation based on 3D
noninvasive imaging
Based on contrast-enhanced
magnetic resonance angiogram of
the abdominal aorta
Coarctation of the Aorta
Significant morbidity (hypertension, aneurysms, stroke) may be
attributed to abnormal hemodynamics in the aorta and its branches
Laminar Flow
Parabolic velocity profile
Laminar
Flow
Parabolic
velocity
profile
Axial and
Radial Flow
Turbulent
Flow
Comparison of laminar flow to turbulent blood flow..
Laminar Flow– all points in fluid move parallel to walls of tube
– Each layer of blood stays at same distance from
wall
– Blood cells forces to center of vessel
Turbulent Flow–
–
–
–
At bifurcations of blood vessels
Pressure drop greater than with laminar (square)
Makes heart work harder
Blood clots and thrombi much more likely to
develop
Effect of turbulence on pressureflow relationship
Turbulence
decreases flow
at any given
perfusion
pressure
Pressure-Flow Relationship
Reynolds's
Number
Reynolds number
above 2000
associated with
turbulent flow
Dimensionless
number,
relates inertial
forces to
viscous forces
Reynold’s Number = density * diameter * mean velocity
Figure 4-4 Effect of the diameter of the blood vessel on the velocity of blood flow.
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© 2005 Elsevier
Systemic
CirculationComprised of
Parallel and
Series Circuits
Parallel and Series Circuits
Arrangements of blood vessels in series and in parallel.
Arrows show direction of blood flow. R=Resistance
Figure 4-9 Systemic arterial pressure during the cardiac cycle. Systolic pressure is the highest pressure measured during systole. Diastolic pressure is the lowest pressure measured during
diastole. Pulse pressure is the difference between systolic pressure and diastolic pressure. (See the text for a discussion of mean arterial pressure.)
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© 2005 Elsevier
Figure 4-1 A schematic diagram showing the circuitry of the cardiovascular system. The arrows show the direction of blood flow. Percentages represent the percent (%) of cardiac output.
See the text for an explanation of the circled numbers.
Downloaded from: StudentConsult (on 9 March 2007 10:18 PM)
© 2005 Elsevier
Law of LaPlace
Vessels are “built to
withstand the wall
tensions they normally
“see”
If intravascular
pressure increases
will increase vessel
wall tension (T)
In response,
vascular smooth
muscle contracts
and T returns to
normal
Law of LaPlace
T = (∆P*r) / µm
Where T = tension in the vessel wall
∆P = Transmural pressure
r = radius of the vessel
µm = wall thickness
May explain critical closing pressure
Law of LaPlace
Law of LaPlace- Relevance
• For given BP, increasing the radius of the vessel
leads to a increase in tension.
• Arteries must have thicker walls than veins
because they carry much higher BP.
• Capillaries also carry significant BP, but unlike
arteries, capillary walls are thin. Small size leads
to reduced level of tension so thick walls not
needed.
• Conclusions: Properties of this relationship helps
us understand the variable thickness of arteries,
veins, and capillaries.
LaPlace’s Law Explains …
• Aneurysms
• Blood vessel distensibility
• Effects of ventricular dilatation on
contraction
End of lecture