Control of cardiac output

Question 1

What is cardiac output?

Answer 1

Cardiac output is the amount of blood ejected from the heart per minute

Question 2

What is Cardiac output proportional to?

Answer 2

CO is proportional to heart rate and stroke volume

Question 3

What does cardiac output determine?

Answer 3

Determines blood pressure and blood flow

Question 4

What is preload?

Answer 4

Stretching of heart at rest

Question 5

What does preload increase and due to what?

Answer 5

Increases stroke volume due to starlings law

Question 6

What is afterload?

Answer 6

Resistance to ejection

Question 7

What does afterload reduce and due to what?

Answer 7

Reduces stroke volume due to laplace’s law

Question 8

What is the energy of contraction and what does it depend on?

Answer 8

Energy of contraction is the amount of work done required to generate SV
Depends on Starling’s law
and contractility

Question 9

What does stroke work carry out?(2 functions)

Answer 9

1) Increases chamber pressure

2) Ejection from ventricles

Question 10

What do preload and afterload do in terms of SV?

Answer 10

Preload increases SV

Afterload opposes SV

Question 11

What is Starlings law of the heart?

Answer 11

“Energy of contraction of cardiac
muscle is relative to the muscle fibre
length at rest”

Question 12

The greater the stretch of ventricle in diastole…

Answer 12

Then the greater the energy
of contraction

And a greater stroke volume
is achieved in systole

Question 13

what is an intrinsic property of cardiac muscles?

Answer 13

More blood in = More blood out

Question 14

Equation for stroke volume

Answer 14

Stroke volume= EDV-ESV

Question 15

Un-stretched fibre(Molecular basis of starlings law in preload)

Answer 15

Overlapping actin/myosin

Mechanical inteference

Less cross-bridge formation available for contraction

Question 16

Stretched fibre (Molecular basis of starlings law in preload)

Answer 16

Less overlapping actin/myosin

Less mechanical interference

Potential for more cross-bridge formation

Increases sensitivity to Ca 2+
ions

Question 17

What are the roles of starlings law?

Answer 17

Balances outputs of the right and left ventricle
which is very important

It is responsible for the fall in cardiac output
during a drop in blood volume or vasodilation (e.g.
haemorrhage, sepsis)

Restores cardiac output in response to
intravenous fluid transfusions

Responsible for fall in cardiac output during
orthostasis (standing) leading to postural
hypotension and dizziness

Contributes to increased stroke volume and
cardiac output during upright exercise

Question 18

After load and what is it determined by?

Answer 18

Afterload opposes ejection of blood from the heart and is determined by wall stress directed through the heart
wall

Question 19

What does stress prevent and due to what reason?

Answer 19

Stress through the wall of the heart prevents muscle
contraction
o More energy of contraction is needed to
overcome this wall stress to produce cell shortening and ejection

Question 20

Relation ship between wall tension, pressure and radius

Answer 20

P=2T/r

Question 21

Relationship between wall tension, wall stress and wall thickness

Answer 21

T=SW

Question 22

How is after load increased?

Answer 22

After load is increased by increasing pressure & radius and reduced by increasing wall
thickness

Question 23

Afterload with a small ventricle radius

Answer 23

Small ventricle radius

Greater wall curvature

More wall stress directed towards centre of chamber

Less afterload

Better ejection

Question 24

Afterload with a large ventricle radius

Answer 24

Larger ventricle radius

Less wall curvature

More wall stress directed through heart wall

More afterload

Less ejection

Question 25

What does Laplace’s law oppose?

Answer 25

Opposes Starlings law at rest

Question 26

What does increased preload do and hence what does laplace’s law state and do?

Answer 26

Increased preload gives increased stretch of chamber (Starling’s law)
o This increases chamber radius and decreases curvature
o Laplace’s law states that this will increase afterload
o Laplace’s law opposes ejection of blood from a “full” chamber
o In a healthy heart, Starling’s Law overcomes Laplace’s
 So you get good ejection

Question 27

What does laplace’s law facilitate?

Answer 27

Facilitates ejection during contraction

Ventricular contraction reduces the chamber radius and increases curvature
o Laplace’s law states there will be less afterload in the “emptying” chamber
o This aids expulsion and increases stroke volume

Question 28

What does laplace’s law contribute to?

Answer 28

Contributes to a failing heart at rest and during contraction

In a failing heart, the chambers are often dilated – increased radius

There is a reduction in ejection as Laplace’s law dictates that there is increased after load opposing ejection

Question 29

What happens when there’s an increase in blood pressure under laplace’s law?

Answer 29

Increased blood pressure will increase wall stress

  Therefore increase      afterload and reduce ejection

Question 30

How are acute rises in blood pressure offset by?

Answer 30

Starling’s law – increased stretch gives increased
contraction and increased SV

There is an intrinsic increase in contractility

 Anrep effect or local positive inotropes (noradrenaline)
 Baroreflex – decreased
sympathetic tone, decreasing blood
pressure

Question 31

What will a chronic incease in arterial blood pressure lead to?

Answer 31

Increased energy expenditure to
maintain stroke volume
 Ultimately decrease in stroke volume

Question 32

What will decreasing blood pressure do?

Answer 32

Decreasing blood pressure will increase the efficiency of the heart

Question 33

Why is it important to keep bp fairly constant?

Answer 33

This is an important reason why blood pressure needs to be kept fairly constant
during exercise

 A high BP will reduce cardiac output

Question 34

Hypertrophy in heart failure(Increased radius)

Answer 34

Heart failure where the heart does not contract properly (MI, cardiomyopathies,
mitral valve re-gurgitation)

 Blood is left in the ventricle leading to eventual volume overload

Question 35

Hypertrophy in heart failure(Increased pressure)

Answer 35

Pressure-overload heart failure due to increased pressure / afterload in chamber
(hypertension, aortic stenosis)

Question 36

What does an increase in either radius or pressure do in the hypertrophy in heart failure?

Answer 36

Increase in wall stress(afterload) which opposes rejection

Question 37

How does the heart compensate for an increase in radius or pressure?

Answer 37

Compensates with ventricular hypertrophy
Increasing wall thickness
o This decreases wall stress per sarcomere and therefore afterload and so
maintains stroke volume and cardiac output

Question 38

What are the downsides of ventricular hypertrophy?

Answer 38

this requires more energy (more sarcomeres used) – greater O 2

 The amount of energy required continues to increase so ultimately
contractility will decrease and produce more heart failure

Question 39

what does an increased venous return lead to during exercise? (Starlings law)

Answer 39

During exercise increased venous return leads to an increase in end diastolic
volume
o This causes an increased preload and more stretch
- This causes a shorter isovolumetric contraction phase and an increase in stroke
volume due to Starling’s Law

More blood goes back to the heart and therefore more blood is ejected from the heart

Question 40

What does a high blood pressure lead to?(Laplaces law)

Answer 40

With high blood pressure (in red) there is an increased afterload
- A longer time is spent in isovolumetric contraction in order to increase pressure in
the chamber to be above that in the aorta to open the valve
- This uses more energy and lowers the force of contraction and stroke volume
o Therefore, end systolic volume increases