Compliance and elastance Learn about compliance (and elastance) of arteries, veins, and lead pipes! Rishi is a pediatric infectious disease physician and works at Khan Academy.
Compliance and elastance
- Alright so you take a little balloon, let's assume, you see it on the table and you can't resist yourself.
- You take that balloon and you start to think about how to do a little experiment with it.
- Maybe that's not what you're thinking, but that's certainly what you should be thinking because we're going to have a lot of fun checking out learning about this balloon by giving it some air.
- So imagine if you put some pressure into that balloon what would happen.
- It would of course get larger, right?
- So you know that the volume of the balloon is going to go up as you put pressure into that balloon. But you actually want to measure it, let's say.
- So you go ahead and give a small amount of pressure, maybe a small breath and this balloon gets a little bit bigger,
- and you note that it's, let's say a small amount of pressure is over there, and it gets a little bit bigger over there so you can put a little X right there.
- Very good.
- Now you go back and give it a little bit more pressure, a little bit bigger breath,
- and you do the same thing you say well there's my pressure and now it's a little bit bigger so I am going to put a little X right there.
- And you do this again with a large amount of pressure, and you notice that the balloon is getting much bigger now, so you figure out that as you put in more pressure the balloon is getting bigger,
- and on top of that, it's happening at a linear rate, right? So the more pressure you're putting in, you're getting a direct amount of volume for that.
- Now not all balloons are going to behave that way, but let's assume for the moment that this balloon does that.
- So it gets bigger and bigger as you put in more pressure. Great. This is my balloon.
- Now you notice that there's one more thing sitting on the table, and you grab it, and it's a plastic wand like this and you dip it in soap and you make a bubble out of the soap.
- So you give it a soft breath just like you did before.
- And you notice that even with a soft breath you get kind of a large volume.
- So that's interesting, right? So kind of a large volume.
- And then you give it a medium sized breath, and you get even a larger volume, let's say something like this.
- Even a large volume. And you can kind of see where this is going to go because I'm going to give it a large breath and maybe it will fill out this entire corner, something like that.
- You get this enormous bubble, and it doesn't burst (let's assume).
- So now you have three little blue exes for the bubble and you connect them just as you did before, and this is my bubble line, and you can already see something kind of interesting, right?
- You can see that the balloon has a smaller slope than the bubble.
- The bubble is rising more quickly, and so thinking about this you can actually say, "well this is a formula for the slope, rise in volume over run," which would be pressure in this case,
- and if you do rise over run you get the slope and in this case we're going to call the slope "compliance".
- Compliance. Really interesting and important word.
- Seems pretty simple, right? I mean it's just "how big does something get when you give it a certain amount of pressure,"
- and you can see in this case that the bubble has more compliance than the balloon. Good.
- So now we've figured out a couple things, and I'm going to add one more new word which is actually just the inverse.
- So what if I flipped it around, what if I put pressure over here, and volume over here.
- I can do that right? I can just take the same data, the same information, and just flip the two axes around.
- And if I did that, then in this case the balloon line would be over here, something like that, right,
- because all I'm doing is just flipping how I look at this chart, and the bubble line would be over here. Something like that.
- So now my bubble and my balloon have switched places because the axes have switched.
- I mean, in a way you could literally just tilt the graph over and you would get the same thing - there's nothing magical.
- But the thing that's different about this is that now, if I'm calculating rise over run, or the slope, I actually have flipped the volume and pressure, right?
- So now my pressure is on top and the volume is down below. And if you have it like this, pressure over volume, we actually call that "elastance".
- So the first one we called compliance, and this one we call elastance.
- And so you can see that elastance and compliance are basically just the inverses of one another.
- They're just the flip of one another. And so these two words, you're going to hear them, but I want you to see how they're very very much the same kind of thing, it's just that one is the inverse of the other one. Ok.
- So now we've gotten that, I'm going to make some space here, like that, and I'm going to bring up one final point.
- And that might be this. What if you have an artery? Instead of balloons and bubbles, let's talk about blood vessels for a second.
- What if you have an artery, like that, and you decide that you want to block it up on one end, maybe with your hand, like this,
- and let's say you do the exact same thing with a vein. You decide you want to take a vein and block it off on one end.
- I'm trying to draw these two to be the same size, so if they look different please assume for the moment that they're the same size and same length.
- Block it off. So that end is blocked off with your hand, nothing can leak out, right? So you only have one open end.
- And now let's assume that you cover up this end, so you cover it up, and you have just one tiny opening here.
- You cover up the vein, you do the same thing, you have one tiny opening here, and this opening, let's have it go down so it looks the same, and this opening is to a bicycle pump.
- I know this is sounding very strange - why in the world would you have a bicycle pump attached to an artery or a vein?
- Well you'll see in just a second. Here's my bicycle pump. And I'm going to actually pump up my artery and pump up my vein much in the same way that I did before with the balloon and the bubble.
- And you're going to start seeing some really interesting parallels I think.
- So let's say I pump up the artery. Immediately what happens: if I put a certain amount of pressure there,
- let's say I put the large amount of pressure that I put in the balloon, I'm going to get something like this where this artery's going to start swelling up, and this goes away.
- So now my artery looks a little fat, like a plump little sausage.
- And if I give that same amount of large pressure to the vein, it's going to do something like this.
- It's going to get enormous, and I have to erase these little lines to make it clear that my vein is getting huge.
- So with a little bit of pressure the artery gets a little bit bigger, but the vein gets a lot bigger.
- So with the same amount of pressure you see a difference in the volume, and this is actually a critical point,
- because the artery and the vein really are behaving just like the balloon and the bubble, and it's actually very very similar.
- So if I was to make a volume-pressure loop, with this, I could actually erase the word 'balloon' and 'bubble' and replace them completely with 'artery' and 'vein',
- I could just write artery and vein and essentially they would be behaving this way.
- Artery up here, artery over here, and then vein in the other two spots.
- So you can now see that the artery has lower compliance than a vein, and higher elastance than a vein.
- And now just speaking to the compliance issue: imagine that you had a really rigid iron pipe, something completely solid, it's not going to budge no matter what you do.
- Well for that solid pipe, you'd actually get something like this, you would have even less compliance.
- So if you're ever thinking about the issue of compliance, when we're talking about stiffness,
- think about these curves and the fact that where the slope is tells you how compliant something is, and that arteries are going to be more
- compliant than a stiff pipe, certainly, but less compliant than the veins.
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