 I want to review the anatomy of the thoracic cavity, and I want you to listen to this next piece, keeping in mind our anatomy that we've already looked at. I'm going to simplify this picture like ridiculously. I'm going to simplify it down to just a mouth, a trachea, and a lung inside the thoracic cavity, and I'm going to draw the whole thing out for you because I think that if we have a simplified version, I think it will help significantly. So let's start with a trachea that is open. We know that the trachea bifurcates, but let's just go no bifurcation. Let's just simplify this thing down. We know that it switches into bronchi, and it bifurcates into bronchioles. I'm just showing you my bronchioles just by showing you this red color, and then we're going to culminate in an alveoli. I mean, an alveolus. Does this work for you? Totally diagrammatic. Now, I'm going to throw in the thoracic cavity. This is, I don't know, look at this. The trachea is out here. It's open. Do you agree to the atmosphere? Here's the atmosphere. That says atmosphere. Of course it does. And it's open, which means the atmosphere can actually go inside your trachea. That's a wild thought, but that's exactly what's happening. The atmosphere can go all the way into an alveolus. And then the atmosphere, after we do some exchanging of gases, can go back out again. You've got a context. So I don't know, this blue line right here, that's probably my clavicles, right? And then the rest of my box is what, my ribs. This is all what defines the thoracic cavity. And this is important. At the base of the thoracic cavity, separating the thoracic cavity from the abdominal pelvic cavity is a structure made out of skeletal muscle. And this thing is the diaphragm. This is very important. It actually is shaped like this. This is the diaphragm. It is skeletal muscle. When it contracts, it does this kind of movement. Can you see how I'm shortening my fingers? But the shape of it means that when the diaphragm contracts, we get some interesting stuff happening here in the thoracic cavity. Now, the thoracic cavity, remember that it actually contains two plural cavities. And so really what I have drawn here is a single lung sitting inside a single plural cavity. And I'm going to draw that for you too. Keeping in mind, because you are brilliant, that my plural cavity is really out of proportion here. My plural cavity is defined by two cirrus membranes, two layers. And this one is called the visceral pleura. And the visceral pleura is a cirrus membrane that covers the lung tissue itself. And it's really important. This is not something that I'm just randomly like, dude, this is cool. No, this is the visceral pleura. This is just a membrane. It's responsible for producing cirrus fluid to reduce friction. We also have cirrus membranes are double layered. So there's also a pleura. This is the parietal pleura. The parietal pleura lines the cavity. It actually is going to line the pleural cavity. And this right here is the parietal pleura. Parietal pleura. So really, you probably would say, ugh, it's a little bit tricky because the heart is in here. There's a lot of anatomy that we're leaving out here. This whole, this box actually probably represents the pleural cavity. And this, there's a space that I've drawn here that looks gigantic, but it actually isn't. This is the intraplural space. And the visceral pleura and the parietal pleura produce pleural fluid. It's a cirrus fluid. And this is a crazy thing. All told, you have like 25 milliliters. Here's one milliliter. So you can imagine 25 of those. It's not very much fluid in there. This space is actually super, super small. They consider it a potential space. And we'll see the importance, like, the function of this space and this fluid. I'm showing it to you like this so that you have an appreciation of the big, we can blow it up so we can see the mechanics that are involved in breathing. This space is really important to ventilation. I think that that is what we need. The next thing that we're going to talk about are some gas laws that are going to, what, regulate, provide rules for ventilation. Gases have to follow a couple of laws. There's actually three of them that we're going to look at. And once we've got our anatomy, we're going to look at the rules we have to follow with the gas laws, and then we're going to put those pieces together and we're going to end up with some mechanisms for how we breathe.