Control of cardiac output Flashcards
What is cardiac output?
Cardiac output is the amount of blood ejected from the heart per minute
What is Cardiac output proportional to?
CO is proportional to heart rate and stroke volume
What does cardiac output determine?
Determines blood pressure and blood flow
What is preload?
Stretching of heart at rest
What does preload increase and due to what?
Increases stroke volume due to starlings law
What is afterload?
Resistance to ejection
What does afterload reduce and due to what?
Reduces stroke volume due to laplace’s law
What is the energy of contraction and what does it depend on?
Energy of contraction is the amount of work done required to generate SV
Depends on Starling’s law
and contractility
What does stroke work carry out?(2 functions)
1) Increases chamber pressure
2) Ejection from ventricles
What do preload and afterload do in terms of SV?
Preload increases SV
Afterload opposes SV
What is Starlings law of the heart?
“Energy of contraction of cardiac
muscle is relative to the muscle fibre
length at rest”
The greater the stretch of ventricle in diastole…
Then the greater the energy
of contraction
And a greater stroke volume
is achieved in systole
what is an intrinsic property of cardiac muscles?
More blood in = More blood out
Equation for stroke volume
Stroke volume= EDV-ESV
Un-stretched fibre(Molecular basis of starlings law in preload)
Overlapping actin/myosin
Mechanical inteference
Less cross-bridge formation available for contraction
Stretched fibre (Molecular basis of starlings law in preload)
Less overlapping actin/myosin
Less mechanical interference
Potential for more cross-bridge formation
Increases sensitivity to Ca 2+
ions
What are the roles of starlings law?
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
After load and what is it determined by?
Afterload opposes ejection of blood from the heart and is determined by wall stress directed through the heart
wall
What does stress prevent and due to what reason?
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
Relation ship between wall tension, pressure and radius
P=2T/r
Relationship between wall tension, wall stress and wall thickness
T=SW
How is after load increased?
After load is increased by increasing pressure & radius and reduced by increasing wall
thickness
Afterload with a small ventricle radius
Small ventricle radius
Greater wall curvature
More wall stress directed towards centre of chamber
Less afterload
Better ejection
Afterload with a large ventricle radius
Larger ventricle radius
Less wall curvature
More wall stress directed through heart wall
More afterload
Less ejection
What does Laplace’s law oppose?
Opposes Starlings law at rest
What does increased preload do and hence what does laplace’s law state and do?
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
What does laplace’s law facilitate?
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
What does laplace’s law contribute to?
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
What happens when there’s an increase in blood pressure under laplace’s law?
Increased blood pressure will increase wall stress
Therefore increase afterload and reduce ejection
How are acute rises in blood pressure offset by?
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
What will a chronic incease in arterial blood pressure lead to?
Increased energy expenditure to
maintain stroke volume
Ultimately decrease in stroke volume
What will decreasing blood pressure do?
Decreasing blood pressure will increase the efficiency of the heart
Why is it important to keep bp fairly constant?
This is an important reason why blood pressure needs to be kept fairly constant
during exercise
A high BP will reduce cardiac output
Hypertrophy in heart failure(Increased radius)
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
Hypertrophy in heart failure(Increased pressure)
Pressure-overload heart failure due to increased pressure / afterload in chamber
(hypertension, aortic stenosis)
What does an increase in either radius or pressure do in the hypertrophy in heart failure?
Increase in wall stress(afterload) which opposes rejection
How does the heart compensate for an increase in radius or pressure?
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
What are the downsides of ventricular hypertrophy?
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
what does an increased venous return lead to during exercise? (Starlings law)
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
What does a high blood pressure lead to?(Laplaces law)
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
