Cardiovascular | Cardiac Cycle

0:07Alright, what we're going to do in this video is talk about the cardiac cycle

0:10Now what is the cardiac cycle?

0:12The cardiac cycle is all the mechanical events

0:15where the blood is flowing through the different chambers of the heart

0:18and on average, it takes about 0.8 seconds.

0:21So what we're going to do is, is we're going to go ahead and go through each one of these

0:24hearts. Alright and we're going to discuss

0:27the differences in the atrial

0:29versus the ventricle pressure.

0:31We're going to discuss the differences between the

0:33arterial versus the ventricle pressure.

0:36We're going to talk about what's happening with the AV valves

0:39or the atrioventricular valves

0:41and then we're going to be talking about what's happening

0:44to the semilunar valves... SLV is for semilunar valves.

0:47So the pulmonary and the aortic. And then we're going to say at what part

0:50in each one of these stages, what will it appear

0:52like on the EKG. Which one of the

0:55components on the EKG would it be like?

0:57Alright, so, I'm going to draw here in green

0:59I got to draw these guys.. this my pulmonary semilunar valves right here

1:02and then this is my aortic semilunar valve, alright?

1:05Alright, so in the first event,

1:08the first event is defined as

1:11mid

1:12to late

1:15ventricular

1:18diastole

1:20Now what is meant by diastole?

1:24Diastole is defined as relaxation. So in other words, we could say that

1:27this mid to late

1:29ventricular relaxation. We're in the

1:31last parts of ventricular relaxation.

1:34So what's happening here? Well, what's happening here is

1:37blood is actually going to be returning to the heart. So, some of the blood

1:39is actually going to be coming from the inferior vena cava

1:42some blood is going to be coming from the superior vena cava.

1:45If you remember, there's the coronary sinus. Some blood is going to be

1:48coming from there right? Where else?

1:49Well here's our pulmonary veins, right. I just drew one here.

1:51...pulmonary veins.

1:54Bloods going to be coming in through these guys and dumping

1:56into the left atrium

1:57So, we've got blood

1:59There's blood coming back to the heart.

2:03And what happens is these valves

2:04the AV valves

2:07Whenever this blood starts accumulating

2:10in the atria from, actually, from all these peripheral veins

2:13like the pulmonary veins and the systemic veins

2:15It actually starts opening up the AV valves

2:19because the atrial pressure is a little bit greater than the ventricular pressure.

2:23So if that happens, these valves open.

2:26So, what valves open? The AV valves

2:29open.

2:31So this would be...in this case, this would be your tricuspid valve.

2:33between the right atrium and the right ventricle.

2:36And this would be the bicuspid valve

2:38or mitral valve which is in between the left atrium and the left ventricle.

2:41So those valves would open. And what would happen is

2:44passively, without contraction,

2:47a good 70 to 80% of the blood

2:51that's coming into the heart

2:53is going to passively flow down by gravity

2:57into the ventricles. So, again, let's go over that really quickly. Again, blood coming from the systemic veins,

3:03the coronary veins, and the pulmonary veins are coming to the atria.

3:06The blood is accumulating

3:07in the atria. The pressure in the atria

3:09is going to greater than the ventricular pressure

3:12so it opens up those AV valves

3:14and about 70-80% of the blood

3:17flows down passively, without contraction

3:20into the ventricles. So that's what happens there.

3:23So, again,

3:24We kind of already defined what is happening here so far.

3:26So what's atrial pressure? Is it greater than ventricular pressure? Yes, it is.

3:30So the atrial pressure in this event

3:33is greater. I'm just going to put P for pressure, right.

3:36Here, I'll actually put

3:37I'll put, put pressure. I'll just write it.

3:40Is greater than the ventricular pressure.

3:44Right, and because of that

3:47the AV valves are going to open.

3:51Now, the next thing.

3:54The ventricular pressure is still not...so,

3:56the blood is accumulating in the ventricles, but the

3:59ventricles are not contracting. They are just taking that blood in.

4:02What's going to happen is the ventricles are accumulating that blood

4:05but the pressure in the actually pulmonary artery

4:08or the pulmonary trunk, pulmonary arteries, and the aorta

4:11is still greater than the pressure in the ventricles.

4:14so these AV valves, they're not going to open.

4:16Remember, valves are one-way. They're not going to allow blood to go out

4:18They are going to prevent blood from coming back in.

4:21So it wants to go out this way. So, again

4:24this pressure in the aorta and the pressure in the pulmonary trunk

4:27is going to be greater than the pressure in the ventricles.

4:29So if that's the case, these valves are going to stay shut.

4:32So, again, what's happening here with the arterial pressure or the aortic and pulmonary pressure

4:37the arterial pressure

4:41is greater than the

4:44ventricular pressure.

4:46And what would that mean, then?

4:48Again, it would mean that the semilunar valves would stay

4:51closed. Because the pressure has to be greater here

4:53to push them open. Cause remember the valves are going to open up like this.

4:57Like this, right?

4:58You have to be able to push the blood like this

5:02up against these valves so they can open up

5:05So the blood can pop through, right. So, we want

5:07to be able open up those valves

5:10But in this case, the pressure isn't great enough.

5:13So the semilunar valves, like the pulmonary

5:15semilunar valve and the aortic semilunar valves

5:18are closed.

5:21Now,

5:22on the EKG

5:23it is extremely interesting. Now, if you remember, I told you one of the first events, when the blood is coming into the heart

5:2970-80% of it is passively flowing down.

5:32But then what happens is toward the last...the end

5:35...the late end of that ventricular diastole

5:39the actual SA node starts firing.

5:42So, if your SA node fires.

5:44it will actually produce

5:46this depolarization of the atria and whenever the atria depolarizes

5:49it contracts. Towards the late end.

5:52And that will push the remaining 20% of the blood

5:55down into the ventricles.

5:57So if atria depolarizes, we know that

6:00from the EKG to show up as a P wave.

6:02So, it's going to show up on the EKG as a P wave

6:05Right, so that's the first thing we know.

6:08And this is basically...

6:09We can also define not only mid to late ventricular diastole

6:12but this is the period of ventricular filling.

6:14Let's actually write that down. So this is the period

6:19right over here. This is the period

6:28of ventricular filling.

6:30Okay, so that's that first event. So, this is

6:33the first part of the the heart phase, right. So, it's mid to late ventricular diastole.

6:36The second part of this...Let's go...What's happening now?

6:39Well, we know that the blood is already accumulated

6:42Let's fix the inferior vena cava.

6:45We know that the blood is sitting here. Right.

6:48The ventricles have already accumulated some blood.

6:51This is actually defined as the EDV - end diastolic volume.

6:54Which we'll talk about when we get to cardiac output.

6:56So, the blood is accumulating here now. The atria have

6:59already supplied and opened

7:01up their actual valves to push the blood down. Now what's

7:04going to happen is that the ventricles are going to start slowly

7:07depolarizing and beginning to squeeze and contract

7:11If you remember the muscular layer, that cardiac muscle,

7:13the myocardium is going to start

7:16squeezing the actual chambers of those ventricles and start trying

7:19to push the blood upwards slowly. But what happens is...

7:22we're going to

7:23name this phase. It's a very, very mportant phase

7:26This phase is called iso-

7:29volumetric

7:32contraction.

7:34And you can say "systole" if you want.

7:36Isovolumetric systole or isovolumetric contraction.

7:39So what's happening? Let's actually denote this that

7:42these little arrows pushing in is the myocardium

7:45beginning to slowly depolarize and contract.

7:49Well, you know, according to, you know, a law,

7:52that as you start squeezing this, right,

7:55as you start squeezing these ventricles,

7:57the blood is going to start to try to move its way up and up

8:00and up. So, what going to happen here? As these ventricles start

8:03squeezing, the blood is going to start rising.

8:06and getting reading to move up

8:09towards the

8:11pulmonary trunk and its arota. Right, and again here is green

8:14is my pulmonary semilunar valve and here is this green

8:17is my aortic semilunar valve. They're closed right now.

8:20But the ventricles are really starting to squeeze. But

8:23here's where it is extremely important.

8:25The pressure in the aorta

8:27is naturally about 80mmHg.

8:30Right.

8:31Pressure here in the pulmonary trunk

8:34is usually about 7-10.

8:36So we're going to just go with an easy number - 10.

8:39Right now the ventricles

8:42these two ventricles, their pressure in this

8:44actual...these chambers is going to be less

8:47than the aortic pressure and the pulmonary pressure. Let's say

8:50it's on average is about 60

8:53Right, 60mmHg. And let's say on

8:55average this one is about 7

8:58mmHg, right.

9:00This one, again, this is

9:0260

9:03mmHg. We know that

9:05this pressure is not greater than this pressure. And you know

9:07this pressure is not greater than this pressure so it can't

9:10open up my semilunar valves. So, those are closed.

9:13So, again, what is happening to the semilunar valves in this point right here?

9:16They are closed. Let's actually draw, like,

9:18a division line down here so we can separate these.

9:22So, at this point in time the semilunar valves are still

9:25closed.

9:28But,

9:29Look what's happening as we kinda try to imagine this diagram.

9:32The blood is moving up and up and up and up.

9:35And it's naturally pushing these

9:38valves. If you imagine like this. Imagine these two valves like this.

9:41they are open and as the blood starts accumulating it

9:45starts pushing these valves

9:47up and back together.

9:49As it starts doing that, what going to happen then?

9:52This ventricular pressure is rising and it's greater

9:55than the atrial pressure. The atrial pressure is going to drop. It's going to drop down to

9:58about zero or 10. So we'll say, let's just say

10:00zero for the sake of it. Zero and zero here.

10:04Then than means that the ventricular pressure is

10:07greater than atrial pressure. And if that's the case then,

10:10those valves will be snapping shut. So the AV valve

10:13or the atrioventricular valves are actually going to be

10:16closing shut now.

10:18So, they're going to close.

10:21Okay.

10:23If they close, then now we define the pressures

10:25because the pressure are what determine the valves closing.

10:28So we already know then that the atrial pressure is not

10:31greater than the ventricular pressure because we already said that the ventricles

10:34are higher than the atrial, so they close the AV valves.

10:37So that means, in this case, that the atrial pressure

10:42is less than the ventricular pressure

10:47Alright.

10:50And then we also know.

10:54that if the aterial pressure - 80, 60 -

10:57The arterial pressure is still greater than the ventricular pressure.

11:00And then 10, 7, look this is still going to be greater

11:03than the actual ventricular pressure, so if that's the case,

11:05that's why the semilunar valves

11:07are still going to be closed.

11:09So this is the same thing here.

11:10The arterial pressure is still at this point in time

11:14greater than the ventricular

11:17pressure.

11:20Alright.

11:21Next thing.

11:23And I want to say thing before I go on to the EKG. When

11:25you're doing auscultation, when you're listening to the different heart sounds,

11:29When you go to this point of the

11:32phase. This phase. This isovolumetric contraction

11:36When that brief moment...no blood is leaving the ventricles,

11:38No blood is leaving the ventricles during this phase.

11:41Alrght, because the pressure isn't greater than the arterial pressure - ventricle pressure isn't greater

11:44than arterial pressure

11:45So because of that you're still going

11:47to hear a sound. Those AV valves are going to snap shut. When the

11:50AV valves snap shut

11:52It actually going to produce a sound which is your first heart sound

11:56And this first heart sound is called "Lub".

11:59So they call this S1.

12:02Alright. Which is your first heart sound. And that going to produce the sound

12:04"Lub." What we remember from this phase

12:07is that it was starting to contract. Well guess what?

12:09It gets to the point, when again, let's say this one is about 10

12:12mmHg, right. Ten

12:15mmHg. And this one is about 80

12:19mmHg, right.

12:22The pressure within this left

12:24ventricle, it starts rising.

12:26and it get up to the point to where it reaches about

12:29one hundred

12:30and twenty mmHg. A hundred

12:33and twenty

12:35mmHg. Now, again, at this point in them the AV valves

12:38are closed.

12:39Here

12:42in the right ventricle, it's not a high pressure system, right.

12:46It's usually a low pressure system in an average, you know,

12:48healthy adults, right.

12:51And even adolescents. So, usually it's not a high pressure system

12:54But the pressure in here is only going to go to about 24-27.

12:57So let's go, let's say 27

12:5925 is an easier number to remember. It's about 25...

13:0224-27. So, we'll say 25.

13:05mmHg, right.

13:07So if you look now, what's the difference?

13:09The pressure in the ventricle is greater than the pressure

13:11within the artieries, right?

13:1425 and 10, 120 and 80.

13:17So the ventricular pressure is greater than the arterial pressure.

13:20What's going to happen, then?

13:21Things like to move from areas of high pressure to areas of low pressure. That's how it works,

13:24right. So now what's gonna...when these ventricles are squeezed

13:27cause what's happening still, they're still contracting.

13:32They're still contracting.

13:35And the pressure rises. Well guess what? These

13:37valves are going to open and blood is going to move

13:41out. Alright, so what I'm going to do real quick before I continue

13:44to keep talking, is that I know that sometimes

13:46whenever I was discussing about

13:47atrial versus ventricular pressure over in the first phase

13:50I know you guys might have been like, "Well where is he going over

13:53here"? So, I'm just trying to make sure that we keep

13:56this same flow visible throughout

13:58all of the phases. Alright, so the next thing. We left off here.

14:01The ventricles are ejecting blood out,

14:03right. Because the ventricular pressure here in both the

14:06right ventricle and left ventricle is greater than the

14:09arterial pressure for the pulmonary trunk and aorta.

14:13So, when that happens it blasts open those

14:15semilunar valves. So, these valves are going to open.

14:20Right, at the same time we already know

14:24that the blood is going up.

14:26The blood is going to still keep these valves closed. Right, again, what are these valves.

14:29These are the AV valves

14:30or the atrioventricular valves: tricuspid/bicuspid.

14:33Or mitral valve, right, for the bicuspid.

14:36Now those valves are still going to be closed. So, we're going to snap those closed.

14:39Right.

14:41Still going to closed.

14:44And we already said

14:46that the ventricular pressure in both the left and

14:48right ventricle are greater than the arterial pressure in the

14:51pulmonary trunk and the aorta.

14:53That's why the semilunar valves open. So in this one we can

14:56say arterial pressure

15:00arterial

15:03pressure

15:05is less than the

15:07ventricular pressure, right.

15:10And then we know that the valves...the blood is still

15:13being pushed by the ventricles and its

15:15a hundred and twenty. In the atria, it's still almost 0.

15:19So here in the left atrium and right atrium

15:21It's still almost about 0 mmHg. So, these guys

15:24are still greater than these pressures here in the atria.

15:27So, again that's why the AV valves

15:29are still closed because the pressure is greater in here

15:32than it is up here in the atria. Right, so again..

15:36The atrial pressure is

15:40less than

15:42the ventricular pressure.

15:45So, this phase, if you can kind of tell is

15:48all about blood being ejected out of the ventricles. So this actually...we can call it...

15:52there's two names for this phase. It's called mid

15:56to late

15:59ventricular

16:02systole.

16:05Alright.

16:07but

16:08another way they describe this is just the phase in which

16:11there is actually ventricular

16:16ejection.

16:18So it's the ventricular ejection.

16:24So, this is the ventricular ejection phase or is the

16:27mid to late ventricular systole phase.

16:30And again, on more thing.

16:31If you think about it, the ventricles are still doing what they were doing over

16:33here in the second phase. They're still depolarizing.

16:36And they are still contracting. So it's

16:38going to be the same wave on the EKG here as it would be

16:42in this phase. So, it's still going to be the

16:45QRS complex, or QRS wave, right.

16:50So the same thing in this one.

16:53Alright. Now let's go into the last phase.

16:56Right, so we're going come over here. So this is going to be our fourth phase.

17:00Now same thing. We're going to follow the same

17:03components here.

17:05We're going to do the same thing. So, we're just going to keep it going.

17:07So, what's happening here? If we look at these...

17:09the actually heart again? We know

17:12that the ventricles have ejected their load.

17:14Again, let's draw these valves. What's right here? This is going to be the

17:18pulmonary semilunar valve. And then right here is going to be the

17:21aortic semilunar valve.

17:24Now, if we look here

17:26we know the ventricles have ejected the blood out,

17:28right. There is still going to be a little bit of blood left. That

17:31blood is actually called the end systolic volume. The blood that's remaining,

17:35you know, after the ventricles have contracted. So,

17:38that's called the ESV. We'll, again, we'll tak about that when we talk about cardiac output.

17:41But a majority of that blood is out here.

17:44Right, so it's out here. It's getting distributed now.

17:47So, 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.

17:53Or even some of it is getting distributed out to the coronary circuit. Which again

17:56we talked that in previous videos.

17:58So now, if that blood is going out there

18:01that pressure here

18:03in the aorta and that pressure here in the

18:06pulmonary trunk, it's going to rise. Cause now it's going to

18:08accumulate all that blood and the pressure is going to rise.

18:11So, that pressure here in

18:13the aorta is going to rise very, very, like

18:16drastically. Right. So, it's going to rise out here.

18:19and it's still going to be...

18:20this pressure is going to be greater, the aortic pressure and the pulmonary

18:23trunk pressure are going to be greater than the ventricular pressure.

18:27Another thing that happens is

18:29these arteries are very elastic so they can stretch.

18:32So when they stretch, imagine it being able to stretch

18:35when the blood is coming in to it. So imagine something like this:

18:38Imagine the aorta here. If you can imagine it.

18:41And blood is coming into the aorta. It's extremely...

18:44and so is the pulmonary trunk. It's extremely elastic.

18:46This is a low pressure system. This is a high pressure system.

18:49When the blood moves in here, it can..

18:52it can actually expand a little bit. It can actually accommodate

18:55or is very compliant or extensible, so it can actually

18:58take on the high pressure. When it

19:00does that it can recoil the blood down,

19:03right and so it can go to all the systemic circuit, or up

19:06to supply the actual, the head

19:08and the actual upper limbs. But, what happens

19:11is some of the blood

19:13can actually go down.

19:16Some of the blood can try to go back. And when it goes back,

19:19it snaps this valve closed. The

19:21aortic semilunar valve. Same thing. This guy is going to stretch

19:25And if this guy stretches a little bit, again it's not a very

19:28high pressure system, but imagine it stretching.

19:30it's going to recoil the blood out this way. Recoil

19:33it out this way. But a little bit is going to come back,

19:36right.

19:37So the pressure in the aorta and the pressure

19:40within the pulmonary trunk are going to rise

19:43become greater than the pressure within the ventricles.

19:46and snap these semilunar valves shut.

19:49When you look at a graph here. And I'll show the graph after. There's going to be a brief rise in

19:54aortic pressure. They actually call it the dicrotic notch.

19:57When there's that brief rise in aortic pressure because it snaps

20:00that semilunar valve closed.

20:02So, again, what happening here.

20:04We already discussed it.

20:06We know that the arterial pressure...

20:09if we come over here and we try to follow it. We know that the arterial pressure is

20:13going to be greater than

20:15the ventricular pressure. So, arterial pressure

20:21is greater than the ventricular pressure.

20:24Right.

20:25So, if that's the case, then again we already discussed

20:27that there's going to be a brief rise in aortic pressure.

20:29Dicrotic notch. And it's going to cause that drop

20:32of the blood to come back down and snap those valves closed.

20:36And so, that means the semilunar valves are actually going to

20:39snap shut. So, then the semilunar valves

20:41are actually going to be closed.

20:45Alright.

20:47Then,

20:49if you think about it, what happens here

20:51is that the ventricle pressure is still a little bit greater. It's going be

20:54going down and down and down. But the atria

20:56still going to be at this point in time, still going to be zero.

20:59So, it's still going to be a zero at this point. Very, very low pressure.

21:03But the ventricular pressure is still going to be great enough

21:05to be able to be greater than the atrial pressure.

21:08So, if that's the case. We know that the

21:11ventricular pressure is still greater than the

21:14atrial pressure.

21:15Or we can rewrite it this way. The atrial pressure is

21:20still less than the

21:21ventricular pressure.

21:25Alright, still greater than the ventricular pressure

21:28So, if that's the case then, the ventricular pressure is still

21:31greater, well then..the..because the pressure

21:34differences and pressure gradients these

21:35valves, the AV valves are still going to be closed.

21:39So, the AV valves here are going to be

21:41closed.

21:44Now.

21:46If you noticed, coming back to what we hit in the second

21:48phase, both the valves are closed. And it's only a brief

21:51moment in which

21:53all four valves are closed again. And if you

21:56noticed before, if you come over here really quick, if we

21:59noticed,

22:00when the actual AV valves closed in the isovolumetric contraction phase, it produced the lub sound.

22:07Well, now, if we come back over here,

22:10look what happened. The semilunar valve snapped

22:12shut, right. And if the semilunar valve snap shut and

22:15they're closing, they're going to produce another sound.

22:18That is going to be the second heart sound.

22:20And that is going to be referred to as dub.

22:23Right. And that is S2.

22:27Now this phase is going to be just like

22:31the second phase. We actually define this phase

22:34as the iso-

22:36volumetric

22:39relaxation phase.

22:42Because the ventricles are beginning to go into

22:44diastole.

22:45They are beginning to relax. The coronary arteries are getting filled.

22:48The blood is going to the muscles, so they can get the oxygen they need.

22:51Right, in order to be able to produce ATP

22:54and undergo contraction.

22:56So, for the next cycle. So, in this part

22:59the venticles are...what did I say...it's that the ventricles

23:02are relaxing. So in other words, the ventricles are repolarizing.

23:05And if the ventricles are repolarizing, if we go back to remember what the

23:08EKG said. The EKG for

23:11ventricular repolarization is the T wave.

23:14So again, this last part of the EKG is actually going to show up

23:17on the T wave.

23:20So, that describes basically what's happening here with

23:24all of these, right. So, if you think about it right

23:26after this cycle, what's going to happen? Right, when this ends.

23:29the ventricles are going to their relaxation.

23:31The blood is getting distributed to the pulmonary

23:33systemic and the coronary circuit. Once it gets

23:36distributed, guess what's going to happen? Let's come back here.

23:38It's going to go all the way back over here

23:41to where the ventricles are getting to the mid, to their

23:44late part of their relaxation phase. And they

23:47start filling again. The cycle continues again

23:49for another 0.8 seconds. So,

23:52that right there describes the cardiac cycle in a nutshell.