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Current time:0:00Total duration:13:38

Video transcript

I want to talk a little bit about the idea of pressure and volume and we're actually going to clear up some kind of misconceptions that I think I may have caused no apologize for them but I think this is a good chance for us to kind of reflect on the things that we've learned and also build up a couple of new ideas so let's draw a volume going that way and pressure going up and one of the things I wanted to start with was the end systolic pressure volume relationship you know we drew something like this we said this is the relationship at the end of systole between the two between pressure and volume and one thing that I wanted to kind of bring up immediately was the idea of increasing volume so as I go up this yellow line I'm increasing volume and sometimes the way I've drawn that actually maybe I'll make a little bit of space on this canvas sometimes the way I draw an increase in volume can be a little bit misleading so I've drawn for example in the past I've drawn a left ventricle like this and I said well as blood goes into my left ventricle it basically does this you have more and more blood kind of filling up the heart and I've drawn this sort of a picture and it really does tell you about a couple things it tells you that you have filling right you're filling happening and that part I'm ok with but the part that I'm not okay with is the idea that it basically seems like you have a fixed volume it looks like you a fixed volume on the heart or the left ventricle anyway and it almost makes it look like you're filling up a glass of water basically that's kind of what it looks like like a glass and really the the correct way to think about left ventricular filling is a little bit more like this you should be thinking of it more along the lines of a picture like this where you basically have a a smaller volume Devendra chol filling up with blood and over time it kind of fills in completely so it starts out like that and then you add more blood and it becomes like that and you finally fill it up like that so that would be the more accurate way of kind of showing what's going on in the left ventricle and of course all three of these then are the left ventricle at different points in time so this second picture also tells you about filling so you get the idea that blood is filling in the heart but it does a better job of showing you that the volume changes the volume of the left ventricle changes so it's not fixed and that's correct this is the better way of looking at it and kind of an analogy might be a balloon you might think of a balloon for this for this filling process so I want to be very clear that the left ventricle is not like a glass it's like a balloon and that it's not a fixed volume it actually changes so this is probably the the more accurate way of thinking about and and I apologize for for doing this sort of a drawing truthfully I didn't mean to confuse anyone but I just wanted to kind of demonstrate filling and I probably just did it in a quick and hurried way and so I want to clarify that point now so this is what it would look like and actually I could stick this a step further and say well what if I was to do this what if I was to take kind of a cross-section like that cut it with a blade you know at these three points in time wouldn't you agree that you would actually get a kind of an interesting cross-section view of it you know if I was to take it like that and I was to erase these top bits out and you were to now look down at the heart you'd basically see kind of an interesting view and I let you try to draw that view out for you and it's helpful actually to do it that way and I'll tell you why so this one would basically look like this and this one would look like this and this one might look much larger than the other two something like this and again this is just looking at a cross-section so it's nothing different at all just looking kind of at the cut surface of it and all three you would expect to be full right so this is how I'm going to use our diagram I'm actually going to use these kinds of images now to show what filling of the left ventricle looks like so we can actually get it get a real kind of sense for it and you'll see an interesting problem that comes up so let's let's do that let's draw a couple of circles here I'm going to draw let's say a big circle here where it's really large and then let's say the volume is a little smaller at this point so let's draw something like that and the volume is really really tiny let's draw something like that over here now if you have these three volumes you might say well okay you know you've colored them in well one thing you'd have to admit and you'd realize pretty soon is that at the the bottom of this curve you have a small volume but it does take a little bit of blood to fill that volume in so you when you have zero volume like right here there's zero blood in there it would be an empty left ventricle and then you'd actually add a little bit of blood to it let's say you you fill it up let's say halfway and now you've got a half full ventricle and then you keep doing it and you have a full ventricle so you basically are kind of going this way along the curve but until you have a full ventricle and this is the point until you have a full ventricle you actually don't have any increase in pressure so previously when I drew out the end systolic pressure volume relationship with that yellow line I drew it the way you see it now but now I'm telling you that the truth is that it actually looks a little bit different especially at the bottom end of this curve so I'm going to erase this and draw it in properly and this is the the more accurate way of drawing it and you basically have almost no or really no increase in pressure I shouldn't say almost no no increase in pressure and then once you get to a full ventricle now you start seeing an increase in pressure and really the way that an increase in pressure looks is that you have a larger volume and that's what you're starting to see right you start to see that larger volume so even a tiny bit of pressure is going to push out on the left ventricle you'd actually notice that because now it gets larger so the left and trabocchi doesn't change in size initially and finally when the pressure starts actually mounting up it starts changing in size so you can start appreciating why I am saying that this first yellow line is incorrect let me erase it completely so it doesn't distract you so I've drawn out the N systolic pressure volume relationship but what I want to do is now add to it our end diastolic pressure volume relationship and you know it goes something like that and let me just label it in yellow color just to be parallel so this is our end diastolic pressure volume relationship right now if I was to say well what would the cross-section look like now let's just kind of choose a couple points and say this is this point and this is this point like that and if I said what would the same volume look like on the other curve I would have to actually just kind of draw a line down and say okay this is this volume right here and this is this volume down here and along those points let me actually just mark it on my other curve those points would be right there and I actually could just similarly draw them out I could say well this is about that and then the other one looks maybe a little bit larger it would be something like that so these are my two curves right and I'm trying to make them look as similar as possible to the other ones and I'll fill them in so that's what the volumes would look like at these points so really when you look at that when you look at the the volumes they look about the same right they don't look any different at all and so you're left wondering well how in the world is it this is actually very very confusing to think about for folks how in the world is it that the pressure is so darn high on the end systolic curve whereas it's low on the end diastolic curve when they look the same they don't look any different and to figure this out I think one easy kind of trick I've been using is to just imagine what's happening at the muscle level so the muscle cells are kind of contracting and pulling in those z-discs right there at the end of systole you've got tons of contraction happening and it's happening here too it's an in fact it's happening at every part of this this curve and if I was to try to simplify this instead of drawing hundreds of arrows like this I could do this for every single point instead of drawing hundreds of arrows you could imagine that I can actually connect all these arrows like this and then I would have a similar effect if I just kind of drew it like this I could simply draw almost like rope or a band imagine a band or a rope that's pulling and tugging this way and this way if I was actually to draw a band like that you can imagine then it would be the same effect as the hundreds of little muscles that are contracting and to take it a step further you can actually even imagine people yanking on that band so this is how I kind of picture it just people yanking on that band these are like two little workers let's say yanking on the band and pulling it in opposite directions and if they were pulling in opposite directions you'd basically have what we think of as contraction you can have little workers that are basically yanking on all these things yanking away and by yanking away what you basically end up with is a force of contraction so this is basically how I imagine contraction having workers yanking in two different directions and if you had them going all around the heart in every direction you could possibly imagine that is what a contracted ventricle is like and because they're yanking so darn hard because they're pulling so hard on this thing you basically have a lot of increase in pressure building up on the inside of these ventricles and you really don't have that happening on the other side because on the end diastolic curve I guess the question is do we have any workers are they yanking and the answer is no you know the muscle cells are completely relaxed right they're relaxed they're just kind of hanging out and taking a nap you can imagine you know your workers are really not yanking at all and as a result you don't have any of that increase in pressure you have kind of just a very very low pressure and so that's the reason you can imagine there's a difference even though the volumes are the same that there's a difference in pressure so a final kind of question that plagues a lot of people and I'm actually going to make a little bit of space to answer it is so why is there blood in the ventricle at the end of systole I mean isn't that the point where all of the blood is exited the ventricle and gone into the aorta why is there any blood in there anyway shouldn't it be empty and to answer this to think about this we can actually draw a pressure volume loop I'm just going to draw it in purple just to create a little difference in color and let's say that I have contraction right here where I have a big purple dot that's where I begin my contraction so I'm going to draw going up from there like that and let's say now my ejection is happening and let's say just rise over my picture of the worker like that let me actually draw one final volume piece and that would be kind of what is the volume here because we know that the volume is not changing there it's constant volume and at this point you begin ejection so this is all ejection right I'm going to write ejection on the curvy part of the curve so this is ejection happening right here over the over the hump like that so ejection is happening between my two white lines and here in the vertical part I could draw a picture like this I could say well my heart will be really full so it'll look in fact let me make it bigger my heart is going to be really really full let me try to illustrate that nicely so I could have something like this I can have something like that and if my ventricle is that big if it's that large let me actually just color it in now then what's actually happening when I have ejection well I'm going to cut and paste this little guy and show you on the top what it would look like so let me just drag this little fella over here and now this if this is how I start out then when I inject blood you're basically going to have something like this you're going to have an amount that goes away and an amount that's left behind so the amount that's left behind is of course the amount that I showed you on the side and I'm cutting it out and this donut hole shape that's left this is actually our stroke volume this is our stroke volume so you actually do have a lot of blood that goes into the aorta of course you know that's important and you have a little chunk that's left so now you can see that you at the beginning of contraction you end up with having a lot of blood here this is where you start and then you lose a lot of blood this is our stroke volume that you lose and then you are left with a little bit of blood here and that's at the end of systole so at the end of systole do have some blood left but you don't have nearly as much as you had when you began systole