 Dog pounds, capillaries are unbelievable. Are you ready for this? These are my fast capillary facts. You, this is crazy. You have 50,000 miles of capillaries in your body. No, that is so many capillaries. That's insane. And if you were to spread them all out and squish them out so that all their surface area was exposed, it would cover two football fields. You have got to be kidding me. There are so many capillaries in your body. And it makes sense because every single cell in your body has to be within at least 0.1 millimeter of a capillary or it will die. If it isn't within that distance of a capillary, it can't get fresh oxygen and it can't get rid of its garbage. And that's how important capillaries are. Whoa. And in the next section, we're going to talk about like how does stuff get exchanged in capillaries. And guess what? There's lots of pressures involved in that. Before we go there, I think it's important to recognize and appreciate the fact that there are different kinds of capillaries and the different capillary structures are associated with their different functions. Most capillaries are considered, I don't know, continuous capillaries. That works for me, whatever. Essentially, continuous capillaries have your endothelial layer of cells. You see how tiny they are. It's just one layer of endothelium. Simple squamous epithelium. There is a basement layer of connective tissue associated or whatever, basement layers are made out of proteins, but there is a basement layer associated with this simple, thin cell. Here's a nucleus of this cell. So you can see that these things are really, really tiny. Most of your capillaries are continuous capillaries. They allow, so you'd imagine this is the lumen, so this is going to be blood in here. And stuff, fluid, water, can move through out of the blood and into the surrounding cells or vice versa. And that's the topic of the next section. Some capillaries are called fenestrated capillaries, and fenestrated capillaries have holes in them. And I mean, dude, what's going to happen if you've got holes in you? You're going to be leaky here. So fenestrated capillaries allow more fluids through the capillary wall. Does that work for you? It's literally like there's these little tunnels going through. It's like little gap junctions, but they're not gap junctions or else they'd call them gap junctions. So it's just holy capillaries, which just means they're going to be leaky here. You find fenestrated capillaries in the glomerulus of the kidney, which is the glomerulus is a capillary bed associated with each nephron. We're going to talk about those guys, and you'll see hopefully that, oh yeah, that makes sense that we would have a leaky capillary associated with the nephron. Then you've got these amazing capillaries that are called sinusoidal capillaries, and that's because they're like, they're beyond leaky. They're like a whole another layer of leakiness. I mean, they have a larger diameter, and it's almost like they're not even really tubes anymore. It's like they've expanded and they're kind of partial, and so blood and stuff can move very easily in and out of a sinusoidal capillary. Let me give you some examples of places where we're going to see this. First of all, the liver, what? The liver filters all your blood, especially from the digestive system, through the hepatic portal system. Your liver touches all the blood that comes from your digestive system before it goes to the rest of your body, and so it rolls through these super leaky sinusoidal capillaries that allow lots of stuff. We're not going to be selective. We're just going to let lots of things go in and out of the liver so that we don't have to deal with managing pressure or whatever. We can just let stuff move. We also see sinusoidal capillaries in the bone marrow, and think about that. The bone marrow is where blood cells are born, so entire blood cells have to squeeze out of the bone marrow and into the capillary. They're not going to fit through a normal capillary. They're going to need to fit through something that has bigger holes ripped in it. So you will also see these in bone marrow, and then the last place that you're going to see them is in the spleen. The spleen is really part of the lymphatic system, which is the last thing that we're going to talk about in this lecture, but the spleen is a place where red blood cells are recycled, so entire red blood cells are going to have to be pulled out of the blood. It's going to be super handy to have this leaky place. You're also filtering the blood for bad guys, so immune cells hang out in the spleen, like macrophages, hang out there, and wait for bacteria and bad guys to come swing and buy, and it's much more convenient. It's much easier to filter the blood if you can get easy access to the blood if you're a macrophage. So the functions of these different structures vary depending on where you're located. If you think about something like the brain, the brain has really, really, really tight capillaries. In fact, there's a whole another layer of cells. Remember our astrocytes, which are glial cells? The astrocytes create a whole another layer separating brain tissue from the blood, and so you can consider capillaries, continuous capillaries in the brain to be super, super, super unleaky. There's lots of stuff that can't get through. The blood alcohol, however, gets right on through. Okay, let's talk about the whole point of the lecture, which was capillary exchange.