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So we've talked a lot about pressure and time, and how pressure actually changes in the left ventricle over time. And I'm going to take that one step further by describing to you what happens in the left atrium over one second. So it's still the same way we've been looking at it with pressure and time on the two axes. And of course, pressure goes up in that direction. Let's say this is about 100, and let's say zero's down there. And I'm going to measure it all in the same units, millimeters of mercury. So let's start with the left ventricle. And we know that the left ventricle pressure rises steadily. And then it contracts really hard. So it has this huge contraction, which increases the pressure dramatically. And then it bottoms out as the left ventricle relaxes. And then it slowly picks up again in pressure as more blood fills in. And of course, then you have your aortic pressure as well. I'll put the aortic pressure in yellow. And the aortic pressure is actually slowly drifting down. And then it picks up along with the left ventricle, goes for a nice ride, and then at some point, the aortic pressure actually exceeds, goes higher than the left ventricle pressure. And that's because of the compliance of the aortic walls. And you get this notch, what we call a dicrotic notch. Let me just make sure I end it in the right spot, something like that. So that's what the aorta's doing. And this is our aortic pressure curve. And this of course is our notch, and the name of our notch is dicrotic notch. So you'll see that word. Now, what's happening with the left atrium? Well, the left atrial pressure is basically the same as our left ventricular pressure for a little while. And I'm actually going to sketch it out on the side so you can get a visual idea of what's happening in the left ventricle and the left atrium during this time. So, you've got the aorta coming off, and let's draw our big ventricle down here. And our atrium, our left atrium, which is right here, is getting blood from two sources, the left and right pulmonary veins. So this is our left pulmonary vein, and of course, then there's the right pulmonary vein. And let me actually finish off the aorta first before I draw the pulmonary veins. So this is our pulmonary vein going from behind. And this is coming from the other lung. So this is our right pulmonary vein. So these are our two sources of blood. This is where the blood is really coming from, and the pulmonary veins have blood that's coming with a little bit of pressure. Little bit of pressure with these veins. And so, blood is coming in this way. And it's staying, at times, in that left atrium. And why is it staying there? Well, because there's a valve here. There's no valve that separates the left atrium from the pulmonary veins. There's no valve there, but there is a valve here. There's also a valve in here. So these valves, we know this one is called the mitral valve. And that separates it from the ventricle. And on the other side, you've got your aortic valve. And at the beginning of this process, what's going on? Well, you've got there's a free space here-- let me make it very clear there's a space here-- between the left atrium and left ventricle. The valves are not closed. And you're actually getting blood flow this way. So really, the way I want you think about it is the left atrium and left ventricle are really one giant chamber at this point. There is no difference in the two, and so of course, the pressure would be basically the same. And so I'm trying to draw it basically the same in those two spaces. So the pressure is basically the same. And maybe a slight increase in the left atrium side, and why would that be? Well, that's where the blood is initially coming in, so there's a high pressure in the pulmonary veins. And so maybe there's a slight increase in pressure in the left atrium over the left ventricle, but it's very similar. So this pressure tracing looks about the same. But then, there's a bump. There's a bump here. Why would there be a bump? Well, there is a process called atrial systole, meaning the atrium actually contracts. That's all it means. So atrial systole just means that just like the ventricle likes to contract, the atria can also contract. And I'm going to actually put a giant red square around this letter A, because we actually end up calling this the A wave. So, the pressure tracing for the ventricle then is not really accurate, it's not really straight, is it? It actually has a little bump as well. So, because the two chambers are continuous, whatever's happening in one is reflected in the other. And you get this little bump there. So then the contraction stops. So atrial systole's over with, contraction stops, and that's why the bump comes back down. So that's what explains why it comes down this way. Now, the ventricle contracts. And that's the part we definitely feel very comfortable with. The ventricle has a huge contraction, we know this, and so of course you're going to get all this pressure building up. And this causes, and this is actually going to get interesting here, these valves, this mitral valve, to all the sudden close up. Now remember, these atrioventricular valves, meaning the mitral valve and the tricuspid valve-- those are the two atrioventricular valves-- they have these interesting chordae tendineae. They have these things called chordae tendineae, basically little tendons or chords, I guess you could use either word. Chordae tendineae. Of course, the mitral valve's going to want to close. There's no doubt about it, because the pressure in here is so darn high in the ventricle. So of course the mitral valve's going to want to close. So that means that this arrow is gone, right? But these chords prevent the valve from flipping backwards. They don't let the valves go into the left atrium and flap in the wind. They really keep the valves tight. Now remember, there's so much pressure in the left ventricle, the valves are going to want to buckle in, but the chordae tendieae keep them firm. But what really happens, interestingly, is that because the contraction happens so darn quick, there's blood that's actually snapping back, just like in the dicrotic notch. Remember we talked about snap back of blood? You could think of it as crashing off of that valve. So now there's actually blood crashing-- I'm using that word specifically-- off the mitral valve. So what would that do? Well, if blood is crashing off the mitral valve, then it's going to increase pressure in the left atrium. You're going to get a little bit of increased pressure in the left atrium. So for a short time, right when the mitral valve is closing, you're going to get a little bump in pressure. So that explains what's happening here. You get this tiny little bump in pressure. And we call that the C wave. So there's this C wave that happens. So it's interesting. You have this A wave and this C wave. Actually, maybe I drew this a little bit too low. And then you have, over time, now that the mitral valve is closed, more blood coming in. So you've got more blood coming in from the two sides, from the left and right pulmonary vein. Of course, that's continuously happening. Except now, there's no continuous space with the left ventricle. So all that blood has only one chamber to sit in, right? Not two, like before. Only has one chamber, which is the left atrium. So the pressure starts rising, rising, rising, rising, rising, rising in the left atrium, because of course, it's taking in a lot of blood. And it really can't release it, because that mitral valve is still shut. Now at some point, that left atrium is going to wait. It's going to wait for the left ventricle to finally have less pressure. It's waiting for the pressure on the left ventricle to decrease. And what is that decrease? Well, this contraction finally relaxes. So all these arrows go away slowly over time. And you have a little bit of contraction left, let's say. But really, it's almost just a little bit compared to how much there was, so the pressure is almost back down. And so the left atrium's filling up with blood. Let me show that filling up with blood. So that's why the pressure is rising here. And simultaneously, the pressure is falling in the left ventricle. So there you have an interesting difference, because on one side, pressure's rising, and the other, pressure's falling. And you are going to actually get a cross here, which means that the pressure in the left atrium is actually going to be higher than the left ventricle pressure. So at that point, of course, if the pressure on one side is higher now than the other, this mitral valve is going to open again. So finally you're going to get opening of the mitral valve again. And blood can now dump in. So at this point, blood dumps in. The left atrium is just so relieved. It's like imagine you have to go to the bathroom, and you finally urinate. Your bladder is so relieved, because all that fullness is gone. The same thing, the left atrium's fullness is gone, and so it basically empties out very quickly and the pressure falls. So you might be thinking well, wait a second. If pressure is falling in the left atrium, that makes sense. But why is pressure falling in the left ventricle? I mean, isn't it filling up with blood? And if it's filling up of blood, doesn't that mean that pressure is rising? That's sort of true. But remember, there's also another process going on. So on the one hand, you have more blood. And you're right, that would definitely cause more pressure. But on the other hand, remember you still have relaxation. You have another process going on. The left ventricle is still relaxing. All those muscles are re-polarizing. So that's going to cause a decrease in pressure. So you have these two counteracting forces. And for a very short time, just for let's say, from here to here, this short period of time, the pressure is actually falling, because the relaxing wins out. That's actually the bigger deal. And so you have a decrease in the left ventricular pressure, because that last little bit of pressure that it was exerting goes away. And finally when that goes away, now you have nothing but filling. So now, you slowly will fill it up more, let's say to that level. And then you'll fill it up more, let's say to this level. And you're just keep filling all the way back up. So you'll slowly fill back up, and the pressure will just keep rising in the left ventricle. And similarly, the left atrium, because of course, there's a continuous space, like we talked about between the two ventricles. So now, you can see that there's actually you could call it a wave, but really it's just a spike in a way, right here, which we call V. And you can remember that by saying well, that's when the left atrium is very full. So very full left atrium is what causes that last little spike. And the fact that there's a quick drop off is because that's when the mitral valve opens. So let me actually box that. So you can remember the three interesting parts of the left atrium pressure wave. The A is when there's atrial systole, so you have a little bump in pressure from the contraction of the muscle. C is from blood crashing off of that mitral valve. So that's really because of the left ventricle squeezing and, of course, the mitral valve snapping shut suddenly. And then V is the very full left atrium. And one final thing I want to point out is that the middle one, this C, this is actually really quite similar to what we described happening with the dicrotic notch. Similar idea, because there, we also had blood crashing off. You had blood crashing off the aortic valve. And there, we talked about the compliance of the aorta. So there's actually some similarities between the two. But this is the left atrium pressure wave now.