Control Of Cardiac Output

Question 1

What is afterload?

Answer 1

The load the heart must eject blood against (roughly equivalent
to aortic pressure)

If pressure in aorta is high, heart must work harder to eject eh blood out.

Question 2

What is the definition of preload?

Answer 2

Amount the ventricles are stretched (filled) in diastole.

It is related to the end diastolic volume or central venous pressure.

Question 3

What is the Total peripheral resistance?

Answer 3

It is sometimes referred to as systemic vascular
resistance – resistance to blood flow offered by all the systemic vasculature.

Resistance can be increased or decreased

The majority of the resistance occurs at the arterioles

Question 4

What happens to pressure of fluid in a tube as it encounters resistance

Answer 4

The pressure that the blood exerts drops downstream as
it flows through ‘a resistance’.

The pressure will increase before the resistance and decrease after the resistance.

Since the arterioles feed blood into the capillaries,venues and veins, this causes a decrease in pressure in those three vessels, while serving the pressure in the arterioles (before the resistance) is greater.

The arterioles offer the greatest resistance

Constriction of the arterioles increases the resistance.

This will cause pressure in the
capillaries and on the venous side to fall but will cause pressure on the arterial side to rise.

Question 5

Why does the majority of the resistance occur at the arterioles?

Answer 5

They have very thick muscular walls

Question 6

What are the effects of changing total peripheral resistance (TPR) when the cardiac output is unchanged?

Answer 6

If TPR falls and CO is unchanged
– Arterial pressure will fall (as the dilation must increase if TPR falls)
– Venous pressure will increase

If TPR increases and CO is
unchanged
– Arterial pressure will increase
– Venous pressure will fall

Question 7

What are the effects of changing cardiac output?

Answer 7

If CO increases and TPR is
unchanged
– Arterial pressure will increase (heart pumping out a lot)
– Venous pressure will fall

If CO decreases and TPR is
unchanged
– Arterial pressure will fall
– Venous pressure will rise (heart is not pumping out as much so venous pressure increases)

Question 8

WHy and how does the heart have to change in demand for blood?

Answer 8

The heart must meet changes in demand for blood

If the tissues need more blood the arterioles and precapillary sphincters will dilate

Question 9

What happens if the tissues need more blood?

Answer 9

The arterioles and precapillary sphincters will dilate

Therefore peripheral resistance falls

The heart needs to pump more so that arterial pressure does not fall and
venous pressure doesn’t rise

The heart ‘sees’ changes in this demand as changes in arterial blood
pressure (aBP) and central venous pressure (CVP)

The heart responds to changes in CVP and aBP by INTRINSIC and
EXTRINSIC mechanisms

This is controlled by autonomic nervous system

Question 10

What is the cardiac output?

Answer 10

Cardiac Output = Stroke Volume x Heart Rate

The volume of blood pumped out of the heart per min (5l for 70kg man)

Question 11

What is the stroke volume?

Answer 11

Stroke Volume = end diastolic volume – end
systolic volume

The amount of blood that is pumped out of the heart per beat (70ml for 70kg man)

This is about 67% of normal EDV (ventricles do not empty out completely)

Can increase SV by increasing EDV or decreasing ESV

Question 12

What happens during ventricular filling?

Answer 12

As the ventricles fill, the ventricular pressure increases.

In a normal ventricle it is 120ml volume (systole) at approx 10mm Hg (diastole)

As the ventricle fills it gets harder to fill more so the roes sure goes up steeply not linearly

The ventricle fills until the walls stretch
enough to produces an intraventricular
pressure equal to the venous pressure

The higher the venous pressure the more the heart fills

Relationship is the Ventricular Compliance Curve

Question 13

Compliance can be increased or decreased in diseased states. What is the consequence of that?

Answer 13

In decreased compliance, eg caused by LV hypertrophy, it makes it harder to fill the ventricles and so the pressure is higher.

In increased compliance, it is easier to fill but the pressure is lower.

Question 14

Explain the Frank – Starling Law of The Heart

Answer 14

Like skeletal muscle – if you stretch the fibres of the heart before contracting, it will contract harder.

The more the heart fills, the harder it contracts (up to a limit)

The harder the heart contracts, the bigger the stroke volume

An increase in venous pressure will fill the heart more
– How much the ventricles fill depends on the compliance

Question 15

What is the relationship between the left ventricular end-diastolic pressure and the stroke volume?

Answer 15

Increasing venous return leads to increased left ventricular end-diastolic pressure (LVEDP) and volume (‘increased preload’).

This causes an increase in stroke volume, so that the extra blood is pumped out of the ventricle.

The ‘normal’ operating point at rest is with an LVEDP around 8-10mm Hg and stroke volume of ~70 ml

Volume that is ejected depends on how much the heart is stretched.

Question 16

What happens if the sarcomere length is too short?

Answer 16

If sarcomere length is too short filament overlap interferes with contraction

In cardiac muscle also get an increase in calcium sensitivity as the muscle fibres are stretched so the graph is steeper than skeletal muscle. Not sure why though.

Question 17

Why is starlings law of the heart important?

Answer 17

It ensures both sides are balanced. These two sides are in series with each other.

The increased stroke volume with
increased filling of the heart is and
INTRINSIC control mechanism

It ensures that both sides of the heart
pump maintain the same output

The pulmonary and systemic circulations operate in series
– The same volume of blood pumped to the body must also be pumped to the lungs

Question 18

What is contractility?

Answer 18

The force of contraction for a given fibre length

Question 19

How do you change the slope of the Starling curve?

Answer 19

Change the contractility

An increase in contractility will increase the force of contraction for a given left EDP

EXTRINSIC factors such as sympathetic stimulation and circulating adrenaline can increase contractility and thus affect the slope

Reducing sympathetic stimulation will
reduce contractility and thus affect the slope

Question 20

What is the effect of increasing arterial pressure on stroke volume?

Answer 20

Afterload is the pressure that the heart has to pump against

This is the pressure in the aorta (aortic impedance)

The aorta is impeding blood flow

Arterial (aortic) pressure is increased when the peripheral resistance is
increased
– This makes it harder for the heart to pump out

Increased TPR also reduces venous pressure and therefore reduces filling of
the heart

Over time you can get an inappropriate increase in arterial pressure. The
heart will have to work harder (see hypertension lecture)

Question 21

What are the factors that determine cardiac output?

Answer 21

How much the ventricle empties (end systolic volume) depends on

1. How hard it contracts
   determined by the end diastolic volume (how much the heart fills) and contractility (increased by sympathetic drive)
2. How hard it is to eject blood
   determined by aortic impedance (roughly arterial pressure - if pressure is too high, harder to get blood out)

Cardiac Output = Stroke Volume x Heart Rate

Contractility and heart rate are controlled by the autonomic nervous system

A decrease in arterial BP will reduce parasympathetic NS activity and
stimulate sympathetic NS increase heart rate and increase contractility

Question 22

What will happen to the TPR, arterial pressure and venous pressure if the metabolism of the body increases?

Answer 22

• If the metabolism of the body increases, then TPR will fall to supply
more blood

• This will result in a fall in arterial
pressure and an increase in venous
pressure

• The heart will respond by pumping
more

Question 23

How does the CVS respond after you eat a meal?

Answer 23

Local vasodilation in the gut

Decrease in TPR

Thus decreased in arterial and increased venous pressure which leads to an increase in stroke volume and cardiac output

The decrease in arterial pressure leads to an increase in heart rate. This increase in heart rate combined with the the increase in stroke volume leads to an increase in cardiac output.

This later leads to an increase in arterial pressure and a decrease in venous pressure to put the system back into the a stable pressure.

Question 24

How does the CVS respond to standing up?

Answer 24

Standing up causes ‘pooling’ of blood in legs due to effect of gravity on a column of liquid

The venous pressure decreases, the cardiac output decreases and the arterial pressure decreases.

Now both arterial and venous pressure have changed in the same direction

This is not very good as the arteriole pressure needs to be high in order to perfuse vital organs

Cannot adjust by intrinsic mechanisms

Baroreceptor reflex and autonomic nervous system increase heart rate AND increase TPR

If reflexes don’t work you get postural hypotension

Question 25

How does the CVS respond to exercise?

Answer 25

Initially muscle pumping and venoconstriction allows increase in venous pressure returns more blood to
the heart

This increases heart rate so venous pressure doesn’t rise to much to overstretch the heart

Then you get increase in contractility and increase in cardiac output

Later decreased TPR also increases
venous return

Very early response of increased heart
rate (decrease parasympathetic drive,
increase sympathetic drive)

Increased contractility (increased
sympathetic drive)

Question 26

What would happen in the CVS during exercise if the venous pressure increased alone?

Answer 26

This would move ventricular function to the
top (flat) part of the Starling curve.

This would cause pulmonary oedema as the left side would not be able to result by contracting more to the increase in blood volume.

Question 27

What is Starlings law controlled by?

Answer 27

The ANS

Question 28

According to the Frank -Starling Law of the heart, what would happen if you increase the venous return to the heart?

Answer 28

Increasing venous return leads to increased left ventricular end-diastolic pressure (LVEDP) and volume (‘increased preload’). This causes an increase in stroke volume, so that the extra blood is pumped out of the ventricle.

Question 29

What can increase contractility?

Answer 29

EXTRINSIC factors such as sympathetic
stimulation and circulating adrenaline can
increase contractility

Question 30

What will reduce contractility?

Answer 30

Reducing sympathetic stimulation will reduce contractility

Question 31

How will an decrease in BP affect parasympathetic and sympathetic activity?

Answer 31

Contractility and heart rate are controlled by the autonomic nervous system

A decrease in arterial BP will reduce parasympathetic NS activity and stimulate sympathetic NS increase heart rate and increase contractility

Question 32

What would happen to to the cardiac output, arterial pressure, TPR and venous pressure is the metabolism of the body increased?

Answer 32

• If the metabolism of the body increases, then TPR will fall to supply
more blood

• This will result in a fall in arterial
pressure and an increase in venous
pressure

• The heart will respond by pumping
more