Now, let’s move to afterload.
Afterload is a resistance.
It's how much you are pushing
against the aortic valve to open it up.
So, you need to open the valve up.
The heart needs to overcome the
resistance that's in the cardiovascular system
and to push out any blood.
If it couldn't open the valve up,
no blood would move.
And the absolute way you quantify afterload
is in the amount of pressure it generates
at a certain wall stress.
A little bit more complex,
but sometimes it’s important to
not just think of the valve opening,
but it’s the amount of pressure the left
ventricle needs to contract on to open that valve.
The most important thing to
think about with afterload
is how it affects the velocity of contraction.
This should make inherent sense to you.
This more weight you have on a muscle
when you try to contract it,
it will contract slower
simply because you're trying
to overcome a greater weight.
The heart is working on a very similar principle.
It is trying to open up the aortic valve.
The more resistance there is
to opening up the valve,
the less velocity the contraction will be.
If there is easy time to open that valve,
velocity will be very rapid.
So, the velocity of contraction
always affected on how much
afterload it needs to overcome.
You can think about this,
even with something like a weight.
So, if you are trying to
lift up, let’s say, a table
and you went to lift that table up
and the table was really light,
you can move it rapidly.
If that table, however,
was a big oak table and had stuff stacked on it
and you were trying to lift it up,
you would lift it – using that same amount of effort,
it would move slowly.
That same principle works for the heart,
all about how the resistance is
to opening up the aortic valve.
So now that we know the effect
on velocity of contraction,
let’s look at how this affects
these Frank-Starling curves.
So, we have the same curve,
which is left ventricular end-diastolic pressure on the x-axis
and stroke volume on the y-axis.
A is our normal curve
and you can see two changes that happen.
The first is increasing afterload.
That would be going from A to B.
When this happens,
even though left ventricular
end-diastolic pressure increases,
we get a decrease in stroke volume.
Also, this increase in stroke –
decrease in stroke volume
increases preload for the next beat.
The other way is going from A to C.
This is decreasing afterload.
you're getting a decrease in
left ventricular end-diastolic pressure
and you get an increase in stroke volume.
This increase in stroke volume also decreases preload.
So, you notice that anytime
we talk about one variable,
we have to bring up the others
they are interrelated.
The afterload also affects how
much ejection of the heart you have.
So not only does it affect the stroke volume,
but it affects the total amount
of volume of blood ejected per stroke.
It affects the velocity of shortening
and this reduces the ejection velocity.
It affects the amount of end-systolic volume
and it changes the amount of stroke volume.
So, simply by having this
fiber shortening difference
affects ejection velocity,
and stroke volume.
So, let's look at pressure-volume loops.
Everybody's favorite, pressure-volume loops.
If we put in our two parameters,
which senses our maximal value and our minimal value,
we put in our normal curve.
Again, we have ventricular filling,
and isovolumic relaxation
to return back to normal.
An increase in aortic pressure means
that there is an increase in afterload.
This increase in afterload
makes you contract harder in isovolumic contraction
before the valve opens.
So, you waste a little bit of energy
having to contract harder to
push the valve open.
In that case,
you’ve wasted energy.
Anytime you waste energy,
now you can't impart it to
the rest of the circulatory system,
and that ends up decreasing stroke volume.
The opposite is true
if you have a decrease in aortic pressure.
That allows you to open up that valve sooner
you can impart more energy to that
blood that's within the cardiovascular system.
In that case,
you increase the amount of stroke volume.