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# Drawing a pressure-volume loop

## Video transcript

so what do you see here you see two figures right on the top figure you've got pressure in time and this is actually the pressure the left ventricle over some period of time in this case I guess that's about one point two seconds and on the bottom you've got volume over time and again I've kind of drawn it also over one point two seconds so you can kind of see exactly what's happening the left ventricle as time passes so let's actually do a step by step we'll kind of go through this very very carefully and figure out for any given point like this point right here what is the pressure and what is the volume so here the pressure looks like it's about I would say I don't know around ten millimeters of mercury I'm just gonna write a circled ten and at that same point in time if I was to kind of just go down that same point in time I would find that the volume is about a hundred and twenty-five millimeters milliliters so that's the volume and that's the pressure at that particular time point right now let's do another one let's do this one up here this is about eighty millimeters of mercury and if I was to drop that line down that gets me to about here and of course that is the same volume really nothing has changed in the volume even though the pressure is shot up so those two are really easy but let's do a slightly tougher one let's do something up here where the pressure is about 120 pretty high pressure in fact it looks on this graph like the highest pressure it ever goes to and dropping the line down I'm gonna get to let's say here and I'm gonna say that X is 75 milliliters so at the peak pressure of 120 the volume is 75 milliliters I'm just gonna keep going this is let's say about a hundred pressure of hundred and you'll see how we can actually kind of use this information to make another graph so we're just going to kind of collect information from these two graphs using volume and pressure and we're gonna make another graph of our own this I'm gonna say it's about 20 and 20 drops down to here and this looks like it's about 50 milliliters right and you can kind of start getting the idea here this is actually probably the lowest pressure here the pressure is about 5 and dropping it down it looks like it gets to about here that looks I'm just gonna say 75 again just kind of round off and then here you have kind of a slightly elevated pressure right this is about 12 I'm gonna say and then this last little bit right here is 10 back where we started and if you drop those down let's see right here the pressure right here right before it kind of maxes out on the volume let's it's about 123 slightly less than completely full and then that final X is gonna be back at 125 right so this is kind of how the volume changes over time and how the pressure changes over time and actually I'm gonna take now our two graphs and I'm gonna try to merge them together I'm gonna make one yes you kind of think of it as a super graph so this one is going to have volume down here using the same units as before we usually measure volume in milliliters and we'll do 50 over here and I'm gonna estimate that's about 125 over there and on this side we're gonna do pressure right so pressure we measure usually in millimeters of mercury although of course you could use pounds per square inch or something else but we're gonna do millimeters of mercury because that's what we usually do and I'm gonna say this is I'm just kind of estimating I'm gonna say that's about 120 so these are the the two new axes week we're going to use and we've got to pick some point to begin at and I'm gonna kind of assume that the the first point I start at this one over here is a good place to begin again so we can start there and the pressure there was about 10 and the volume was about 125 so I'm gonna make a little red X there that's our starting point right a new graph and from there it went up to a pressure of 80 well 80 is about over here and the volume did not change so I'm gonna do a little red X there and I'm gonna connect the two lines like this basically you just kind of show the two connecting like that and we know that over time changes are happening when it changes happening between the first and second X and it takes a little bit of time and how much time exactly I'm gonna write out remember it was about 0.05 seconds that's about how much time it took to go from the first spot to the second spot and let's just keep track of that so if you go forward in time you should go up on our graph at least in the beginning so what happens after that well then you get to a point where the pressure is really high 120 and the volume is about 75 so it's I'm gonna draw a little red X there somewhere there right and that's when the blood is actually leaving the left ventricle and entering the aorta so this is gonna be the next spot right and then the pressure falls again it goes down to about a hundred and the volume is still about 50 so volume has gone down pressure has gone down too so it goes back down like that so this yellow chunk that I've divided into two just kind of to make sure I include the very very high pressure of 120 but that entire yellow chunk takes time of course and how much time does it take let me mark it so this whole bit takes some time and we said that whole bit combined takes about 0.25 seconds so about 1/4 of a second to do that bit so it's interesting right because the first bit happened really really fast point oh five seconds and the second bit takes five times as long but on our graph it doesn't look it's not like you see a line that's five times longer right because again this is pressure and volume and these graphs do not actually show you the I'm which is why I have to separately show it to you just to convince you that some segments do in fact take longer than others now the next part the volume stays the same but the pressure falls and it goes from we said it goes down to a pressure of about 20 so let's say 20 is about there on my graph right so it's gonna fall and I'm gonna try to keep the colors consistent so we've got green now this is when the left ventricle is relaxing so the pressure falls at that point right and how long does that take well if you remember this bit right here takes about 0.15 seconds 0.15 seconds so again compare that to the contraction the first part which took only a third of the time but the two lines in many ways look very similar right one's going up one's going down but the time is different now let's continue and see how the pressure Falls really low right about five and we said the volume at that point is about seventy-five so the pressure Falls down to somewhere like this and then it starts to rise again so let me show that so we've got kind of a decline here in fresher and then you've got a rise so then it starts rising again and remember there's a point where it hits twelve and so it's gonna do something like that rise and then it's gonna go up just slightly at the end and then go like that so that would be our pressure volume loop now most times when you see it drawn you'll see it drawn something like this but almost never do you see this last little bump so I'm just gonna draw it even though you know that it's there and I know that it's there just to kind of show the way that most people draw it in books is like that so that's what the pressure-volume loop looks like in most places and actually let me put that last little bit of time in there so let's let's do this little bit right here and this takes how long this takes about 0.55 seconds so the majority of the time is spent on that last bottom part of the loop and so in in some this is our pressure volume loop