 Okay, so I have a definition for you up here for what homeostasis actually is, and it's good to have a sense. So let's dissect this definition so that we can break down the nuances. Homeostasis is the maintenance of a relatively constant internal environment, and organ systems work together to maintain homeostasis in the human body. So notice that we're talking about a relatively constant internal environment. So in a homeostatic system, it isn't like your body temperature is maintained at 98.6 all the time, and if you go to 98.7, you are now out of homeostasis. You're in a relatively constant maintenance, which means you kind of go all over the place, you kind of range within a set range, and the parameter that you're maintaining in the internal environment stays mostly constant in that zone. That's an important point. Another important point, possibly more important for our original discussion of homeostasis is this concept of the internal environment. What is that? Hopefully you are sort of going, it's inside the body, yeah, but we can actually define that the internal environment very specifically. The internal environment, okay, well, let's start out by looking at the fact that basically you are a bag of fluid. So I'm going to draw you a picture of the human. Look, the human, and watch. Oh, yes, that is the human. Now, in the human, approximately 60% of the human is fluid of some sort. And the other 40%, what is the other 40%? Now think about this for a second. Think about connective tissues. Oh, go back to your anatomy days and think of what is a connective tissue? Remember that connective tissue is cells in a matrix of ground substance and fibers. And the ground substance was fluid. It's a specific kind of fluid. Think about your cells themselves. Your cells are little water balloon bags of fluid. You have cytoplasm, that's a kind of fluid. Think about your blood. Your blood is the extracellular fluid that's found surrounding blood cells and flowing through your veins. So about 60% of you is fluid. That fluid, that is your internal environment. However, you might be like, dude, that was easy, perhaps, perhaps it is so. But I'm going to show you that actually the fluid that we're talking about, the 60%, there's different kinds of fluid. And we're going to break it down because functionally, scientifically, there's a couple of fluid compartments that are easier to monitor and measure than others. And so when we're talking about the internal environment, we're going to talk about a specific part of the fluid in the human body. So I'm just looking at the fluid now. You know that the other 40% is like, dude, it would be the fibers. It would be the matrix, the stuff that isn't watery. Dude, you've got lots of stuff that isn't watery. Think about your organelles. Think about the cell membrane. You could deflate it. And then you'd have a little solid pile of deflated cell membrane. That stuff is not fluid. Okay, the fluid breaks down into two flavors. And the two flavors are separated by a structure that I'm going to draw, and you have to guess what it is. All right, ready? Ready? Okay. Go ahead, start trying to guess what is this structure. Mm-hmm. You might not be able to guess it yet, especially if you don't know me very well. Don't worry. If you know me at all, what am I drawing? What? The little people with legs standing on their heads. Oh, now I'm drawing people standing on the legs of the people who are standing on their heads. What? What is that? Come on, you can do it. It's a cell membrane, right? The cell membrane? Yeah, those are my phospholipids. Yeah, yeah, yeah, the phospholipid bilayer. You know you've got that. The cell membrane separates my two fluid compartments. So what do you think my fluid compartments are? Well, 66% of the 60% of fluid that makes up the human body, 66% of that is intracellular fluid. It's found inside the cell. So I'm going to say intracellular fluid. And cytoplasm is another name for this intracellular fluid. We're going to call it ICF, but it's just cytoplasm. 66% of the fluid in your body is intracellular fluid, so two-thirds of it. But functionally, you can't really tell a whole lot about the state of the intracellular fluid. Different cells with different functions have different characteristics in their intracellular fluid. So maintaining a relatively constant internal environment really doesn't deal with the intracellular fluid. If you wanted to measure a quality of the intracellular fluid, you'd have to consider where you are, and then you'd have to measure every single cell. And that's kind of crazy. Why not just measure, what do you think? The 34%, which is the extracellular fluid. Extracellular, that says extracellular fluid, and I'm just going to tell you, I'm not going to write fluid, but it's ECF. Now, extracellular fluid is found outside of the cells. The ground substance in connective tissue is extracellular fluid, but extracellular fluid has two different flavors. So if we were to take our extracellular fluid and do a little blow-up version, what? A blow-up version? Nobody knows. You're going to end up with two kinds of extracellular fluid. One of them, approximately 25% of the extracellular fluid is blood plasma. So we refer to that as plasma. So if you hear us talking about plasma, we're talking about blood plasma. And that's an important extracellular fluid, which means 75% is a type of fluid called interstitial fluid. And interstitial fluid, I would argue that blood plasma is actually a specialized type of interstitial fluid, but interstitial fluid is the fluid between cells in the tissues. So you'll have your intracellular fluid, your cytoplasm, and your extracellular fluid, which we have two flavors that we sort of keep track of. Back to homeostasis. Homeostasis is the maintenance of a relatively constant extracellular fluid. And I just want to make a note that this right here was the fluid in the body. So we have the human, the fluid that makes up the human, and the extracellular fluid. And we've broken those down. The extracellular fluid is where we maintain homeostasis. Now, if we're going to maintain homeostasis, and I told you that organ systems do this, they have to have a way of coordinating action. And so I want to talk about that theme in Physiology in just a second.