Stored elastic energy in large and middle sized arteries

So let's say you're looking at the heart during systole. I'm going to write systole here. And the heart, of course, is going to pump blood out, and it's going to go into the aorta. So this is the aorta, and I'm not drawing it very accurately because we know that the aorta is not long and stretched out like that. It's got a lot of branches and definitely doesn't go off to the left like that. But just humor me for a second because I want to show you something really cool. Now, as the blood goes out into the aorta in systole, we know that the shape of the aorta does not stay like that at all, right? I mean, it's actually going to balloon out. And I'm actually going to even show you the old shape with a dashed line, just so you remember it. And then I'll show you the new shape just so that we can stay kind of consistent with the changes that I'm talking about. The new shape is going to look like this. It's going to be like a big balloon. And we know that this happens because we talked about the idea of compliance. You recall we talked about how if you have a certain amount of pressure and you actually want to look at the volume over pressure, that for an artery, like the aorta is an artery, you actually have changes like this. So basically as the pressure goes up, the volume goes up. And so we know that the arteries, specifically the aorta, is going to balloon out a little bit like that. Now here's what I want to show you and here's what I want to ask you, really, is to think for a moment about energy, OK? So think about energy and think about where the energy is. We know the heart is putting a lot of energy into each pump, right? We know this. So where is it going, exactly? Well, obviously a lot of it is going into movement, so there's a lot of movement energy. And we also know that there is a certain pressure energy. So if I was to actually just check the pressure in here, if I was to check the pressure, I know that there would be some pressure at that spot. So there's definitely pressure energy and movement energy. But see if you can think about where the third type of energy is. And I'll give you a clue, that there's blood actually rushing into this space, right? The reason that it's ballooning out like it is is because blood is pushing it out. And so if blood is pushing it out it's a little bit like a balloon. And a balloon has an elastic energy, and so do the aorta and other arteries. They have an elastic energy. So think about the walls of this balloon, if you want to think it is a balloon, as having elastic energy . Now, when I say elastic energy, one thing you might think of to help you kind of make this more concrete is this. Think about something you may have played with as a kid. I certainly used to play with these things, and I would chase-- actually scare parrots that were chewing on my peanuts away from the peanut tree by flicking stones at them. So I would load up on a slingshot with lots of elastic energy, and they wouldn't know what I was doing, obviously. But they would soon figure it out, because what I would do is I would aim my slingshot, and I would let go. I would let this snap back. And when it snaps back, you lose all that elastic energy, but your little rock is way over here. It's actually flown over here, and that's a great example of movement energy. So really what you're doing when you let go, in that one second that you let go of a slingshot, is you're converting all that elastic energy that you built up by pulling back into movement energy. And the same thing is happening here. So in diastole, you have a very similar situation. You have, again, your heart. And your heart is now refilling or getting a chance to kind of relax for just a moment. And that aorta begins to kind of collapse back down to its original shape. It doesn't actually take the exact original shape. And that's because there is some pressure still here, pushing it out. There is still some pressure. But we know that in diastole the pressure is lower, so of course, the volume is going to be lower. It's going to be more like it's-- it's going to be more collapsed, you could say. But this balloon has really done the same thing as my slingshot. The elastic energy of the walls has pushed back right here, pushed back all of this blood on both sides. And all the blood is moving up here as movement energy. So you have here movement energy again, same as before. But this time it's coming from conversion of this elastic energy, just like the slingshot. So your elastic energy gets converted to movement energy. And of course, there is still a little bit of potential energy as well. So you still have your potential-- pressure energy. Sorry, I said potential, I meant pressure energy. So you still have three energy sources but the big key is that you've really converted a lot of this elastic energy into movement energy when you go from systole over to diastole. So that's the beauty of your aortic and arterial walls.