The circular reasoning trap in what controls cardiac output

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The circular reasoning trap in what controls
cardiac output
How Can we analyze this?
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Arthur Guyton found the answer
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In his model the heart pumps blood out of the
venous reservoir and puts it in the arterial
reservoir.
Blood gets back to the veins by being forced
across the peripheral resistance.
Cardiac Output SV Heart Rate
Peripheral flow AOP/Peripheral Resistance
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Blood will accumulate in the arterial reservoir
until the pressure is high enough to push blood
across the peripheral resistance at the same rate
as the cardiac output.
AOP Arterial Vol / Arterial Compliance
Cardiac Output SV Heart Rate
Peripheral flow AOP/Peripheral Resistance
Cardiac output Peripheral flow
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A change in pressure in a compartment must be due
to a change in volume. Since there is a fixed
amount of blood in the system any rise in volume
on one side must be at the expense of volume on
the other side.
AOP Arterial Vol / Arterial Compliance
Venous Vol Total Vol Arterial Vol
Cardiac Output SV Heart Rate
Peripheral flow AOP/Peripheral Resistance
Cardiac output Peripheral flow
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The arterial reservoir has less compliance than
the venous side. Thus a large change in arterial
pressure causes a much smaller change in venous
pressure
Venous pressure Venous Vol / Venous compliance
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The venous pressure is the filling pressure for
the heart (a primary determinant of stroke
volume)
How can we analyze this?
AOP Arterial Vol / Arterial Compliance
Venous Vol Total Vol Arterial Vol
SV f(Venous pressure)
Cardiac Output SV Heart Rate
Peripheral flow AOP/Peripheral Resistance
Cardiac output Peripheral flow
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A rat is sacrificed and half of class studies
his heart while the other half study the vascular
system by replacing the heart with an electric
pump
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The half with the heart rediscovers the
ventricular function curve They find that
cardiac output is determined by LVEDP
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• The other half finds that both arterial and
  venous pressure vary as a function of cardiac
  output.
• At zero cardiac output venous and arterial
  pressure are the same.
• When the pump is started the two pressures
  separate.

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It is noted that both graphs have the same axis
Venous pressure LVEDP
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Take the vascular curve and swap the axes so
pressure is on the horizontal.
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Expand the pressure scale and throw away the
arterial curve.
venous pressure curve
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If we connect the heart on the right to the blood
vessels on the left what would the cardiac output
be?
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The solution is a simple simultaneous equation
that we can solve graphically. There is only one
cardiac output that can satisfy both systems. The
crossing point determines that cardiac output.
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What happens if contractility changes?
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If contractility is increased (symp. nerve
stimulation) then cardiac output will rise and
venous pressure will fall.
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If contractility is decreased then cardiac output
will fall and venous pressure will rise.
Heart Failure
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A fall in contractility raises venous pressure!
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What happens when blood volume changes?
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If total blood volume (TBV) is increased the
venous pressure curve will be shifted up in a
parallel fashion
Cardiac Output
Venous Pressure
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Decreased blood volume (hypovolemia) will cause a
parallel downward shift of the venous curve.
Increased blood volume (hypervolemia) will cause
shift in the curve upwards.
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Loss of blood (hypovolemia) reduces cardiac
output and venous pressure. If blood loss is
large nothing will restore cardiac output except
volume replacement
Its the low filling pressure that reduces the
cardiac output.
hemorrhagic shock
The venous pressure distinguishes low
contractility from hypovolemia
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Increased blood volume would have the opposite
effect
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What happens when venous tone changes?
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At any given time most of the blood volume is in
the veins.
Constriction of the smooth muscle in the walls of
the veins decreases venous compliance and raises
venous pressure for any given venous volume.
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Changing venous tone has the same effect as
changing blood volume, a parallel shift in the
venous pressure curve.
Increased Venous Tone
Decreased Venous Tone
The lower curve shows venodilation where venous
compliance (VC) is increased. While the upper
curve is a venoconstriction.
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Constricting the veins increases venous pressure
and cardiac output.
Venoconstriction is a common way for the body to
increase its cardiac output
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What happens when arteriolar tone changes?
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Resistance is given by ?P/Flow .
The major site of resistance in the periphery is
in the arterioles.
Only a small of the blood volume is in the
arterioles so changes in diameter do not shift
the venous curves.
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At zero flow increasing resistance has no effect
on venous pressure
When the heart is pumping increasing resistance
will raise arterial pressure and lower venous
pressure. The change will be proportional to the
flow.
Thus the rotation around the mean systemic
filling pressure.
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Decreasing the peripheral resistance causes a
clockwise rotation of the venous pressure curve
around the x axis intercept.
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Decreased peripheral resistance by itself would
increases filling pressure and increase cardiac
output
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Because the aortic pressure will fall, the heart
will have an increased stroke volume for any
filling pressure.
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Because the aortic pressure will fall, the heart
will have an increased stroke volume for any
filling pressure.
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The overall effect on venous pressure is
therefore ambiguous.
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These are the only 4 maneuvers the body can use
to alter the cardiac output!
Contractility Blood volume Venous tone Arteriolar
tone
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Three levels of Cardiac output regulation
Stroke volume
Is determined by
Aortic venous pressure
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At the heart only
contractility
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The sympathetic nervous system cannot separately
constrict the arterioles separate from the small
veins. Yet arteriolar constriction reduces
cardiac output while venous constriction
increases it. What is the body trying to
accomplish?
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What would the control for activating the
sympathetic nerves be labeled?
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Increase Blood Pressure!
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In this diagram the heart is represented as the
right heart , the lungs and the left heart all
together.
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The two sides are automatically synchronized. If
output from one side falls then the ventricle
behind it will force blood into its atrium until
its output matches the other ventricle.
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CO heart rate stroke volume
Theoretical
In the normal range 50-180 b/min CO is
independent of heart rate because cardiac output
is primarily determined by the peripheral
vasculature
Real
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CO heart rate stroke volume
Theoretical
CO falls dramatically at very low heart rates (lt
50 b/min). Stroke volume is OK but the rate is
too low
Real
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CO heart rate stroke volume
Theoretical
CO falls off at very high heart rates (gt 180
b/min). Stroke volume decreases because
diastolic filling time becomes too short.
Real
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In heart failure myocardial contractility is
reduced to the point that the heart can no longer
meet the needs of the body
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The rate at which the kidney loses sodium is
determined by the blood pressure
Output curve
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If cardiac output is low due to failure, blood
pressure will be low and the kidney will retain
fluid in an attempt to restore it.
Output curve
Chronically low pressure
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The chronically low blood pressure causes the
kidneys to retain sodium (and thus fluid) which
increases the blood volume.
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If the left heart fails then the right ventricle
will actively pump blood into the left atrium in
an attempt to restore the cardiac output. This
can raise left atrial pressure to dangerous
levels within seconds of the onset of LV failure.
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• 3 factors raise venous pressure in heart failure
  
• direct effect of reduced contractility
• 2) increased blood volume
• 3) Strong ventricle forces blood into the atrium
  of the weak ventricle.

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Guyton used the model we have discussed and
incorporated everything that was known about
blood pressure control in the 1970s and came up
with the following analysis.
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A.C.Guyton Ann Rev Physiol 1972
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End