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Cardiac Cycle Mechanics
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0:07
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.
0:31
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?
0:57
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
1:15
ventricular
1:18
diastole
1:20
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.
1:45
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.
1:54
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.
2:23
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
2:57
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
3:24
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.
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Right, and because of that the AV valves are going to open.
3:51
Now, the next thing.
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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.
4:16
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
4:41
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
5:10
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.
5:21
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.
5:52
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
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right over here. This is the period
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of ventricular filling.
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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.
6:56
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
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name this phase. It's a very, very mportant phase This phase is called iso- volumetric
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contraction.
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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
8:03
squeezing, the blood is going to start rising.
8:06
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.
8:39
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.
9:13
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.
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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
9:55
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.
10:21
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
10:42
is less than the ventricular pressure
10:47
Alright.
10:50
And then we also know.
10:54
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.
11:20
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
11:45
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?
12:09
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
12:19
mmHg right.
12:22
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
12:42
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
13:11
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.
13:32
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
13:58
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.
14:20
Right, at the same time we already know
14:24
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
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pulmonary trunk and the aorta. That's why the semilunar valves open. So in this one we can say arterial pressure
15:00
arterial pressure is less than the ventricular pressure, right.
15:10
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
15:40
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
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ventricular
16:02
systole.
16:05
Alright.
16:07
but another way they describe this is just the phase in which there is actually ventricular
16:16
ejection. So it's the ventricular ejection.
16:24
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.
16:50
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.
17:00
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
17:26
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.
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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.
18:11
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
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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
19:00
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.
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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.
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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
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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.
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Alright.
20:47
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
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ventricular pressure is still greater than the atrial pressure. Or we can rewrite it this way. The atrial pressure is
21:20
still less than the ventricular pressure.
21:25
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.
21:44
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
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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.
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Now this phase is going to be just like the second phase. We actually define this phase as the iso- volumetric
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relaxation phase.
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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.
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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.
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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
23:44
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.
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