 Now, there are three main ways that stuff gets out of the capillaries and into, I don't know, the rest of your body. So in order to do this concept justice, it's critical to our understanding of everything else. I'm going to, what? Draw you a picture. I know, I know, shocking. What did I just draw you? I'll draw you a blood vessel, dog pounds. It's a blood vessel. Now, I want you to remember this is the edge of my blood vessel and what's it made out of? It's made out of endothelium. I don't want you to think about the leakiness, whatever. What are my dots I'm drawing? Those are just nuclei, just to remind you that this is a layer of cells. Inside my blood vessel, I've got blood. So I'm going to have red blood cells and there's going to be a bajillion, four million of them. That's a real word. Those are my red blood cells. And of course, you're also going to have white blood cells. So we're going to have, I'm just going to draw one of them because that'll be fun. That's a white blood cell. And then we've got holy fluid madness. So we've got our plasma, which is just liquid. And our plasma has little plasma proteins in it, which is going to be significant in this picture. I'm kind of wishing that I hadn't drawn the little water lines that look like upside-down seagulls. I was just trying to show you that it's a fluid in there. But the little dots are plasma proteins. Maybe I should label those as PPs. Associated with the endothelium, of course, basement layer. I'm just doing all of this just to remind you. Now, stuff can diffuse out. For example, if there's a great deal of oxygen, if there's a high concentration of oxygen inside this blood, oxygen is just going to diffuse out to who? Who are we diffusing to? What cells out here? Here's... these are Joe's cell out here doing Joe's cellular things. Why, hello, Joe's. And let's just say these little guys are hanging out and they've used up all their oxygen. So there's a low concentration of oxygen or preview for respiratory system, a low partial pressure of oxygen. So the oxygen is going to move out of the high concentration zone and into the low concentration zone. Nice and easy and that is logically, that makes perfect sense. That's just the process of diffusion. And some things simply diffuse. The concept that I think is a little bit trickier is a concept called bulk flow, bulk flow. This relates to blood pressure. Now, do you agree that my heart is somewhere out here? My heart is somewhere out there and I'm just going to tell you what direction my heart is pumping through this capillary. And so blood is traveling from left to right. Does that work for you too? I hope so. It's going this direction through my capillary from the heart and then back to the heart again. That works for you, right? There's blood pressure, right? And when you're talking about bulk flow, the heart is providing a pressure that is called hydrostatic pressure. So bulk flow is a factor of hydrostatic pressure and hydrostatic pressure always is going to push fluid. So let's just say water. Now, substances like oxygen can diffuse. And this really, I don't know, if we want to call it osmosis, this right here isn't osmosis. It's hydrostatic pressure pushing water out. But water is going to squeeze out of the capillaries. It's going to happen more in leaky capillaries and that totally works for you, right? The higher the blood pressure, the more hydrostatic pressure there is pushing blood out. Does that, pushing water out. Blood cells don't come out, proteins don't come out. Just the water. Okay, I want to change my line. I want you to see that hydrostatic pressure at the beginning of the capillaries is big. Hydrostatic pressure, as we move through the capillary, it decreases. Why? It actually decreases because blood pressure decreases, because of friction. So the heart pumps in the aorta and you have 120 millimeters of mercury of pressure in the aorta. But you agree that by the time you get to the capillaries, you certainly aren't pumping 120 millimeters of pressure. You're not pumping blood at 120 millimeters of mercury pressure through your capillaries. Your capillaries would explode and you'd be a sad state of deadness. So pressure is going to drop due to friction over time. But look at this. I think I have numbers somewhere. Looks like I don't have numbers on this diagram. So maybe we'll bring in the numbers in class. So you can see that essentially we're going to have, we do have net filtration that takes place of water. Filtration, take a deep breath. Filtration happens when fluid from the blood is pushed out of the blood. It's filtered out. Where did it get filtered to? You got filtered into the interstitial space, right? It's out here in between the cells and in between it's outside. So it was in the plasma and it moved out into the interstitial place. That's filtration. It's not the end of the story because, dude, is anyone worried about all the fluid that's getting filtered out due to hydrostatic pressure? We got to have something that's going to keep the blood coming back in. That's what that is. It's osmotic pressure, you guys. And if anybody was hoping osmosis would go away and never come back, your sad story has begun. We are going to see osmosis basically from now until the end of the semester over and over again. Osmotic pressure is going to pull fluid back in. Now, even though my arrows are, what, orange, it's water that's being pulled back in. Why? What is creating osmotic pressure in the blood? And the color that I picked gives you a hint. Hydrostatic pressure is created by the heart. Osmotic pressure is created by plasma proteins. And that's going to pull water back in. Guess what? Overall, there's more filtration than reabsorption. And this is called absorption. The pulling of the fluid back in is absorption. So I'm going to write that down. Absorption, back into the blood, or back into the tube that you're interested in. So we've reabsorbed what we filtered out. And overall, your system filters about three liters of fluid every single day. So filtration happens a little bit more than reabsorption happens. And think about that for a second. We had hydrostatic pressure decreasing over time due to friction. Osmotic pressure doesn't change. So all the way across the whole thing, you're going to have the same amount of osmotic pressure pulling the fluid back in. The only thing that might change it is if, for some reason, the proteins were somehow the capillaries became so leaky that the proteins left. That's going to be a problem. And that's something that you can... There's clinical cases that we could look at to see symptoms of such phenomena. Okay, are you good? Both of these things balance and result in a net filtration of three liters every day. What happens to that? Because I'm telling you right now, if you had three liters of fluid every day, you added it to your interstitial spaces. You would be like the blueberry girl in Willy Wonka's Chocolate Factory movie. Violet Beauregard. We just watched that movie in my house. Violet Beauregard would be you because you would be swelling every day. So, where's it go? We dump it into the lymphatic system and we'll talk about that next. But if you remember properly, we've had one, two different mechanisms for getting stuff in and out of the capillaries. I've got one more for you and it's called transcytosis. Transcytosis. Are you ready? Transcytosis is the phenomenon that happens in the endothelial cells where endocytosis happens first on the luminal edge of the epithelium and exocytosis happens next on the basolateral edge of the epithelium. So, stuff comes into the cell on the luminal edge and stuff, the same stuff exits the cell on the basolateral edge and now you have stuff in the interstitial space. You can then endocytose that stuff into the cell and now you've got stuff in your cell through the process of transcytosis. What? Shall we talk about the lymphatic system next? Let's do it.