 The respiratory system is essentially this giant series of bifurcating tubes. And bifurcate might be one of my favorite words of all times. And all it means is split into two different pathways. So the opening to the respiratory system is your mouth. And the first tube is the trachea. And you can feel your trachea in your neck. It's surrounded in cartilage. And then the trachea travels down and begins the bifurcation. So of course, I mean, is there any other way to do this? No. I must draw you a picture. So I'm going to draw you the trachea. And I'm going to draw a little cartilage rings on it to let you know that it is surrounded by little cartilage rings. And then we hit our first bifurcation. And as soon as the trachea bifurcates, you have a bronchi. And the first bifurcation, these are primary bronchi, whatever. The bronchi are still surrounded in cartilage. I will change color when that changes. And watch the amazingness. The bronchi essentially feed into the lungs. That's a lung. It's puffy. It's squishy. It's awesome. It's like this amazing spongy tissue that you can blow up by putting air into it. It's actually pretty phenomenal. So of course, it's not just like a balloon. It has a lot of bifurcations inside this balloony structure. So in typical Wendy style, I am going to show you all these branching bronchies. And there's all different levels, and each level of branching bronchi has its own name. And I think from start to finish, there are, in each lung, there are 22 bifurcations, where here's a bifurcation, here's another bifurcation, and they bifurcate 22 times. What? This crazy talk. I love facts like that. The bronchi continue to be surrounded in cartilage. I just gave away the hint. Eventually, bronchi bifurcate into structures called bronchioles. And I'm going to draw a couple of them up here. And this right here is a bronchiole. And I've drawn them in red to indicate that they're different. They're structurally different from the bronchi and from the trachea. Bronchioles are not surrounded in cartilage. They are surrounded in smooth muscle. And you can imagine, you know, we're definitely down in the 19s or 20s of all of our bifurcations. And so the tube, the diameter, the lumen of the tube is really, really small at this point. And then the bronchioles terminate in structures called alveoli. Alveoli, we're going to talk about the structure of these guys in the next lecture or the next section. So alveoli, this is where gas exchange occurs. This is where the magic actually happens. And so it's important to understand the structure of the alveoli. But do you see how essentially I could start here and put puffy, fluffy, floofy lung tissue all around that. And all I've done is shown you a blown up version of what's actually inside the squishy, puffy, cool lung tissue. Let's look at the alveoli. I've got to write that down first so that you do not forget it. Alveoli. Let's go talk about the structure of the alveoli. K.