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By this point, you may be getting kind of sick of these pressure volume loops. But I assure you they're very, very worthwhile getting to know. In fact, let's learn one more thing out of these pressure volume loops. Let's try to squeeze all the knowledge we can out of these things. So I'm going to draw one for you here very quickly. And on this side, we've got pressure in millimeters of mercury. And on this side we've got milliliters, right? We're just going to use the same units we've kind of gotten very familiar with. And we're going to do 50, and 100, and 125 or so on this axis. That's a pretty normal set of terms or numbers. And then on this side, we've got 0, 50, and I'm going to go up to 120 or so up here. So this would be a pressure volume loop. And I like to kind of start out where the pressures are low and the volumes are really, really high, so somewhere around here. I'm going to imagine that this is kind of the end of diastole, where my left ventricle is full of blood, right? It's going to start rising in pressure slowly. And as it rises, it's going to get to let's say about this point. And then the blood is going to start entering the aorta. And as it enters the aorta, I've got to get to a nice high pressure. I know that that's the target or thereabouts. And so the pressure kind of rises even higher. And then the volume starts to fall as volume enters the aorta, and so it leaves the left ventricle. And then the left ventricle continues to lose its blood to the aorta. And the blood in the left ventricle that's left is around 50. And then you start having relaxation. So you relax all the way down and the pressure falls to about that point. And then it continues falling, but now there's a little bit of blood kind of entering into the left ventricle. So it's starting to fall in pressure but continues to now start rising in volume. And it continues to rise until it's ready to do the whole thing all over again, right? And now if this is kind of our overall pressure volume loop, what I want to do is kind of focus in on one particular point. Let's focus in on this point right here. And this is the end of systole, right? We talked about the end of systole being right here. This is where it begins to start relaxing. And when I say "it," I mean the muscle cells. So you've got a muscle cell over here. And I like to draw them kind of branched just to remind you that they're muscle cells. And you've got now this cell completely contracted down, right? So it looks like this with the actin and myosin completely overlapping, right? Because that's what you expect to happen at the end of systole. And, of course, I'm drawing it this way really just to remind you what's going on inside of the cell, although you know that, of course, a cell has many, many, many sarcomeres, not just one. And of course, this one I've drawn just having one. But you get the idea that there's a lot of overlap between the actin and the myosin. In fact, that's what these little red lines represent, just the major proteins instead of heart cells. So these are my heart cells full of protein, and they're completely contracted, right? So sometimes they like to relax, and sometimes they like to contract. And at the end of systole, where I've drawn that orange arrow, these muscle cell are completely contracted down. I'm going to write "contracted" just to kind of remind you that that's what's going on. And what are they waiting for? I mean, what's next? What are they hoping will happen next? Well, they are hoping that they can now get rid of all that calcium, and so they can relax. So you've got a lot of calcium in this space. And they're kind of hoping that the calcium will go away and they can relax. And so imagine now that the calcium-- I'm going to draw calcium as a white circle, right? And I'm going to fill it in. This is my calcium, right? And imagine that I've got calcium in here. I'm just going to draw little white circles in here. And instead of allowing my heart cell to relax, I'm going to kind of do something interesting. And I remember calling it a trick last time. I guess I can call this a trick. And the trick is I'm going to trick my heart cell into not relaxing. I will basically not allow it to relax, because I load up this cell-- imagine I can somehow do this-- with lots and lots of calcium. And so I just fill it with calcium. It's chock full of calcium, and it has no way to really get rid of it. So it is going to continue to be contracted, right? It's just going to continue to be contracted if I can somehow fill this with calcium and not allow it to go into relaxation. So my heart cell's completely contracted. That's the key, right? I have done this. And so as a result of doing this now, of course, this won't happen, this relaxation bit. This won't happen and neither will the next bit. So basically I'm kind of forcing myself to continue to remain contracted, and all this kind of disappears. And so what happens is that now my heart is basically full of blood, right? I've got a heart full of blood. I'm going to draw it over here. I'm going to just kind of ignore for the time being the left atrium and the aorta. But this is my left ventricle full of blood. So this is chock full of blood. And what I'm going to do is I'm going to take a little needle-- watch this. I'm going to take a little needle, and I'm going to try to add a little bit of blood or take a little bit of blood away. And I guess I'm going to start by taking a little bit of blood away just to see what will happen, right? So I take a little bit of blood away just to see what will happen. And let me choose a different color. Let's choose a green color. And at this point now, my heart is-- again, it's full of blood, right? And I'm going to now take a needle, and I'm going to take some blood off of the heart. I'm going to actually just pull it off like that. So now I've got blood in my syringe. And as a result, what have I really done? Well, I've lowered the volume, right? I've lowered the volume. Actually, let me switch to a green color and show you that I've lowered the volume. And if I lower the volume, basically, what will happen? Well, if I lower the volume, the pressure will start to fall, right? The pressure goes down. So it goes something like that. And I can do this again, and I could see if the pressure falls. And, oh, it does. And I could do this again. In fact, I could take all the blood out of the left ventricle, and I could see that basically my pressure will go down to zero. So I basically have now a few dots. I can connect my dots. And I can see that I create this basically kind of a line, right? And so this line is what you would get if you just keep reducing, and reducing, and reducing the volume in the heart. Now, what if I did the opposite? What if I-- instead of reducing the blood, what if I actually added blood? And of course, it might be kind of tricky to think about adding blood. But just remember you can always add air to a balloon if you try hard enough. And similarly, you could actually push blood into a full left ventricle if you have enough pressure pushing down. So I could actually do this. I could actually try to do this. In fact, I'm going to add, let's say, a little bit of volume here, right? And I'm going to notice that the pressure goes up. In fact, it goes up even more than it ever did before, right? It actually rises above the line that I had drawn in blue. And in fact, I might even do it again. I could say, well, let's just add some more volume. Let's just see what happens. And the pressure goes up even higher. So I could connect these lines. I could say, OK, well, let's see what these lines look like. And basically, it's forming a nice straight line. In fact, to see it a little bit easier, let me just get rid of this blue stuff. Let's get ride of all this stuff. And you can see that you get this nice straight line that relates volume to pressure. And so this relationship between volume and pressure or pressure and volume is happening with the muscle cells contracted. Remember, all this time my muscle cells are bathing in calcium. They're completely contracted down. They have not been able to relax. They're contracted. And so you could even say, well, this is basically the relationship between pressure and volume at the end of systole. So you could say this is the end systole pressure volume relationship. And this is kind of the long way of saying it. People actually shorten all of this down. They don't say all these words. They usually say ESPVR-- End Systolic Pressure Volume Relationship. And all it means is that if you could get a situation where your heart cells are completely contracted, completely contracted down-- and that's why I kind of made up the bit about filling it up with calcium, because I guess that's one way to do it, but completely contracted, and you just kind of added volume or took it away, what would the pressure do? How would it change? And this line tells you that. In fact, one final point I want to make about this is if you remember, there's a relationship now between elastance-- remember the term elastance-- and pressure and volume. In fact, if you take pressure divided by volume, that gives you elastance. So with this line that we've drawn, you can actually say, well, there's a slope to this line, right? I tried to draw a straight line so imagine there's a straight line between all these points, right? Well, the slope of that line-- slope is elastance, right? That is the elastance. In fact, you might even see the term-- wherever the pressure volume loop falls on this ESPVR, sometimes you even see the line actually labeled, and you see it called E0. In fact, you'll often see that. So E0 refers to the slope of the line that is formed at the end of systole.