 Now, in a pathway, if the action, the response, put a big ol' red circle around this thing, this is my response. Ooh, I don't need an arrow there because that's misleading. If the response to, at the end of the pathway, the response feeds back and decreases the stimulus. That's a negative feedback loop. Do you see my loop? The response goes back to the original stimulus and says, slow your roll, we're done. It removes the stimulus. The response makes the stimulus go away. If the stimulus is holy hotness in here, the response is, sweat up a storm and the sweating cools me down so that it isn't so hot in here. And the stimulus, the hotness, has been removed. Sometimes, now, if the response removes the stimulus in a negative feedback loop, it is possible if it is a homeostatic pathway that we can maintain homeostasis that way. We can sort of, if you go out of the range, you can negatively feed that back and get back to the set point. Not all negative feedback loops are homeostatic. But if there is a feedback loop, if there is a homeostatic pathway, there most likely will be negative feedback. I say that and I always love the general statements that I make where it's always this way and it's never true in physio. There's always exceptions, but I wonder if there are any exceptions to that. If there are any homeostatic pathways, they don't exhibit negative feedback. So if you can think of one, hala, hala, because I want to know. I can't think of any. So we have negative feedback loops that can be homeostatic, but not all negative feedback loops are. You have jillions of negative feedback loops that have nothing to do with homeostasis because they have nothing to do with the extracellular fluid. Awesome. You also have positive feedback loops. Sometimes, okay, so think about this. What's a positive feedback loop? Dude, it's when the response increases the stimulus. Think about that. So the response, if a positive feedback loop was happening when I got hot, I get hot, I start to shiver. The shivering, the response, this would be a bad idea. The shivering response actually increases my body temperature. So now, my body temperature has increased even more, which initiates the whole pathway, which causes the response more shivering, which increases my body temperature even more. That's a positive feedback loop. The response increases the stimulus in a positive feedback loop. Most of the time, positive feedback loops are bad news. Like, they're not, you're going to die. If my shivering increased my body temperature and then increased body temperature caused shivering, I mean, that's like, that's crazy and I'm going to die, which is why we don't have critters that have strategies like that because they all died. There are some positive feedback loops, though, in your system and you didn't die. Thankfully, I've done it twice. Birth, holy positive feedback loop. In the process of giving birth, you actually, I mean, it's insane, so thank your mother for going through that process for you because that's how you got here. The uterine contractions push down on the little human that's in your belly so that the little human's head, like, smashes into your cervix, which is the opening at the base of your uterus. How you get out of your very first apartment in your mama's uterus and that pressure on the cervix causes stretching in the cervix, which is a stimulus, which has a response of increasing uterine contractions. Brilliant, which causes increased stretching, which increases contraction, which increases stretching, which increases contraction, which is how you end up, like, what? Pushing a giant thing out of you. And it comes out eventually. So there is a cut-off point to the positive feedback loop because you get it out. And at that point, once you get it out, then you're golden. Then you're like, the whole system stops you. The stimulus has gone away because it came out. Thank goodness. Birth, thank you for positive feedback loops. Blood clotting is another example of a positive feedback loop where the chemicals in the blood that actually cause the clot to form initiate additional production of chemicals that cause the blood to clot, which initiates additional chemical production that causes the blood to clot. And you can imagine that that would be a total disaster if that one got out of control. Thankfully, we have strategies in the blood vessels that prevent that, like, crazy, like, peace out, blood clotting forever because that would be terrible. Okay, so we're going to mess today in lab. We're going to have pathways. Like, the whole day is pathway fun times. Fun times, I say. And so you're going to get lots of practice with this concept. You're going to get lots of practice with this concept for the rest of the semester. And now I'm out of here. Bye-bye.