 Now, I need you to consider what is going on in that intraplural space. Here's what I want you to think about. No, maybe here's what I'm going to tell you. You ready? I don't know how to write this. Always, unless something bad happens. Always, intraplural pressure. The pressure of the intraplural space, pips. I just made that up, so don't ever tell anybody that because it's totally made up. It is always less, maybe three millimeters of mercury, less than alveolar pressure. Always. How'd that happen? What actually happened in utero? So much happens in there. Dude, your first apartment rocked. When you were in your mama's belly, your thoracic cavity, your pleural cavity, grew faster than your lungs grew. That created, like this, it created a vacuum. And a vacuum really is just kind of the absence of pressure. If out here, and the only way I can do this is by drawing pictures. Imagine that this intraplural space is filled with people. And they're running into the wall like they always do, especially when they're in my class. And they run into the wall, I don't know, four times in this because there is a pressure in there. Remember that the pressure inside this space isn't actually three millimeters of mercury. It's three millimeters of mercury less than the pressure in the alveoli. So you could essentially imagine the air molecules in the alveoli hitting the walls. Look, there's more of them hitting the walls. The pressure's higher in here. Do you agree? What are the consequences of that going to be? I had a little bit of trouble drawing all my arrows. They're still bouncing off the walls. Look, this bounce and this bounce, they even each other out. If the pressures were equal, they would have no movement of the wall of the alveoli. They're not equal. There's more people inside the alveoli pushing out than pushing back, which means the alveoli are going to stay expanded. If, for some reason, oh boy, George here, were to get stabbed with a knife and his intraplural space were to be punctured. And that's a knife. Air was to perhaps rush in. Atmosphere was to rush in and remove that pressure difference. Then now I'm going to have more molecules bouncing into the wall and my lung is going to collapse. What? That's pneumothorax, dog pounds. That's what happens when you have a collapsed lung. Your lung will collapse if the vacuum in this intraplural space is removed. Does that work for you? I could probably come up with like a million more analogies to do. I kind of feel like I would like you to stand up and push against a wall and have someone else push against the other wall and see who wins, who has more pressure. That could be dangerous. The other line is that the difference, the fact that intraplural pressure is always lower than alveolar pressure means that in a healthy system, the lungs stay inflated and they literally collapse if that is not the case anymore. And there's all sorts of things besides knife stabs with, that looks like a butter knife to me. You could collapse someone's lung with a butter knife. That's a horrible thought. The last thing that we want to do is we want to look at some volumes of air that we can actually measure. You've got ventilation down. You know how we breathe now. Now we're going to look at, okay, what kinds of breaths can we take? For example, our little tidal breaths that are, you don't even really think about it. You just are breathing in and out and you're really not moving a whole lot of air as opposed to like a giant deep breath. And we have names for all those things. We're up back.