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Hemodynamics
Objectives
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Define resistance and understand the effects of
adding resistance in series vs.in parallel in total
resistance and flow.
Describe the relationship between pressure, flow
and resistance in the vasculature.
Explain how Poiseuille’s law influences resistance to
flow and define the factors that determine
resistance.
Describe the change in pressure along vascular tree
and explain how flow to any organ is altered by
change in resistance to that organ.
Explain types of flow, laminar versus turbulent and
the transition between them; Reynold’s number.
Distribution of Cardiac Output at Rest
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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
Hemodynamics: Factors affecting blood flow
How much blood flow and what
determines how much?
Blood Flow: Volume of blood flowing through any
tissue in a given time period (mL/min)
Relations of pressure, flow and
resistance
Flow =
Change in Pressure
Resistance
P
F=
R
Flow is:
Directly proportional to
pressure gradient
Inversely proportional to
resistance
Pressure gradient is pressure difference between beginning
and end of a vessel
Blood flows from area of higher pressure to area of
lower pressure
P1 = 90 mmHg
P2 = 40 mmHg
∆P = P1 – P2 = 50 mmHg
( the greater the ∆P, the greater the flow)
Flow  ∆P
Path of Blood Flow
in the Circulatory
System
Heart (left ventricle)
aorta
arteries
arterioles
capillaries
venules
veins
vena cava
Heart (right atrium)
Resistance to BF
Resistance is measure of opposition of blood flow through a
vessel
 Resistance arises due to
 interactions between the moving fluid and the stationary tube
wall
 interactions between molecules in the fluid (viscosity)
 (the higher the R, the smaller the flow).
Flow  1/R
 Factors determining the resistance:
 Vessel length
 Vessel radius
 Blood viscosity
1. Blood vessel length
Resistance to Flow is directly proportional to
the length
the longer the length  the higher the
resistance
e.g. Obesity
2. Blood viscosity
Resistance is directly proportional to blood
viscosity
depends on:
 ratio of RBCs to plasma vol.
 conc. of proteins in plasma.
-  viscosity ( dehydration, polycythaemia)
-  viscosity ( RBCs or  Plasma prot.)
3. Size of the Blood vessel lumen (vessel radius)
The resistance to flow is
inversely proportional to the
fourth power of the radius
R  1/d4
( the smaller the diameter the
greater the resistance -----if the diameter  by ½, the
resistance  16 times)
Therefore vessel radius is a major determinant of resistance
to flow (happen in arterioles-vasoconstriction and
vasodilatation)
Poiseuille’s Law
}r
F
P
F=
R
8l
R=
 r4
l
(FLOW)F =
DIFFERENCE
IN PRESSURE
(P ) r
VISCOSITY
4
8nL
LENGHT
RADIUS
Some Implications of Poiseuille’s
Law
P
 r4
=
F=
R
(P)
8l
If P is constant, flow is very sensitive to tube radius
% decrease in radius
r
(10 - r/10)*100
Q/X
10
0%
10,000
9
10%
6,561
5
50%
625
1
90%
1
% decrease in flow
[1 - (Q/Qr=10)]*100
0%
35%
94%
99.99%
What Can the Body Regulate to Alter Blood
Flow and Specific Tissue Perfusion?
F=
P
R
=
 r4
8l
(P)
P = Mean Arterial Pressure – Mean Venous Pressure
P, not subject to significant short term regulation
8l
R = Resistance
R=
 r4
8, , l,  are not subject to significant regulation by body
r4 can be regulated especially in arterioles, resistance vessels
Arteriolar Vasoconstriction and Vasodilation
Organization in the Circulatory
System
SERIES AND
PARALLEL CIRCUITS
RESISTANCE TO FLOW IN SERIES VS
IN PARALLEL
Rt = R1 + R2 + R3….
SERIES RESISTANCE
1/Rt = 1/R1 + 1/R2 + 1/R3… PARALLEL RES.
SERIES
R1
R2
R3
R1
PARALLEL
R2
R3
If: R1 = 2; R2 = 4; R3 = 6 PRU’s
Then a series arrangement gives:
RT = R 1 + R 2 + R 3
RT = 12 PRU’s
But a parallel arrangement gives:
RT =
1
1 + 1 + 1
R1
R2 R3
=1.94 PRU’s
WHAT REALLY HAPPENS IN THE CVS?
LOWER R
HIGHER R
LOWER R
CAPILLARIES
ARTERY
ARTERIOLES
Velocity of blood flow
Flow is a measure of volume per unit time
Velocity is a measure of distance per unit time
Velocity = Flow/Cross sectional area
CROSS SECTIONAL AREA
AND VELOCITY
A= 2cm2
F=10ml/s
a
V= 5cm/s
10cm2
b
1cm/s
V=F/A
1cm2
c
10cm/s
Blood Vessel Diameter and Blood
Velocity
LAMINAR VS TURBULENT FLOW
THE REYNOLD’S NUMBER
LAMINAR
FLOW
TURBULENT
FLOW
Nr = pDv / n
laminar = 2000 or less
p = density
D = diameter
v = velocity
n = viscosity