Hemodynamics
Purpose of blood flow?
Maintain homeostasis
Nutrient and waste exchange
Purpose of control mechanisms of blood flow?
Blood flow must match metabolic needs of tissue
Blood flow to brain and heart must be maintained
Insufficient blood volume to perfuse all tissue
simultaneously
AJ Davidoff
Important to maintain
adequate perfusion
pressure in order to
control blood flow
MAP = CO x TPR
HR x SV
MAP = mean arterial pressure
TPR = total peripheral resistance
CO = cardiac output
Sherwood Fig 10-1
Blood flow depends on
pressure gradients and
vascular resistance
Capillary exchange is the sole purpose of the circulatory system
Sherwood
Relationship between blood flow, pressure and resistance
Ohm's Law: V = I*R
or
I = V/R
V = voltage (potential difference)
I = current
R = resistance
Blood Flow:
 P = Q*R
or
Q = P/R
Q = flow (mL/min)
 P = pressure gradient (mm Hg)
R = resistance (mm Hg/mL/min)
The major mechanism for changing blood flow is by
changing arterial resistance (e.g., TPR or in a single organ)
Pressure gradients
Pressure difference affects flow
not absolute pressure

Sherwood Fig 10-3
Resistance to Blood Flow
Poiseuille equation
R = 8L
r4
R = resistance
 = viscosity of blood
L = length of blood vessel
r4 = radius of blood vessel raised to the fourth power
If radius decreases by one half,
resistance increases by 16-fold (= 24)!!!
(r4 = area)
Radius profoundly affects blood flow
R~ 1/r4
Q = P/R
Sherwood Fig 10-4
Flow ~ r4
Series Resistance
(93 mm Hg)
(4 mm Hg)
Total resistance equals the sum of the individual resistances
Total flow is the same at each level, but pressure decreases
progressively
Q = P/R
Why?
Costanzo Fig 4-5
Parallel Resistance
Needs work
5 L/min
5 L/min
Flow in aorta is equal to the flow in the vena cave (steady state)
Flow to each organ is a fraction of the total blood flow
Total resistance is less then any of the individual resistances,
therefore no significant loss of arterial pressure to each organ
Velocity of Blood Flow
v = Q/A
Costanzo Fig 4-4
v = velocity of flow (cm/sec)
Q = flow (ml/sec)
A = cross-sectional area (cm2)
Total cross sectional area of systemic blood vessels
Costanzo Fig 4-3
v = Q/A
Laminar flow and Turbulence
quiet
noisy
Costanzo Fig 4-6
Laminar flow is
parabolic, highest
velocity in center (least
resistance), lowest
adjacent to vessel walls
Turbulent flow is
disoriented, no longer
parabolic, energy
wasted, thus more
pressure required to
drive blood flow.
Turbulence is  velocity of blood flow
diameter of blood vessel
1/ viscosity of blood
Ganong Fig 30-8
Mohrman and Heller Fig 6-6
Bernouilles Principle (in a single vessel)
Total energy = distending pressure (PD) + kinetic energy (KE)
Higher velocity through a constriction
KE PD
Bad for plaque regions
Why?
Total energy is actually not
conserved completely because
of heat loss
Bernouilles Principle
Bad for aneurysms
Why?
PD
KE
Compliance of blood vessels
• Compliance is a slope
• At low pressures, veins
have a greater compliance
than arteries
C = V/ P
C = compliance (mL/mm Hg)
V = volume (mL)
P = pressure (mm Hg)
• At high pressures,
compliance is similar in
veins and arteries (but
volume is much greater in
veins)
Cardiovascular Physiology Concepts
http://www.cvphysiology.com/Blood%20Pressure/BP004.htm
Compliance changes related to vasocontraction or aging
With vasocontraction:
• Venous volume
decreases and pressure
increases
• Venous compliance
decreases
Similar effects in arteries with aging
Blood
Vessels
Arteries
Conduits
Martini Fig 21-2
Pressure reservoir
Elastic recoil continues to drive blood toward arterioles during diastole
Sherwood Fig 10-6 & -7
B&B Fig 17-11
MAP = diastolic pressure + 1/3 pulse pressure
(at rest)
80 mph for 40 min
120 mph for 20 min
Sherwood Fig 10-7
2/3 time in diastole
1/3 time in systole
Dampening pulse pressures
Arterial pulse pressure
influenced by:
elasticity
rigidity
resistance
 resistance,  pulse pressure
G&H Fig 15-6
Cardiac Output (CO) = MAP
TPR
What does systolic pressure tell you?
What does diastolic pressure tell you?
Sherwood Fig 10-9
CO & TPR
TPR
Aortic pressure changes
rigid
G&H Fig 15-4 and B&B
Aortic pressure changes
G&H Fig 15-4
G&H Fig 23-4
Mean arterial pressure (MAP) is the main driving
force for blood flow through capillaries
G&H Fig 14-2
Basis of auscultatory method for measuring BP
(Sounds of Korotkoff)
Turbulent flow is noisy
Mohrman and Heller Fig 6-9
Why should cuff be placed at heart level?
What effects on BP measurement would
the presence of obesity cause?