 So let's talk about tonicity. Tonicity, if we're going to throw a definition into the mix, tonicity is how a cell responds when placed in a solution. And first of all, there are some things that you need to think about. Number one, here's Joe, Joe the cell, because you know, all cells are named Joe. Here's Joe's nucleus. This is Joe's cytoplasm. Tell me, what is the osmolarity of the cytoplasm? What is osmolarity again? It's concentration. It's just the concentration, and I can't actually remember if I told you this or not, but it's just the number of particles, the moles of particles in a solution, the number of liters. Did I tell you what molarity was? Because molarity was the moles of molecules. Osmolarity is the moles of particles, and I'll explain that in a second. The cytoplasm in your cell is not pure water, right? I mean, of course it isn't. It's salty, there's sodium, there's potassium, there's proteins all over the place, there's chloride ions, there's all sorts of crap in the cytoplasm of your cell. It does not have an osmolarity of zero. In fact, it has an osmolarity of about 280 milli osmoles. You can commit that to your memory. I would also say 0.28 osmoles. I mean, either way is fine. I like milli osmoles because we tend to talk about things. I mean, 280 is an easier number to kind of throw around than 0.28. And we have ranges in physiology land. Different places, like your kidneys, we have a range from like 100 milli osmoles down to 1200 milli osmoles. And these places in your body stay at about 280 milli osmoles. And if you think about that, if a cell's cytoplasm has that concentration, you would expect the extracellular fluid to have that concentration. Because if it didn't, osmosis would happen, right? And we eventually are going to reach osmotic equilibrium, right? And so the concentrations would eventually change, which is what happens when you are dehydrated or you haven't, or you are overly hydrated, like you change the concentration of your fluids. Okay. Next question. The 280 milli osmoles indicates there are particles in your cytoplasm. Are they penetrating or non-penetrating? There has to be particles, right? Because the particles are the only way that we can have concentration, right? Because otherwise we'd have a concentration of zero. So there's got to be 280 milli osmoles of particles inside there. Yes, there are. Are they penetrating? The answer is no. All of those particles that are contributing to the 280 milli osmoles are non-penetrating and you can take it to the bank. It says penetrating. No, no penetrating. If they could penetrate the cell membrane, they would and they'd get out and they don't because they're non-penetrating. Does that work for you? So you can always assume that a cell has a concentration. It has stuff going on in there. Now, I'm going to give you a couple scenarios here. I've got three beakers going on. And I'm going to tell you, it's going to get more complicated than this, but of course it is. But I'm going to tell you that this situation has a concentration of 280 milli osmoles. And I'm also going to tell you that these particles, whoa, that was a, whoa, those are crazy particles. They're getting crazy. I'm going to tell you that these guys are non-penetrating. What would happen if they were penetrating? No, I mean, I haven't done anything yet. I'm going to drop the cell in. I wish that I could just draw that cell in there, but just imagine it, that I just dropped that cell in there. It has a concentration in its cytoplasm of 280 milli osmoles. I dropped it in to a beaker with a concentration of 280 milli osmoles of non-penetrating particles. Are they the same particles that are in the cell itself? No doggies, green particles, red particles, not the same. However, the concentrations are the same and they're non-penetrating. So they can't go in. They would want to. If they could go in, half of them would go into that cell so that they were in equilibrium. If they could, half of these green particles would come out. Like, they would totally reach equilibrium in their own concentrations. In fact, this is a great analogy. Particles are like teenagers. They only care about themselves. This set of particles is not going to care about the particles in here. They, like whatever, who cares? Nobody cares about those. They're only going to care about particles that are like them. They're going to reach equilibrium if diffusion is happening. Does that work? But if we drop in the cell into these non-penetrating particles, what's going to happen? Is water going to move? No. There's going to be no movement of water. Do you agree with that? The concentrations are the same so this is actually an isotonic solution. There's no change in that cell. You want your extracellular fluids to be isotonic so that your cells don't swell. Sometimes your cells swell. If they get put into a hypotonic solution, they're going to swell. And I'm going to write that down even though I haven't given you the scenario yet. A hypotonic, a hypotonic solution, is any solution, I don't care what it is, any solution that is going to cause that cell to swell. So give me a scenario, like what's it going to look like? What kind of solution could I drop this cell into and have water go into the cell? How about a solution with, let's say, what's a concentration of maybe 100 milliosmoles? If the concentration here is 100 milliosmoles, it's a lower concentration, right? And the water, particles can't move. They're still non-penetrating. The water is going to have to go into the cell if we want to try and equalize concentrations. If we want to reach osmotic equilibrium, water is going to go into the cell. If water goes into the cell, what's the cell going to do? Swell. Oh. That was some swelling. All right, that works. Because we still had non-penetrating particles, but what happens if holy o-dector, holy days of particles? I cannot do this for the whole thing, okay? Because, well, I could, but do you really want to sit and watch me do this all day long? I feel like this lecture has been 8 billion years long, but you're happy that it's long because these concepts, particularly the osmosis one, are tricky. And aren't you glad that I am filling this in? Oh, you know what? This is actually getting bad because I could do the whole thing. How are you feeling right now? Are you like, dude, rigs? Put a lid on it. Okay, I will. How about if we say that this was 400 milliozmols and I filled up the whole thing, okay? Because you guys kept yelling at me so I couldn't really fill up the whole thing. Drop your cell in there. What's going to happen? We're super concentrated. We want osmotic equilibrium. How are we going to get it? Do you agree that it's less concentrated in here? So in order to try and reach equilibrium, the only thing that can happen is that water has to leave the cell. Does that work? Water will leave, which will make the inside of the cell more concentrated, which will try to dilute the outside of the cell with each osmotic equilibrium. That, my friends, is what kind of a solution. That would be our very well-known hypertonic solution. Now, I'm not going to tell you all the coolnesses. No, hypotonic solutions, no matter what the solution is or how it does it, if the cell swells because water rushes into it, then it is a hypotonic solution. No matter concentrations or irrelevant, all that matters is what happens to the cell. In osmolarity, we can actually... Okay, I'm going to do it. We can actually label osmolarities, this cell compared to this solution. This cell is iso-osmotic, isosmotic to this solution. This cell is hyper-osmotic to this solution. Do you totally agree with this? This cell is hypo-osmotic to this solution, right? You're totally cool with that because all we're doing is comparing concentrations. What happens to the cell when we drop it in the solution? That's going to determine tonicity. Drop the cell in the solution. If the cell shrinks, it's hypotonic. I'm going to throw some examples at you in class that will challenge this because what I'm telling you is that everything changes if you have penetrating particles. Your tonicity changes if the particles can penetrate. What you think would be the case is no longer the case if particles can penetrate. So think on that. I've got about enough of this talking to myself in my office at this hour of the evening. So therefore, I'm done. And you are going to study osmosis more and more and more. All right, talk to you later.