Alright, what we're going to do in this video is we're going to 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 we're going to go ahead and go through each one of these hearts, right? And we're going to discuss the differences in the atrial versus the ventricular pressure.
We're going to discuss the differences between the arterial versus the ventricular 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? Right, so I'm going to draw here in green.
I've got to draw these guys. This is my pulmonary semilunar valve right here. And then this is my aortic semilunar valve, right? All right, so in the first event, the...
Our 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 is the 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 in there, right? Where else? Well, here's our pulmonary vein. right I just drew one here pulmonary veins blood is going to be coming in through these guys and dumping into the left atrium so we got blood bringing 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 your, 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 the 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 ventricle. So again let's go over that real 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 and the pressure in the atria is going to be a little bit greater than the ventricular pressure. So it opens up those AV valves and about 70 to 80 percent of the blood flows down passively without contraction. into the ventricles, right? So that's what happens there.
So again, we kind of already defined what's happening here so far. So what's the atrial pressure? Is it greater than the ventricular pressure? Yes, it is. So the atrial pressure in this event is greater.
I'm just going to put P for pressure, right? I'll put it actually, I'll 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, next thing, the ventricular pressure is still not, so the blood is accumulating in the ventricles, but the ventricles aren't contracting, they're just taking that blood in. What's going to happen is the ventricles are accumulating that blood, but the pressure in the actual pulmonary artery or the pulmonary artery is going to be a little bit more intense. 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, so they're going to allow blood to go out. They want 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 are going to stay closed because the pressure has to be greater in here to push them open. Because remember, the valves are going to open up like this. They're like this, right? You have to be...
able to push the blood like this up against these valves so that they can open up so that the blood can pop through right so we want to be able to open up those valves but in this case the pressure isn't great enough so the semi lunar valves like the pulmonary semi lunar valve and the aortic semi lunar valve are closed Now, on the EKG, it's extremely interesting. Now, if you remember, I told you, in the first events, when the blood's coming into the heart, 70% to 80% of it's passively flowing down. But then what happens is, towards the last end, the late end... of that ventricular diastole, the actual SA node starts firing.
So if your SA node fires, it actually will 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 with the P wave, right? So that's the first thing we know and this is basically, we can also define not only the mid to late ventricular diastole but this is the period of ventricular filling.
Let's actually write that down here. So this is the period, we'll write it over here. This is the.
period of ventricular filling. Okay, so that's that first event. So this is the first part of the heart phase, right?
So it's mid to late ventricular diastole. The second part of this is let's go, what's happening now? Well we know that the blood is already accumulating. Let's fix this inferior vena cava right here. So we know that the blood is already accumulating.
that the blood is sitting here right we've the ventricles have already accumulated some blood and this is actually defined as the edv right the end diastolic volume which we'll talk about when we get into cardiac output so the blood's accumulating there now the atria have already applied and opened up their actual valves to push the blood down. Now what's going to happen is the ventricles are going to start slowly depolarizing and beginning to squeeze and contract. If you remember the muscular layer, the 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 important phase. This phase is called iso volumetric.
And you can say systole if you want to, right? Isovolumetric systole or isovolumetric contraction. So what's happening?
Let's actually denote this is that these little arrows pushing in is the myocardium beginning to slowly depolarize and contract. Well you know according to a law that as you start squeezing this, right? As you start squeezing these ventricles the blood's going to start trying to move its way up and up and up. So what's going to happen here?
As the ventricle starts squeezing, the blood is going to start rising. And getting ready to move up towards the... pulmonary trunk and into the aorta, right? And again here in green, here's my pulmonary semilunar valve and here in this green is my aortic semilunar valve and they're closed right now still. But the ventricles are really starting to squeeze, but here's where it's extremely important.
The pressure in the aorta is naturally about 80 millimeters of mercury, right? Pressure here in the pulmonary trunk is usually about seven to ten, so we're gonna go just easy number, ten. Right now the ventricle ventricles, these two ventricles, their pressure in this actual, these chambers are going to be less than the aortic pressure and the pulmonary pressure. Let's say it's on average, let's say this one's about 60, right? 60 millimeters of mercury.
And let's say on average, this one's about seven millimeters of mercury, right? And this one's again, this is 60 millimeters of mercury. Well, you 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... semi-lunar valves, so those are closed.
So again, what's happening to the semi-lunar 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 semi-lunar valves are still closed. But, look what's happening, as we kind of try to imagine this diagram, the blood's moving up and up and up and up, and it's naturally pushing these valves, if you imagine it like this, imagine these two valves like this, they're open, and as the blood starts accumulating, it starts pushing these valves up and back together.
As it starts doing that, what's 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 ten. So let's just say zero for the sake of it.
Zero and zero in here. Then that means that the ventricular pressure is greater than the atrial pressure. And if that's the case then those valves would be snapping shut. So the AV valves 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 can define the pressures because the pressures are what determine the valves closing. So we already know then that the atrial pressure, it's 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.
All right? And then we also know... That if the arterial 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 this before I go into the EKG.
When you're doing oscillation, right, 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 bloods leaving the ventricles no blood is leaving the ventricles during this phase right because the pressure isn't greater than the arterial pressure the ventricular pressure isn't greater than the arterial pressure so because of that you're still going to hear a sound those AV valves are going to snap shut when the valves snap shut it's 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 right which is your first heart sound and that's going to produce the sound lub well we remember from this phase is that it was starting to contract well guess what it gets to the point what again let's say that this one's about 10 millimeters of mercury right 10 millimeters of mercury and this one's about 80 of mercury, right? The pressure within this left ventricle, it starts rising and it gets up to the point where it reaches about 120 millimeters of mercury.
120 millimeters of mercury. Now again, at this point in time, the AV valves are closed. Here, In the right ventricle, it's not a high pressure system, right? So usually a low pressure system in average, you know, in healthy adults, right? And even adolescents.
So usually it's not a high pressure system, but the pressure in here, it's only going to go up to about 24, 26. So let's say 27, 25 is an easier number to remember, but it's about 24 to 27, so we'll say 25 millimeters of mercury, right? So if you look now, what's the difference? The pressure within the ventricles is greater than the pressure within the ventricles. the arterial arteries right 25 and 10 120 and 80 so the ventricular pressure is greater than the arterial pressure well what's going to happen that things like to move from areas of high pressure to low pressure that's how it works right so now it's gonna when these ventricles are squeezed because 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's going to move out. All right, so what I wanted to do real quick before I continue to keep talking is I know that sometimes whenever I was discussing a... atrial versus ventricular pressure over there 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 the phases. Alright so next thing so we left off here the ventricle are ejecting the 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 the aorta, right? So when that happens, it blasts open those semilunar valves. So these valves are going to open.
Right? And at the same time, we already know that if the blood is going up, the blood is still going to 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.
So those valves are still going to be closed. So we're going to snap those closed, right? They're still going to be closed. And we already said that the ventricular pressure in both the left and right ventricle are greater than the... arterial pressure within the pulmonary trunk and in 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 these valves, the blood is still being pushed by the ventricles and it's 120. In the atria, it's still almost about zero.
So here in these left atrium and the right atrium, it's still almost about zero millimeters of mercury. 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 these atria, right?
So again, the atrial pressure is... Less than the ventricular pressure. So this phase, if you can kind of tell, it's all about blood being ejected out of the ventricle. So this is actually, we can call it, there's two names for this phase.
It's called mid to late ventricular systole, right? But another way that they describe it is this is just the phase in which there is actually ventricular ejection. So it's the ventricular ejection phase also. So this is the ventricular ejection phase or is the mid to late ventricular systole phase, right?
And again, one 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're 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 that QRS wave, right? So same thing in this one.
Alright, now let's go into the last phase, right? So we're gonna come over here, this is gonna be our fourth phase, alright? Now same thing, we're gonna follow the same components here. We're gonna do the same thing so we're just going to keep it going here so what's happened here if we look at these these the actual heart again we know that the ventricles have ejected their blood 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 be the aortic semilunar valve right now if we look here we know the ventricles have ejected the blood out right there's 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 that's called the ESV and well again we'll talk 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 it's some of its actually getting distributed out here to the systemic circuits, some of it's getting distributed out to the pulmonary circuit, right?
Or even some of it's getting distributed out into the coronary circuit, which again, we talked about that in previous videos. So now if that blood's going out there, that pressure here in the aorta and that pressure here in the pulmonary trunk, it's going to rise because now it's going to accumulate all that blood and that 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.
Because 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 into it. So imagine something like this. Imagine the aorta here, if you were to imagine it, and blood's coming into the aorta. It's extremely, and so is the pulmonary trunk, it's extremely elastic, but this is a low pressure system.
This is a high pressure system. When it the blood moves in here it can it can actually expand a little bit it can actually accommodate or it's very compliant or distensible so it can actually take on that high pressure when it does that it can recoil the blood down right and so go all the systemic circuit or up to supply the actual the head and the actual upper limbs right 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.
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 called the dichroic notch when there's that brief rise in aortic pressure because it's Perhaps that semilunar valve is closed.
So again, what's 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 in this state, right? So if that's the case, and again, we already discussed it, there's going to be the brief rise in aortic pressure, dichroic notch, and it's going to cause that drop of the blood to come back down, 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, right? Then, if you think about it, what happens here is the ventricle pressure is still a little bit greater. It's going to be going down and down and down. But the atria is still going to be, at this point in time, it's still going to be zero.
So it's still going to be zero at this point in time. 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.
Still greater than the ventricular pressure. So if that's the case then, the ventricular pressure is still greater, well then because of 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, we're 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. If you noticed before, if we come over here real quick, if we notice... when the actual AV valves closed in the iso volumetric contraction phase it produced a lub sound.
Well now if we come back over here look what happened the semi lunar valve snap shut right and if the semi lunar valve snap shut and they're closing it's going to produce another sound that is going to be the second heart sound and that's 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 going to diastole, they're beginning to relax, the coronary arteries are getting filled, the blood's going to the muscle so they can get the oxygen it needs, right, in order to be able to produce ATP and undergo contraction. So for the next cycle.
So in this part, the ventricles are, what did I say, it's that the ventricles are relaxing. So in other words, the ventricles are repolarizing. If the ventricles are repolarizing, if we go back to remember what the EKG said, EKG for ventricular repolarization is the T wave. So again, this last part of the EKG here 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 the relaxation, the bloods 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 and the cycle continues again for another 0.8 seconds right so that right there describes the cardiac cycle in a nutshell