Hello anatomist and physiologists Michele Glass here and thank you for joining me for another one of our chapter one videos and the topic for this presentation is that long-awaited conversation on homeostasis. So when we talk about the body and when we talk about every body system we'll see that the main thing that's happening, the main physiological principle for the body, is this idea of homeostasis. And when we think about regulating homeostasis in the body even though it is the primary principle and all organ systems are involved we can really think about this as being the job of the nervous system and endocrine system. And remember the difference, one of the big differences, between these systems is that the nervous system is doing that fast adjustment whereas the endocrine (which I misspelled there yikes) endocrine system is more long-term regulation. And so what do we actually mean by homeostasis? What we actually mean by homeostasis is the internal body conditions these are regulated around what we call a set point. So the set point is the like optimal working conditions for the chemistry of your body. So there's a set point for your temperature right we already talked about that in humans as being you know on average 37 degrees Celsius or 98.6 degrees Fahrenheit. So this is an example of a set point. We'll see the pH of different parts of the body will have a set point so the stomach pH will be different from the small intestine which is different from the bloodstream and so forth. We'll have a fluid set point, we'll have electrolyte set point, we'll have blood pressure set point, we'll have you know an oxygen saturation set point. Right so all these different components of the body that influence the way that our cells work have to be regulated around a set point and so when we talk about homeostasis what we really see is we're going to keep it keep these conditions we could say balanced around a normal range. So if this is our set point here let's say this is our body temperature textbook average 37 degrees Celsius and let's say you were walking from your car in a hurry to get to class on time and so you're all wrapped up in your coat you're walking fast and your skeletal muscles are generating heat. And so your body heats up just a little bit and as you come into class and you have a seat and then the air conditioning is on and you've sweated a little bit in your walk to class and so your body starts to cool down. And so maybe then you put your jacket back on and then you're getting closer to your stable set point. Right? And so realistically we are fluctuating around that set point all the time. Another example of this is for me this morning I've been drinking quite a lot of coffee right which is a diuretic and so I have dehydrated my body a little bit. I've been making videos so I've been talking quite a lot and so my mouth is feeling very dry. My body is very dry so I need to take a drink. Just so you know I'm reusing my straw here. So I have a little scrubby brush and I clean it and so this unavoidable straw becomes a reusable straw right. Okay and so I'm rehydrating right and if I take on more fluid than I actually need, right? If I just gulp it down and take care of that thirst but I have more fluid than I actually need I just make a little trip to the bathroom. And so it's all being regulated in this way right so this is what we're talking about when we're talking about maintaining homeostasis. Now there's two different mechanisms that are involved in the regulation of homeostasis. So these mechanisms that are involved are called your negative feedback mechanism and your positive feedback mechanism. Now sometimes we get tricked up here because in English language we use the term negative to mean like "ew that's undesirable" right "that's bad" and then we use the word positive like "yay everything's happy and great" "we love it" and it's important to pay attention here that this is not in any way shape or form how these words are being used. In this case so when we talk about negative feedback what we're really doing is saying that we're negating this stimulus. Okay so when we say the term negative here we mean that we are negating the stimulus or we're going in the opposite direction. This is how some people will phrase it opposite direction of the stimulus or in our textbook it says that we are reversing the deviation which means from the set point. So what we're really saying here is the example of body temperature right where I said you jump out of the car you're racing to class you've got all your coat on and everything. And as your skeletal muscle is working to get you to class on time maybe too the pressure of like "oh I'm gonna be late" you know your body starts to heat up, right? And so a response to that would be some sweating, right? And so that sweating is important because it's helping to cool your body. Also your blood is going to move to your extremities into your skin so that some of that heat can be evaporated out or released out into the atmosphere, right? And so then as you sit down and you start to kind of cool down in the classroom because of those physiological changes right and the body starts to cool down and then maybe you need to warm yourself back up, right? And so you can make a change a conscious change to put that coat back on to warm your body back up. So in each case you were too hot and then there are changes to cool your body down so that's going the opposite direction. You're too cool and you make changes to warm the body up you're going the opposite direction. When we talk about negative feedback mechanisms I like to think about my set point here. And I go above and I bring it back down to the set point which in some cases takes me too low. And I come back up to my set point so you should think about or I like to think about a negative feedback mechanism as like a figure eight. And it's important to pay attention here that this is happening all the time. This is just a regular mechanism of the body so it doesn't necessarily mean, you know, you're way outside of that set point. It could just be these little adjustments based on your activity, based on what you've eaten, based on what you had to drink, and so forth. Okay on the other side of things we have positive feedback mechanism. And so this is kind of like the opposite sort of response. Now it's equally important. It's equally common in the body but in this case instead of saying we're going to go to the opposite direction we're going to go in the same direction of the stimulus. (I feel like that needs to be rewritten.) Okay we can also talk about it as intensifying the change but it is important that at some point it's going to end. So it's going to be a response that needs an end point. Now there are several classic examples that are given for positive feedback mechanism. The example I want to give is defecation because this is an example that we're feeling. And we're feeling this probably every day or so in our bodies. And we've been feeling this you know for all of, all of our time on the planet. Right so in the case of a positive feedback mechanism you can think of it as sort of like a snowball effect. So you're going along just fine, right? And then you might feel that fecal material has moved in toward the rectum. And as that happens you're going to feel pressure building. And that pressure information makes its way to the brain so you become consciously aware of it. As this response continues you have muscle contraction moving more fecal material down into the rectum. And so you're becoming more and more aware, more and more aware, more and more aware, right? And you're continuing this response so the signaling is more contraction, more movement. More contraction, more movement. More contraction, more movement. Until you're able to make your way to the bathroom, right? (I don't know why it looks like that) And end the stimulus, right? So upon defecation all that feeling, all of that buildup of pressure dissipates. Take a look in your textbook, remember it's linked in the video. Read the section on homeostasis and there are several great examples of both negative feedback mechanisms and positive feedback mechanisms. Remember this is the primary physiological process of the body so as we go through the course we'll be investigating specific examples of negative feedback and we'll be investigating specific examples of positive feedback. So this is something that you definitely want to have a good foundation for as you move forward in the course. Stay tuned also for our next video and as always take care of yourselves and each other.