Hemodynamics
Khuram Chaudhary
OT Physiology, 1999
Readings: 417-419
OBJECTIVES
1. Know the relationship between blood velocity and vascular cross-sectional area.
2. Know Poiseuille's equation and the relationship between the variables in the equation pressure, flow and resistance.
3. Describe how the arrangement of vessels, in parallel and series,
resistance and the distribution of blood flow.
I.
will influence
Introduction
A.
Hemodynamics is the study of the physical factors which determine the flow of
blood
B.
Knowledge of hemodynamics is necessary in order to understand the
determinants of blood pressure and the flow of blood to organs and tissues
C.
Definitions
1.
2.
3.
Blood flow is the volume of blood which moves through a vessel in a
unit period of time
a)
Often measured in liters per minute or milliliters per second
b)
Can be measured by an indicator dilution technique
Blood velocity is the distance a component of the blood, for example a
red cell, moves in a unit period of time,
a)
Often measured in cm/sec
b)
Can be measured by Doppler ultrasound
Blood pressure is the force per unit area exerted by the blood
a)
Usually measured in mm Hg
b)
Can be measured by a blood pressure cuff
c)
Cardiovascular pressures are usually referenced to
atmospheric pressure, meaning that 0 mm Hg is actually
atmospheric pressure and -10 mm Hg is 750 mm Hg absolute
(this assumes that atmospheric pressure is 760 mm Hg).
Hemodynamics
Page 2
Flow (ml/s)
60
60
60
2
Area (cm )
10
20
30
6
3
2
A
B
C
Velocity (cm/s)
D.
II.
The continuity equation
1.
The flow through a vessel is dependent on the velocity of blood in the
vessel and the crossectional area of the vessel
2.
F=v·A
3.
The flow through non-branching vessels, connected end-to-end, is
everywhere the same
(where: F = flow, v = velocity, A = area)
The relationship between pressure and flow
A.
A force must be applied to blood in order to make it flow
B.
The force that makes blood flow is the pressure difference (P) which exists
across the blood vessels, this pressure difference is produced by the heart
C.
Blood flow through a vessel is directly proportional to the pressure difference
(P) across the vessel (F  P)
1.
Example #1 - pressures at points A and C are equal so that blood flow
is zero in B
Flow = 0
100 mm Hg
A
100 mm Hg
B
C
Hemodynamics
Page 3
2.
Example #2 - the pressure difference between points A and C is 50
mm Hg and the flow in B is 100 ml/min
Flow = 100
150 mm Hg
A
3.
100 mm Hg
B
C
Example #3 - the pressure difference between points A and C is still
50 mm Hg so the flow in B is still 100 ml/min
Flow = 100
50 mm Hg
A
4.
0 mm Hg
B
C
Example #4 - the pressure difference between points A and C is 100
mm Hg so that the flow in B is now 200 ml/min
Flow = 200
100 mm Hg
A
III.
0 mm Hg
B
C
The relationship between blood flow and vascular resistance (R)
A.
Resistance (R) is the ratio: P/Flow (R = P/Flow)
1.
The units of resistance are often peripheral resistance units ( PRU’s)
or (dynes/cm2/cm3) or dynes·cm -5
Hemodynamics
Page 4
2.
B.
IV.
Note that resistance, as it is used in physiology and medicine, is not a
force!
The determinants of vascular resistance (R)
1.
Resistance is directly proportional to the length of a vessel (L) R  L
2.
Resistance is inversely proportional to the radius of a vessel (r) to the
fourth power R  1/r4
3.
Resistance is directly proportional to the blood viscosity (  )
R 
The Poiseuille equation describes the relationship between pressure, flow, and the
determinants of resistance
P = (Flow · L ·  · 8) / (r4 · )
or
4
Flow = (P · r · ) / (L ·  · 8)
A.
B.
Blood flow in a vessel is increased when:
1.
The pressure difference across a vessel is increased
2.
When the radius of a vessel is increased
3.
When the length of a vessel is decreased
4.
When the viscosity of the blood is decreased
The factors which determine total blood flow in the body can be understood
by rewriting the Poiseuille equation
MBP = CO · TPR
1.
The pressure difference (P) is the difference between the aortic
pressure and the pressure in the great veins adjacent to the right
atrium
a)
The right atrial pressure is normally close to 0 mm Hg
b)
Therefore, the pressure difference across the systemic
vasculature (P) can be estimated by using the mean
(average) pressure in the proximal aorta, which is called the
mean arterial blood pressure (MBP)
2.
The flow is called the cardiac output (CO)
3.
The total resistance of the systemic vasculature is called the total
peripheral resistance (TPR)
Hemodynamics
Page 5
4.
V.
The determinants of mean blood pressure are cardiac output and total
peripheral resistance
Layout of the cardiovascular system
A.
Blood vessels are both in series and in parallel within the cardiovascular
system
B.
Calculation of the total resistance of vessels in series
R1
C.
R2
R3
1.
The total resistance (RT) of resistances in series is the sum of the
individual resistances
2.
RT = R1 + R2 + R3 ...
Calculation of the total resistance (R T) of vessels in parallel with one another
R1
R2
R3
1.
The total resistance of resistances in parallel is the reciprocal of the
sum of the reciprocals of all of the individual resistances
2.
1/RT = 1/R1 + 1/R2 + 1/R3 ...
Hemodynamics
Page 6
3.
D.
The total resistance of resistances in parallel with one another is
always less than the resistance of any individual resistance
Comparison of vessels in series and in parallel
1.
2.
Non branching vessels in series with each other
a)
Must have equal flows
b)
Have different inlet and outlet pressures and usually different
pressure gradients
Vessels in parallel
a)
Must have equal pressure gradients
b)
May have different flows