Alright, what we're going to do in this video is talk about the cardiac cycle
Now what is the cardiac cycle?
The cardiac cycle is all the mechanical events
where the blood is flowing through the different chambers of the heart
and on average, it takes about 0.8 seconds.
So what we're going to do is, is we're going to go ahead and go through each one of these
hearts.  Alright and we're going to discuss
the differences in the atrial
versus the ventricle pressure.
We're going to discuss the differences between the
arterial versus the ventricle pressure.
We're going to talk about what's happening with the AV valves
or the atrioventricular valves
and then we're going to be talking about what's happening
to the semilunar valves... SLV is for semilunar valves.
So the pulmonary and the aortic. And then we're going to say at what part
in each one of these stages, what will it appear
like on the EKG. Which one of the
components on the EKG would it be like?
Alright, so, I'm going to draw here in green
I got to draw these guys.. this my pulmonary semilunar valves right here
and then this is my aortic semilunar valve, alright?
Alright, so in the first event,
the first event is defined as
mid
to late
ventricular
diastole
Now what is meant by diastole?
 
Diastole is defined as relaxation. So in other words, we could say that
this mid to late
ventricular relaxation. We're in the
last parts of ventricular relaxation.
So what's happening here? Well, what's happening here is
blood is actually going to be returning to the heart.  So, some of the blood
is actually going to be coming from the inferior vena cava
some blood is going to be coming from the superior vena cava.
If you remember, there's the  coronary sinus. Some blood is going to be
coming from there right? Where else?
Well here's our pulmonary veins, right.  I just drew one here.
...pulmonary veins.
Bloods going to be coming in through these guys and dumping
into the left atrium
So, we've got blood
 
There's blood coming back to the heart.
And what happens is these valves
the AV valves
Whenever this blood starts accumulating
in the atria from, actually, from all these peripheral veins
like the pulmonary veins and the systemic veins
It actually starts opening up the AV valves
because the atrial pressure is a little bit greater than the ventricular pressure.
So if that happens, these valves open.
So, what valves open?  The AV valves
open.
So this would be...in this case, this would be your tricuspid valve.
between the right atrium and the right ventricle.
And this would be the bicuspid valve
or mitral valve which is in between the left atrium and the left ventricle.
So those valves would open. And what would happen is
passively, without contraction,
a good 70 to 80% of the blood
that's coming into the heart
is going to passively flow down by gravity
into the ventricles.  So, again, let's go over that really quickly. Again, blood coming from the systemic veins,
the coronary veins, and the pulmonary veins are coming to the atria.
The blood is accumulating
in the atria.  The pressure in the atria
is going to greater than the ventricular pressure
so it opens up those AV valves
and about 70-80% of the blood
flows down passively, without contraction
into the ventricles. So that's what happens there.
So, again,
We kind of already defined what is happening here so far.
So what's atrial pressure? Is it greater than ventricular pressure? Yes, it is.
So the atrial pressure in this event
is greater.  I'm just going to put P for pressure, right.
Here, I'll actually put
I'll put, put pressure. I'll just write it.
Is greater than the ventricular pressure.
Right, and because of that
the AV valves are going to open.
Now, the next thing.
The ventricular pressure is still not...so,
the blood is accumulating in the ventricles, but the
ventricles are not contracting. They are just taking that blood in.
What's going to happen is the ventricles are accumulating that blood
but the pressure in the actually pulmonary artery
or the pulmonary trunk, pulmonary arteries, and the aorta
is still greater than the pressure in the ventricles.
so these AV valves, they're not going to open.
Remember, valves are one-way.  They're not going to allow blood to go out
They are going to prevent blood from coming back in.
So it wants to go out this way. So, again
this pressure in the aorta and the pressure in the pulmonary trunk
is going to be greater than the pressure in the ventricles.
So if that's the case, these valves are going to stay shut.
So, again, what's happening here with the arterial pressure or the aortic and pulmonary pressure
the arterial pressure
is greater than the
ventricular pressure.
And what would that mean, then?
Again, it would mean that the semilunar valves would stay
closed.  Because the pressure has  to be greater here
to push them open.  Cause remember the valves are going to open up like this.
Like this, right?
You have to be able to push the blood like this
up against these valves so they can open up
So the blood can pop through, right.  So, we want
to be able open up those valves
But in this case, the pressure isn't great enough.
So the semilunar valves, like the pulmonary
semilunar valve and the aortic semilunar valves
are closed.
Now,
on the EKG
it is extremely interesting.  Now, if you remember, I told you one of the first events, when the blood is coming into the heart
70-80% of it is passively flowing down.
But then what happens is toward the last...the end
...the late end of that ventricular diastole
the actual SA node starts firing.
So, if your SA node fires.
it will actually produce
this depolarization of the atria and whenever the atria depolarizes
it contracts. Towards the late end.
And that will push the remaining 20% of the blood
down into the ventricles.
So if atria depolarizes, we know that
from the EKG to show up as a P wave.
So, it's going to show up on the EKG as a P wave
Right, so that's the first thing we know.
And this is basically...
We can also define not only mid to late ventricular diastole
but this is the period of ventricular filling.
Let's actually write that down. So this is the period
 
right over here. This is the period
 
of ventricular filling.
Okay, so that's that first event. So, this is
the first part of the the heart phase, right.  So, it's mid to late ventricular diastole.
The second part of this...Let's go...What's happening now?
Well, we know that the blood is already accumulated
Let's fix the inferior vena cava.
We know that the blood is sitting here. Right.
The ventricles have already accumulated some blood.
This is actually defined as the EDV - end diastolic volume.
Which we'll talk about when we get to cardiac output.
So, the blood is accumulating here now.  The atria have
already supplied and opened
up their actual valves to push the blood down. Now what's
going to happen is that the ventricles are going to start slowly
depolarizing and beginning to squeeze and contract
If you remember the muscular layer, that cardiac muscle,
the myocardium is going to start
squeezing the actual chambers of those ventricles and start trying
to push the blood upwards slowly.  But what happens is...
we're going to
name this phase.  It's a very, very mportant phase
This phase is called iso-
volumetric
contraction.
And you can say "systole" if you want.
Isovolumetric systole or isovolumetric contraction.
So what's happening? Let's actually denote this that
these little arrows pushing in is the myocardium
beginning to slowly depolarize and contract.
Well, you know, according to, you know, a law,
that as you start squeezing this, right,
as you start squeezing these ventricles,
the blood is going to start to try to move its way up and up
and up. So, what going to happen here? As these ventricles start
squeezing, the blood is going to start rising.
and getting reading to move up
towards the
pulmonary trunk and its arota. Right, and again here is green
is my pulmonary semilunar valve and here is this green
is my aortic semilunar valve.  They're closed right now.
But the ventricles are really starting to squeeze. But
here's where it is extremely important.
The pressure in the aorta
is naturally about 80mmHg.
Right.
Pressure here in the pulmonary trunk
is usually about 7-10.
So we're going to just go with an easy number - 10.
Right now the ventricles
these two ventricles, their pressure in this
actual...these chambers is going to be less
than the aortic pressure and the pulmonary pressure.  Let's say
it's on average is about 60
Right, 60mmHg. And let's say on
average this one is about 7
mmHg, right.
This one, again, this is
60
mmHg.  We know that
this pressure is not greater than this pressure. And you know
this pressure is not greater than this pressure so it can't
open up my semilunar valves.  So, those are closed.
So, again, what is happening to the semilunar valves in this point right here?
They are closed.  Let's actually draw, like,
a division line down here so we can separate these.
So, at this point in time the semilunar valves are still
closed.
But,
Look what's happening as we kinda try to imagine this diagram.
The blood is moving up and up and up and up.
And it's naturally pushing these
valves.  If you imagine like this.  Imagine these two valves like this.
they are open and as the blood starts accumulating it
starts pushing these valves
up and back together.
As it starts doing that, what going to happen then?
This ventricular pressure is rising and it's greater
than the atrial pressure.  The atrial pressure is going to drop.  It's going to drop down to
about zero or 10. So we'll say, let's just say
zero for the sake of it.  Zero and zero here.
Then than means that the ventricular pressure is
greater than atrial pressure.  And if that's the case then,
those valves will be snapping shut. So the AV valve
or the atrioventricular valves are actually going to be
closing shut now.
So, they're going to close.
Okay.
If they close, then now we define the pressures
because the pressure are what determine the valves closing.
So we already know then that the atrial pressure is not
greater than the ventricular pressure because we already said that the ventricles
are higher than the atrial, so they close the AV valves.
So that means, in this case, that the atrial pressure
is less than the ventricular pressure
Alright.
And then we also know.
that if the aterial pressure  -  80, 60 -
The arterial pressure is still greater than the ventricular pressure.
And then 10, 7, look this is still going to be greater
than the actual ventricular pressure, so if that's the case,
that's why the semilunar valves
are still going to be closed.
So this is the same thing here.
The arterial pressure is still at this point in time
greater than the ventricular
pressure.
Alright.
Next thing.
And I want to say thing before I go on to the EKG.  When
you're doing auscultation, when you're listening to the different heart sounds,
When you go to this point of the
phase.  This phase. This isovolumetric contraction
When that brief moment...no blood is leaving the ventricles,
No blood is leaving the ventricles during this phase.
Alrght, because the pressure isn't greater than the arterial pressure - ventricle pressure isn't greater
than arterial pressure
So because of that you're still going
to hear a sound.  Those AV valves are going to snap shut.  When the
AV valves snap shut
It actually going to produce a sound which is your first heart sound
And this first heart sound is called "Lub".
So they call this S1.
Alright. Which is your first heart sound.  And that going to produce the sound
"Lub."   What we remember from this phase
is that it was starting to contract.  Well guess what?
It gets to the point, when again, let's say this one is about 10
mmHg, right.  Ten
mmHg.  And this one is about 80
mmHg, right.
The pressure within this left
ventricle, it starts rising.
and it get up to the point to where it reaches about
one hundred
and twenty mmHg.  A hundred
and twenty
mmHg. Now, again, at this point in them the AV valves
are closed.
Here
in the right ventricle, it's not a high pressure system, right.
It's usually a low pressure system in an average, you know,
healthy adults, right.
And even adolescents.  So, usually it's not a high pressure system
But the pressure in here is only going to go to about 24-27.
So let's go, let's say 27
25 is an easier number to remember.  It's about 25...
24-27.  So, we'll say 25.
mmHg, right.
So if you look now, what's the difference?
The pressure in the ventricle is greater than the pressure
within the artieries, right?
25 and 10, 120 and 80.
So the ventricular pressure is greater than the arterial pressure.
What's going to happen, then?
Things like to move from areas of high pressure to areas of low pressure. That's how it works,
right. So now what's gonna...when these ventricles are squeezed
cause what's happening still, they're still contracting.
They're still contracting.
And the pressure rises. Well guess what? These
valves are going to open and blood is going to move
out.  Alright, so what I'm going to do real quick before I continue
to keep talking, is that I know that sometimes
whenever I was discussing about
atrial versus ventricular pressure over in the first phase
I know you guys might have been like, "Well where is he going over
here"?  So, I'm just trying to make sure that we keep
this same flow visible throughout
all of the phases. Alright, so the next thing.  We left off here.
The ventricles are ejecting blood out,
right.  Because the ventricular pressure here in both the
right ventricle and left ventricle is greater than the
arterial pressure for the pulmonary trunk and aorta.
So, when that happens it blasts open those
semilunar valves.  So, these valves are going to open.
Right, at the same time we already know
that the blood is going up.
The blood is going to still keep these valves closed.  Right, again, what are these valves.
These are the AV valves
or the atrioventricular valves: tricuspid/bicuspid.
Or mitral valve, right, for the bicuspid.
Now those valves are still going to be closed.  So, we're going to snap those closed.
Right.
Still going to closed.
And we already said
that the ventricular pressure in both the left and
right ventricle are greater than the arterial pressure in the
pulmonary trunk and the aorta.
That's why the semilunar valves open.  So in this one we can
say arterial pressure
arterial
pressure
is less than the
ventricular pressure, right.
And then we know that the valves...the blood is still
being pushed by the ventricles and its
a hundred and twenty.  In the atria, it's still almost 0.
So here in the left atrium and right atrium
It's still almost about 0 mmHg.  So, these guys
are still greater than these pressures here in the atria.
So, again that's why the AV valves
are still closed because the pressure is greater in here
than it is up here in the atria.  Right, so again..
The atrial pressure is
less than
the ventricular pressure.
So, this phase, if you can kind of tell is
all about blood being ejected  out of the ventricles.  So this actually...we can call it...
there's two names for this phase.  It's called mid
to late
ventricular
systole.
Alright.
but
another way they describe this is just the phase in which
there is actually ventricular
ejection.
So it's the ventricular ejection.
So, this is the ventricular ejection phase or is the
mid to late ventricular systole phase.
And again, on more thing.
If you think about it, the ventricles are still doing what they were doing over
here in the second phase.  They're still depolarizing.
And they are still contracting.  So it's
going to be the same wave on the EKG here as it would be
in this phase.  So, it's still going to be the
QRS complex, or QRS wave, right.
So the same thing in this one.
Alright.  Now let's go into the last phase.
Right, so we're going come over here.  So this is going to be our  fourth phase.
Now same thing.  We're going to follow the same
components here.
We're going to do the same thing. So, we're just going to keep it going.
So, what's happening here? If we look at these...
the actually heart again?  We know
that the ventricles have ejected their load.
Again, let's draw these valves.  What's right here?  This is going to be the
pulmonary semilunar valve.  And then right here is going to be the
aortic semilunar valve.
Now, if we look here
we know the ventricles have ejected the blood out,
right. There is still going to be a little bit of blood left.  That
blood is actually called the end systolic volume.  The blood that's remaining,
you know, after the ventricles have contracted.  So,
that's called the ESV. We'll, again, we'll tak about that when we talk about cardiac output.
But a majority of that blood is out here.
Right, so it's out here.  It's getting distributed now.
So, some of it is actually getting distributed out here to the systemic circuit.  Some of it is getting distributed out to the pulmonary circuit, right.
Or even some of it is getting distributed out to the coronary circuit.  Which again
we talked that in previous videos.
So now, if that blood is going out there
that pressure here
in the aorta and that pressure here in the
pulmonary trunk, it's going to rise. Cause now it's going to
accumulate all that blood and the pressure is going to rise.
So, that pressure here  in
the aorta is going to rise very, very, like
drastically.  Right. So, it's going to rise out here.
and it's still going to be...
this pressure is going to be greater, the aortic pressure and the pulmonary
trunk pressure are going to be greater than the ventricular pressure.
Another thing that happens is
these arteries are very elastic so they can stretch.
So when they stretch, imagine it being able to stretch
when the blood is coming in to it.  So imagine something like this:
Imagine the aorta here.  If you can imagine it.
And blood is coming into the aorta.  It's extremely...
and so is the pulmonary trunk. It's extremely elastic.
This is a low pressure system.  This is a high pressure system.
When the blood moves in here, it can..
it can actually expand a little bit.  It can actually accommodate
or is very compliant or extensible, so it can actually
take on the high pressure.  When it
does that it can recoil the blood down,
right and so it can go to all the systemic circuit, or up
to supply the actual, the head
and the actual upper limbs.  But, what happens
is some of the blood
can actually go down.
Some of the blood can try to go back.  And when it goes back,
it snaps this valve closed.  The
aortic semilunar valve.  Same thing.  This guy is going to stretch
And if this guy stretches a little bit, again it's not a very
high pressure system, but imagine it stretching.
it's going to recoil the blood out this way.  Recoil
it out this way.  But a little bit is going to come back,
right.
So the pressure in the aorta and the pressure
within the pulmonary trunk are going to rise
become greater than the pressure within the ventricles.
and snap these semilunar valves shut.
When you look at a graph here.  And I'll show the graph after.  There's going to be a brief rise in
aortic pressure.  They actually call it the dicrotic notch.
When there's that brief rise in aortic pressure because it snaps
that semilunar valve closed.
So, again, what happening here.
We already discussed it.
We know that the arterial pressure...
if we come over here and we try to follow it.  We know that the arterial pressure is
going to be greater than
the ventricular pressure.  So, arterial pressure
is greater than the ventricular pressure.
Right.
So, if that's the case, then again we already discussed
that there's going to be a brief rise in aortic pressure.
Dicrotic notch.  And it's going to cause that drop
of the blood to come back down and snap those valves closed.
And so, that means the semilunar valves are actually going to
snap shut.  So, then the semilunar valves
are actually going to be closed.
Alright.
Then,
if you think about it, what happens here
is that the ventricle pressure is still a little bit greater.  It's going be
going down and down and down.  But the atria
still going to be at this point in time, still going to be zero.
So, it's still going to be a zero at this point.  Very, very low pressure.
But the ventricular pressure is still going to be great enough
to be able to be greater than the atrial pressure.
So, if that's the case.  We know that the
ventricular pressure is still greater than the
atrial pressure.
Or we can rewrite it this way.  The atrial pressure is
still less than the
ventricular pressure.
Alright, still greater than the ventricular pressure
So, if that's the case then, the ventricular pressure is still
greater, well then..the..because the pressure
differences and pressure gradients these
valves, the AV valves are still going to be closed.
So, the AV valves here are going to be
closed.
Now.
If you noticed, coming back to what we hit in the second
phase, both the valves are closed.  And it's only a brief
moment in which
all four valves are closed again.  And if you
noticed before, if you come over here really quick, if we
noticed,
when the actual AV valves closed in the isovolumetric contraction phase, it produced the lub sound.
Well, now, if we come back over here,
look what happened.  The semilunar valve snapped
shut, right.  And if the semilunar valve snap shut and
they're closing, they're going to produce another sound.
That is going to be the second heart sound.
And that is going to be referred to as dub.
Right. And that is S2.
Now this phase is going to be just like
the second phase.  We actually define this phase
as the iso-
volumetric
relaxation phase.
Because the ventricles are beginning to go into
diastole.
They are beginning to relax.  The coronary arteries are getting filled.
The blood is going to the muscles, so they can get the oxygen they need.
Right, in order to be able to produce ATP
and undergo contraction.
So, for the next cycle.  So, in this part
the venticles are...what did I say...it's that the ventricles
are relaxing.  So in other words, the ventricles are repolarizing.
And if the ventricles are repolarizing, if we go back to remember what the
EKG said. The EKG for
ventricular repolarization is the T wave.
So again, this last part of the EKG is actually going to show up
on the T wave.
So, that describes basically what's happening here with
all of these, right. So, if you think about it right
after this cycle, what's going to happen? Right, when this ends.
the ventricles are going to their relaxation.
The blood is getting distributed to the pulmonary
systemic and the coronary circuit. Once it gets
distributed, guess what's going to happen?  Let's come back here.
It's going to go all the way back over here
to where the ventricles are getting to the mid, to their
late part of their relaxation phase.  And they
start filling again.  The cycle continues again
for another 0.8 seconds. So,
that right there describes the cardiac cycle in a nutshell.
