 Now, this is a nice visual of a bronchiol culminating in multiple different clusters of alveoli. Each one of these little nubbins is an alveolus. And you can see it if we cut down the middle of it, we can see that they're actually like little open balloons. I know that this is a bronchiol because it's surrounded in these ribbons, these pink ribbons. It's actually representing smooth muscle. And remember that that was what distinguished bronchioles from bronchi. Bronchioles are surrounded by smooth muscle. You can imagine if you're surrounded by smooth muscle, if that muscle contracts, you're going to have constriction or decreasing of the diameter. And that's bronchoconstriction. If those muscles relax, you're going to dilate the diameter of the bronchiol. And that's bronchodilation. Bronchoconstriction and bronchodilation can regulate the amount of air that can get into the alveoli or out of the alveoli in the first place. So that's the site where we can actually regulate the amount of carbon dioxide and oxygen concentrations in your blood. Okay, so let's look at the structure of the alveolus. Another thing I want you to notice is how it looks like a little salamander tail to me. But this is a blood vessel. And you can see that we've got little arteries and little veins surrounding and little capillaries surrounding our alveoli. And they've counted the number of blood vessels that are connected, like literally touching the alveoli. And it's, I don't know, like 80% of the entire alveolus is in contact with the capillary. Which means that you're going to have a very efficient exchange of gas, which like I said is the next topic we're going to talk about in the next lecture or not today. So let's look a little bit closer at the structure of the alveoli. That's the big picture, but I want to draw you one just all by itself. Because the alveolus, I hope that you could actually guess what kind of tissue it might be made out of. I'm going to draw a bronchi. Just so you remember that this is a tube. Dude, seriously. We're going to go this way because we need to terminate. And I've just taken like a cross-section of a bronchi surrounded by smooth muscle, which is why I made it red. And now I hope that this does not shock you to find out that, dude, the whole purpose of this thing is gas exchange. We better have a structure that enables this function. And what structure, what kind of tissue, would you put in the alveolus? Well, 95% of alveolar cells are type 1 alveolar cells, and they are simple squamous epithelial tissue. Look how thin these things are. Thank you for being so thin. And in fact, what you'll find is that they're so thin and they're right next to a capillary, which the capillary we learned in the last lecture was so thin lined by simple squamous epithelium called endothelium that, yeah, gas exchange is going to happen super quick between these things. If 95% of them are type 1, what's the other 5%? You're so good, they're type 2. And look, the type 2 cells are different. I'm going to go back and I'm going to finish my type 1s so that you can see that most of them are definitely type 1s, but the type 2s, oh my gosh, the function of the type 1s is just to allow exchange of gases. Done. The function of the type 2s, these guys are rock stars. Number one, they produce a substance called surfactant and they squirt it out into the alveoli. And so surfactant decreases surface tension. And hopefully, this is another Bio 1 review fact. Hopefully you remember that water molecules, doggies, how cool is this? Water molecules were polar molecules, so they had a partially negative side and a partially positive side, which means that they form hydrogen bonds with each other and so they kind of stick together. And that fact that water molecules stick together actually means that it's like a plastic bag. If you stick a plastic bag in water and then you try to open it up and like put something in it, the whole plastic bag like sticks together. And that's because both sides of the plastic bag are covered with water molecules that stick to each other and cause the whole bag to stick together. Your alveoli would do the same thing if you did not have surfactant in there, breaking down the surface tension and making it easier to inflate the alveolis. So imagine if your alveoli actually slapped together and stuck, then every time you took a breath, you would have to work really hard to actually inflate the alveolis because of the hydrogen bonds that water forms with each other, water molecules form with each other. So surfactant is really important. Interestingly, little babies in their mama's bellies, they don't produce surfactant until like six weeks before they're about to be hatched. And so if they are born early, then they have a condition where they didn't have enough time to make surfactant. And so when they breathe, those poor little pumpkin pies, they have to work really, really hard to get a breath in to inflate their lungs because there's no surfactant yet. There's all sorts of strategies for treating that, but that's because that's how important surfactant is. These guys also do something else. They actively transport, okay, they actively transport solutes, that says solutes, out. They take grime and crud that's inside the alveolis, and they say, dude, we got this. Let's just get it out of there. Now, think about that for a second. Apply your brilliance, your osmosis brilliance to this concept. If you take solutes out of this space, you're going to have fluid in there, but if you take the salts, the ions, the dirt, the crud, and you actively pump it out of the alveolis, what's water going to do? Water's going to follow it. If you're pumping solutes out, water's going to follow. It keeps your alveoli from filling up with water. It doesn't keep them dry. Alveoli from filling, dude, with water. I hope you listened to me instead of, because alveoli from filling with water. Alveoli are amazing. This oxygen in, carbon dioxide out. That's gas exchange. Next lecture. Breathing happens because of the structures in the thoracic cavity. So let's talk about that next.