 Let's look at the trachea. The trachea is a tube, and the difference between the trachea tube and the digestive system tubes is that the trachea tube is actually rigidly open. Does this make sense to you? The structure of the trachea keeps it open, which enables its function. The structure of the esophagus, is it important to have some sort of structure to hold the esophagus open? No. In fact, it's probably better to have the esophagus kind of flop closed if there's nothing in it, because if it was just staying open, maybe stuff from below would easily come out. Let's try to avoid that. So the main difference between the trachea, which is just a tube, it has all the same layers as the digestive tubing. So we're going to have a mucosa. We're going to have a laminopropria, a submucosa. The main difference is that we have to have something that holds the trachea open. And guess what it is? It's cartilage. It's actually, I'm going to tell you, the tissue type. It's hyaline cartilage. Hyaline cartilage. And the good news is, we're going to talk about hyaline cartilage. We're going to spend an entire day being cartilage and bone tissue. And so this is a little bit of a preview to that. But we end up with these hyaline cartilage C-shaped rings that have the C, the open-ended C part of the ring on the posterior surface. So check it out. You can totally see that here. Here's my C. It's coming around. This is all cartilage. And this is the open part of my C. And in fact, that is connected. There's a muscle in here called the trachealis muscle that connects the two ends of the C. And when the trachealis muscle contracts, it decreases the diameter of the trachea. This happens if you cough. It's a reflex that if you start coughing because you're like smoking or something and so you should quit smoking. It makes it easier to cough stuff up. It makes it easier to generate more force to cough stuff out of your trachea because your trachea has a smaller diameter. Like if you tried to blow through a really small hole, I can generate a great deal of force in the air that's coming out. But if you try to blow through a really big hole, that doesn't generate the same amount of force. In fact, I generated force by making the hole smaller with my lips, which is whatever. The idea is that if you make the diameter smaller, it's going to, you're going to be able to generate more force to cough out whatever is in your trachea and making you cough. Did you understand what I just said? I hope so. The trachealis muscle, which connects the two ends of my C, it also allows the trachea to kind of stretch. In fact, it allows you to swallow a giant mouthful of ice cream and that ice cream going through your esophagus, which is posterior to your trachea, that giant ball of food can actually push in on the trachea and not like take up more space as it passes through. Does that make sense? You had rigid cartilage all the way around in your trachea and then you swallowed a giant bite of ice cream that would like get squished and stuck and you couldn't go through fast enough and you'd freeze your brain. Aren't you glad that that didn't happen? Okay, trachea. Let's look at the histology of the trachea with my favorite histology tool from the University of Michigan. Ho, ho, ho, ho, ho. Look, I'm going to show you what Kylan cartilage looks like and all cartilage looks very similar, but let's zoom in. Where are you going to expect? What looks like where would the lumen do you think be? This is actually my lumen. Let's go in and take a look at our... I want to cry right now because that's how much I love histology. Look at how beautiful, what? What kind of tissue is this? This is crap. Look, it's not crap, it's blood. This person probably was smoking and had bloody cough or else there was blood when they killed this rat or whatever it was that they chopped out its trachea. But look, pseudo-stratified, ciliated, columnar epithelium. Don't you just want to cry? Look, an incredible, like, can you get a prettier basement membrane? No. And who's this? My friend, the submucosa. And let's zoom out a little bit more and see if we can find, I mean, that was my laminopropria. And now we're in the submucosa. And what is this? Really? Does it... I mean, quintessential. Those are tracheal glands. They're producing mucous to help you cough up all the crap that you're smoking. Quit smoking. And they're embedded in the submucosa. Piece of cake. And now what in the world is that? That's hyaline cartilage. And I'm telling you, that might be one of my very favorite tissues of all time next to pseudo-stratified alumnus-siliated epithelium and transitional epithelium and smooth muscle. I love histology. Look at this thing. That is your tracheal cartilage. Wow, really? I mean, that's really awesome. And we'll look at cartilage in more detail, but can you identify that? Can you just be like, dude, that looks like nothing I've ever seen in my life. We have to be in the trachea. We have to be. And then go ahead and take a look at the pseudo-stratified-siliated columnar epithelium, and then you know you're in the trachea. Now, the trachealis muscle is... Okay, there is one little end of my C. The trachealis muscle actually travels all the way over to the other end of my little C. I can get there. Look at all those glands. There it is. There's the other end. This looks like my skeletal muscle, or my trachealis muscle. Holy glands, I love it. And what kind of tissue is this? Yeah, adipose tissue. Okay, I'll stop because I could seriously do histology all day long. I'm a nerd like that. Let's talk about your lungs.