 We're going to talk about three ways that stuff can get in and out of the capillaries. So to illustrate our three ways, oh jeez, come on little buddy, we have to draw ourselves a picture of a capillary. We're going to make the assumption that all of this, all three of my in and out strategies are happening in a continuous capillary, like I'm just going to visualize a continuous capillary. Knowing that in a leaky capillary, all three methods of exchange still happen. They just happen differently, faster, different characteristics, but these three methods are doable in all kinds of capillaries. So let's remind ourselves of the fact that capillaries have cells lining like they're made of simple squamous epithelial cells. So don't forget that because that could get confusing if you forget that, yeah, we actually have cells here. You're made of cells. What? I know that's shocking. Okay, strategy number one. Strategy number one is diffusion. Remember, our goal, diffusion. Our goal is just to get stuff in and out. But what? Some things can just move out of the blood or back in. Give me an example. Who would that be? Carbon dioxide and oxygen. These guys, I mean, we don't have to mess around. We don't have to play games. We don't have to have different tools. They're just going to zip in and out based on their, essentially, on their concentration gradient, on their partial pressure gradient. So that's easy. Okay, one, okay, move on. Strategy number two is something called transcytosis. Transcytosis. And what does that kind of sound like? Barfing and yumtializing. Transcytosis is barfing on one end and yumtializing on the other end and you move things through cell membranes. Okay, okay. So, transcytosis, hmm, maybe blue. Transcytosis is in reference to my endothelial cell and it involves endocytosis, okay, see this, on one edge of the cell membrane, on the luminal edge of the cell and then exocytosis on the basolateral edge. So, a substance comes in. I made this so freaking tiny because I wanted the rest of the space for the next one. But I think transcytosis is really logical and intuitive. You endocytose something from the lumen of the blood vessel. That something is now inside the endothelial cell and then you exocytose it on the basolateral side and that literally transfers stuff from the blood into the intracellular, I mean the extracellular fluid or the interstitial fluid. That's transcytosis. The third technique, the third strategy is called bulk flow and this one is by far the most complex but all the mechanics of this you already know. Really it's just about putting together pieces of what you would expect to happen. Bulk flow relies on two pressures. And again, all it is is stuff is going to move in and or out of the blood vessel and bulk flow we can assume that we're talking about bulk flow of fluids. So I think it's helpful if you think of it that way. First of all, let's remind ourselves what is going on here. Dude, this is a capillary and blood is flowing through it. We actually have a net pressure that is pushing fluid out. It's actually, there's a net pressure that is, perhaps I shall repeat that, pushing fluid out. So what is that pressure? Like what would be a pressure? What would generate a pressure that would push fluid out of a capillary? Capillaries are thin even though we have differing degrees of leakiness, even a continuous capillary is leaky. Who's responsible for that? You know this? Aw, your heart. Blood pressure is pushing fluid out. It's generating a literal pressure called hydrostatic pressure in the capillaries that pushes fluid out. Now the interesting thing is that hydrostatic pressure changes over the length of the capillary, it gets smaller. Why? Look, I'm going to do one more over here. Why does hydrostatic pressure decrease over the length of the capillary? It's because of friction. Remember the like sliding down the shag carpet slide? Friction, you have a higher pressure in the arteries than you do in the veins because that pressure is lost due to friction as you move through the vessels. Does that work? I mean that's logical, right? So you can imagine that you're going to end up changing hydrostatic pressure. Nonetheless, that hydrostatic pressure is pushing fluid out. If hydrostatic pressure was the only pressure involved in this whole crazy scene, then you wouldn't have any blood and you wouldn't have any volume in your blood because all the fluid would get pushed out into the interstitial spaces because of the heart pressure. So something has to be pulling back in. This process that I just illustrated right here is called filtration. And I'm going to, I don't know, you'll see why this is important, filtration. Filtration is the process of taking fluid and filtering it out of the blood vessel somewhere else. Where actually blood pressure generates a net filtration in your capillaries. Now, thankfully, you got somebody else, you got somebody else doing some work in here. Get out your yellow pens, doggies, because we got to have some yellow people in the blood. They're not people. You might want them to be people. They're not. They're proteins. These are my plasma proteins. They're PPs. Are plasma proteins the only things in there? No, no, no, no, no. Look, that's a red blood cell. We got our white blood cells. Yeah, yeah, yeah. I got to make it look huge and gigantic. We got all sorts of stuff happening inside blood. We're good with that. The plasma proteins are significant. Plasma proteins are only in blood plasma. They're not in the interstitial fluid. So you tell me, what is water going to want to do? We just pushed out a whole bunch of fluid. What's the pressure that the plasma proteins are going to apply? They're going to be like, fluid, come back, dog. Come on back. Now, fluid's going to be pulled back in because of the osmotic pressure generated by the plasma proteins. Is that going to change over time? Like, the way that blood pressure decreased over the length of the capillary is this other pressure going to change? No, it doesn't change. It stays exactly the same, but thankfully it's pulling stuff back in. And guess what that pressure is? This is osmotic pressure. I got to say that's not my favorite color of yellow. Can you guys even see it? It's kind of faint and not very pretty. It's my favorite color. Osmotic pressure is pulling back in. Hydrostatic pressure is pulling, pushing out. What is the net... Oh, osmotic pressure is responsible for reabsorption of the fluid. Does that work? If you want to just call it absorption of fluid, my brain thinks of it as, dude, I just squished it out and now I want to get it back in. So I want to reabsorb what I just squished out. In your capillaries, all three of these things allow us to move stuff back and forth. In your capillaries, there is a net filtration of about, oh, is it three liters? Three liters. That's a lot. One and a half, two liter pop bottles. That's a lot of fluid that is filtered out net every single day. So do you think, like, what would happen? What would happen to you if that was just in your interstitial fluid? Now, three liters of fluid hanging out in your interstitial fluid. Dude, you're going to start looking like, oh shoot, who's the blueberry girl and Willy Wonka, Veronica. So I'm going to look it up and then I'm going to come back and tell you who that lady is. Her last name starts with an S, I think. Salt. We have a net filtration of fluid coming out of the three liters and we better do something with that. So we're going to dump it into the lymphatic system and guess what we're going to talk about next? The lymphatic system and Veronica something.