 So, we want to look at simple squamous epithelium now and we're going to use a couple of microscope slides to help us figure out what simple squamous epithelial tissue looks like and what some of its functions are. So again, simple squamous. And to think about simple squamous epithelial tissue, I want you to think about a piece of paper to start with. And if I have a piece of paper in front of me and in front of you, you can see as I hold it like this that it's a square piece of paper, but you really can't tell if you're looking at it straight on if I'm holding one piece of paper or one entire ream of paper. But that's what we would call a frontal view or looking at the surface of the cell. But consider what the edge of the piece of paper looks like. And if you were looking at that straight on, you would just see a little sliver of the edge of the paper if I could hold it perfectly straight. Well, that's the way to think about simple squamous epithelial cells. So if we wanted to draw a simple squamous epithelial cell, we would just draw something that approximates the shape of a square. And we would draw a circle in there, maybe make it a little more oval shaped for the nucleus. So we've just drawn a simple squamous epithelial cell. But if we wanted to look at it in cross section or look at the edge of that cell, it would look something like this. This would be the nucleus of the cell. So we'll picture this piece of paper with something a little bigger than a golf ball attached to it. And that would be similar to a simple squamous epithelial cell. This is just looking at it in cross section. So using the microscope over here, I have a slide of a lung. And we want to look at this lung slide. Now, of course, the lung is involved with gas exchange within humans. And we take oxygen in and we exhale carbon dioxide. So within the lungs, we see all of these little spaces. And if you look on this microscope slide, you can see all these hundreds and hundreds of spaces. Well, those are the air sacs within the lungs called alveoli. And the cells that make up these air sacs are simple squamous cells or simple squamous tissue is what the alveoli are composed of. And if you think could picture the alveoli, you just picture sort of a little cluster of grapes perhaps. You have an air passageway leading into each little cluster of alveoli. But instead of being made of grapes, they're hollow on the inside and the skin, so to speak, or the covering is made of these simple squamous cells, very, very delicate tissue. Of course, that's important for the respiratory gases to diffuse across the membrane. So we want to go up and look at this on a higher power. We can start to see the cells take shape when we look at it on 40X. And this is a pretty good microscope, and I was able to get away with oil immersion without putting oil on it a second ago. And so here we can really see the cells that make up the alveoli. And so here, for example, is an alveolus, and we could go around and sort of trace some of these cells that we see, and see there's a square shaped cell with the nucleus. Because these are all sort of mashed together in a hodge podge, it's hard to pick them out and just see cells where you're looking at them perfectly straight on, but I'm just picking out a few here that you may see. In this area, you can see no nuclei, and that's just because of the way the slide was made. Now one important function of simple squamous epithelial tissue that we didn't mention when we were talking about epithelial tissues in general is diffusion. Because this tissue is so thin, it makes it much easier for substances to move across that membrane by diffusion. For example, if this was the inside of an alveolus, and this was the outside, then oxygen could move across that membrane or carbon dioxide, could move in the other direction. So very thin tissue, good for diffusion.