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Summary how filling pressure and inotropic state govern cardiac output
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Summary of how filling pressure and inotropic state govern cardiac output
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How Filling Pressure and Inotropic State Govern Cardiac OutputKey Points
Outline of the mechanisms involved in controlling stroke
Energy of contraction (stroke work), Starling’s law, Laplace’s law
What is contractility?
Effect of arterial blood pressure on stroke volume
What is Cardiac Output
Cardiac output (CO) is the amount of blood ejected from the heart per minute. It is
proportional to how often the heart beats per minute (heart rate, HR) and how much
blood is ejected per beat (Stroke volume, SV).
It is important to remember that the output from the right side (via pulmonary artery)
and left side (via aorta) are the same.
CO = HR x SV
These notes are concerned with stroke volume
What Controls Stroke Volume?
There are three things which control stroke volume; stretching heart muscles at rest
(preload) – Starlings law, the relationship between pressure, wall stress and radius –
La Place’s law and the strength of contraction at a given resting stretch – Contractility.
Both Starlings law and La Place’s law are based on intrinsic properties of the heart
itself, i.e. if you removed the heart from the body it would still obey the two laws.
Starlings law is based on filling pressure, which is linked to the idea of preload.
The heart receives blood from the venous system, contracts and then ejects the blood.
Preload concerns the amount of blood in the ventricles that causes stretching of the
heart muscle at rest, the more blood there is coming back to the heart, the more
stretch these cardiac myocytes will be under.
So because we are thinking of relaxation, we are thinking of phase 1 of the cardiac
cycle with filling of the ventricle, at the end of phase one we have the end diastolic
volume, this EDV tells us how much stretch the cardiac myocytes are under. This is the
preload (how much stretch you’re under) and this is an important governing factor in
how much we eject. This stretch is controlling the energy of contraction.
More volume -> More stretch = more preload -> more ejection
However there is also an opposing force which comes into play when the
aortic/pulmonary valves open, this is afterload.
This afterload on the heart is caused by resting blood in the arteries pushing back on
the heart, this afterload puts stress on the heart wall and opposes ejection.
, Preload = Stretching heart, encouraging ejection
Afterload = Stressing heart wall, opposing ejection
In a healthy heart, preload “wins” so we get ejection of blood from the heart, but the
important point is that this balance is going on between preload and afterload,
governing stroke volume.
As an example, if we increased central venous pressure, we would increase venous
return to the heart, there would be an increased EDV, a greater preload and greater
ejection from the heart due to Starlings law.
La Place’s law is governing what is going on in our heart chambers, the relationship
between the pressure in our chambers, the amount of wall stress and how big that
chamber is (the radius). (La Place’s law is linked to afterload)
We then have contractility, which is controlled by our sympathetic nerves and
circulating adrenaline increasing [Ca2+]I
What is Energy of Contraction?
Energy of contraction is the amount of work required to generate stroke volume, the
amount of energy depends on Starlings law and contractility.
Stroke work does two important things in the cardiac cycle –
It increases the chamber pressure so it is greater than the aortic pressure i.e.
isovolumetric contraction AND it causes ejection.
So the energy put into both isovolumetric contraction and ejection must be balanced, if
too much is put into increasing the pressure so the semilunar valves open, there won’t
be enough to be put into ejection to counteract the afterload. Therefore stroke volume
would decrease.
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