 Okay, so organ systems work together to maintain homeostasis. That awesome. However, not every process carried out by an organ system is homeostatic. And so you have to have some tools in order to be able to tell what things are homeostatic and what things aren't. So first of all, all homeostatic systems have a regulated variable. And that just means there is something like pH, like blood pressure, like ion concentration, blood glucose concentration. There's some variable that is related to the extracellular fluid that's maintained in a relatively constant state. This is the first thing. The regulated variable must be related to the extracellular fluid. This, if you can't make a connection, there are all sorts of processes that have nothing to do with the extracellular fluid. And we're still going to study them, and they're awesome and super interesting, but they're not homeostatic. So it's really important to get that piece in your brain. Do you think I think it's important? Yes, I do. It must relate, there must be a regulated variable related to the extracellular fluid. The regulated variable must be, okay, the regulated variable, any changes in the regulated variable? That's a delta signal, and it means change. And so changes to the regulated variable must be detected by some sort of sensor. Now, this is vague. Holy cow, it could be any kind of a sensor. I'm just going to draw you a picture because my brain processes visually, and this is like a little receptor. This is Joe the Cell. There's his nucleus. And this is a cellular receptor. In my scenario here, whatever the regulated variable is, it's changes in the regulated variable. So changes in blood glucose concentration, changes in blood pressure are detected by a sensor. This sensor has to then generate some kind of a signal. It has to generate a signal, some sort of communication message as a result of detecting the change. So change happens, and the sensor has to generate a message. It's often referred to as an error signal because the message isn't going to be sent if you are in a homeostatic range. It's if you go outside of that, if things are looking bad, if you're going outside of the range, a signal is generated. The sensor sends a message. You tell me, who are we going to send the message to? This hopefully looks sort of familiar to you. We're actually going to send the message to a control center. What's the control center, you guys? Those of you who have had me for anatomy know that I'm obsessed with these kinds of pathways. This is your integrator if we're talking about a neural pathway or even any other kind of pathway. The integrator is the place where the error signal is sent. The message, the afferent path delivers often the error message. The integrator somehow has, it has to have a set point. So the error message comes in. The integrator has to have a way of saying, okay, that temperature is a little bit low from my set point. Set point implies that it's a one number, it's a range. Then we already talked about that. The integrator then sends or coordinates an output order or an output message, which is a command. It's a message to whom? Who are we sending the next message to? We're going to send the message to someone who's going to do something. And that is either my effector or target, excuse me. And the target is going to do something. And the do something is going to somehow impact the regulated variable. So the effect, the action that happens has to inform the variable that we're talking about in the first place. In a homeostatic system, it has to bring the regulated variable back into the normal zone so that the error signal is not, the error signal stops. We won't send that message anymore. That's actually a negative feedback loop. If the output, if the action carried out by the effector or the target is functions to remove the original stimulus, which was the change detected by the sensor, then that's a negative feedback loop. There are positive feedback loops. The positive feedback loops are going to increase the stimulus. Positive feedback loops I don't think are ever homeostatic. And we can talk about that more later. Okay, so in the next section what I want to talk about is like what kinds of things are regulated variables? Like what are we talking about here? Because you have to have some sense of what these, what are these regulated variables you speak of?