 Okay, so all living things, not only do they demonstrate this hierarchical organization, all living things maintain homeostasis. Maintain homeostasis, homeostasis. What is that? Homeostasis is going to be a word that describes something alive. So can carbohydrates maintain homeostasis? No, they cannot because they're not even alive. You can't maintain homeostasis unless you're alive. Homeostasis specifically means the maintenance of a relatively stable internal environment. Okay, ready? It's the maintenance of a relatively stable internal, what it says, internal environment. Living systems maintain a relatively stable internal environment. Give me an example. I am a living system. Tell me an example of my living system maintaining a relatively stable internal environment. Dude, we could go on all day long. Temperature, body temperature. I tend to run on the cold side, but even when I'm cold, my body temperature doesn't drop down to 50 degrees. I'd be a done deal if it did. I maintain a relatively stable, like 98.6-ish internal body temperature. Like my blood is that temperature. My blood, my blood is like my internal environment. Do you kind of, can you visualize that? And so I need to maintain like glucose concentration in my blood. If it goes too low, I start feeling all light-headed and funky. If it goes too high, I start getting all wonky and woodly doodly. And you might be wondering frequently during this class if my blood sugar is out of balance because clearly I am woodly doodly often. Blood pressure is an example of a characteristic that has to be maintained in a homeostatic range. Who's doing this? Who maintains homeostasis of a critter? Oh, you ready? Watch. I'm going to do a like this color, star, next to my maintaining homeostasis. Now we're going to go look at the picture that we just drew. And I'm going to tell you who's doing this, dog pounds. It's the organ systems. It's at the organ system level of organization and any critter, okay, I say that, that's human because there are some living critters that don't have organ systems, but they definitely have to maintain homeostasis as well. So for human-ish critters, then we're maintaining homeostasis at the organ system. Our tissues, one of the purpose of tissues is to help maintain homeostasis beyond just the cell. So actually all of these living levels of critter must somehow maintain a constant internal state. Otherwise you're at a complete whim of the environment. If the environment changes, you're screwed unless you can maintain your own space. And sometimes the environment changes and you are screwed and then you die. If you can't maintain homeostasis, you die. The inability to maintain homeostasis is why we get sick. Homeostasis, we'll study it a little bit. We'll bring it back. We'll talk about it a little bit more in this course, but really that's the purview of human physiology. And if when you take that class, you will spend the entire course talking about how the organ systems maintain homeostasis. There's more. The next character that we're going to talk about is the fact that living systems use energy.