Biology: Mechanisms of Homeostasis

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I want to ask you guys a question. Did you ever wonder why bees don't freeze in the winter? Think about that. Bees live all in northern climates and yet they don't freeze in the winter. Well, you're probably saying, "Why do I care and what does this have to do with anything?" But remember here, we're talking about homeostasis, and homeostasis is this whole idea of regulating yourself. I'm going to tell you; I'm going to answer that question about the bees in the winter, but not right now, so you just stay tuned and maybe you'll come across the answer for that. You can amaze your friends and relatives with that trivial bit of knowledge.
Well, anyway, let's talk a little bit about homeostasis continuing to spiral in to this whole idea of--remember what it means--stable. You want to keep your multi-cellular system, your thick system, with all these cells, stable.
Let me use another analogy for you here. You're in a room. Maybe it's a party and there are hundreds of people in this room and you're all packed just person to person, kind of like cells in a tissue. You decide that you want a shrimp, and the shrimp are way over on the other end of the room. So what are you going to do? Are you going to push your way through? I don't think so. What you're going to do is you're going to do communication. You're going to talk to the person next to you and you're going to say, "Excuse me, I'd like a shrimp." And they're going to talk to the person next to them, and the person next to them, and you're going to have this little chain reaction, and somebody, probably me, will be standing next to the shrimp, because I'm very wise about where I position myself at a party, and maybe, just maybe, I'll be able to pass you a shrimp. That's homeostasis. We're getting along. We're communicating, you see. We're talking to each other, and I'm helping you out. You're going to owe me later, but that's okay. We'll talk later about that. But you see, homeostasis is about communication to maintain stability.
Now you're saying, "All right, come on Wolfe, what's stability?" Your stability may be somebody else's insanity. What's up with stability? What does that mean? Well, you see, multi-cellular organisms have something called the set point, and it is this set point that is defined as stability. In fact, the set point is what we call the optimal condition for any given situation. The set point is optimum. Who determines optimum? Natural selection. If you don't meet the optimum condition, chances are you might die. Sorry, but that's the way it works.
You know, organisms never would have survived or evolved on this planet if when you violated the set point they died. It's homeostasis again, because what happens when you get to that set point and violate it is you've got to fix it and if you don't fix it, you can die. Your environment may beat you. So let's take a look at what happens generically, generally, when you violate the set point. Let's talk about violating the set point.
Violation--it sounds horrible, doesn't it? But you violate a set point. What set point? We'll see some examples later. Well, a violation of the set point will usually cause some kind of environmental change, or it may be the violation itself, which is the environmental change. It could be an internal environmental change. Whatever, things are going to change. So you're going to get what is called an environmental change. By the way, the definition of that we call a stimulus. So some kind of stimulus is going to happen. The stimulus is usually going to go to a receptor. Again, it could be internal, it could be external. The receptor may have attached to it or may connect to a control center. You guys already have probably figured out that often in our bodies will be our brain, but it could be a gland. And then that control center will cause effect, and the effect is effected by an effector, which kind of makes sense. So violation of the set point by some kind of stimulus, goes to a receptor, goes to a control center, and we get an effect. Let's take a look at some of this.
In homeostatic mechanisms there's two different ways we can do this. I want to talk about an extreme change, something that we are going to call a "positive feedback loop." Now, I'll show you why it's called positive in a minute. You know what? I've got a better idea. Let me give you a definition of what happens in positive feedback and then you can tell me why it's called positive feedback. I'm going to tell you that in positive feedback the stimulus causes a response that exaggerates the stimulus. Well, does that make sense? What kind of homeostatic mechanism is that? You've got a stimulus, it's changing your environment, so you're going to make your environment change more? It doesn't sound very sensible to me. Well, maybe when we give you this example you'll feel better about this.
I want to talk about something that all of you have been through. There's not a person on this planet that hasn't been through what I'm about to talk about. Everyone watching me right now has been once born, and most of you came from a mother--I would assume all of you came from a mother, I hope. And the point is that you were once this thing growing inside of her uterus, and some of you may have these things growing inside of you right now, and what's happening to your uterus is it's stretching--or what you did to your mother's uterus, you bad children, is you stretched her uterus. Well, what's going to happen is you are going to get to the point where your mother goes into labor. What happens is her uterus stretches. So here comes the stimulus. Watch this. Distortion of the uterus. Well, guess what? When the muscle fibers of the uterus distort, there are receptors, and there are receptors called "stretch receptors" in the uterine walls. And these stretch receptors connect to the brain. And the brain is connected to the endocrine system. But here's your control right here. And then what happens is the brain sends a message to the uterus. What does it say? Get that thing out of here. Squeeze. So what starts to happen is the uterus starts to squeeze and so you get contractions. Because the endocrine system produces this hormone--you'll hear about oxytocin another time, but it causes these contractions, and guess what that does? It causes the uterus to distort some more, which causes the stretch receptors to say, "Oh my God," and so the brain and contraction, and round and round and round, it becomes what is called a "positive feedback loop." Look back at our definition. What was the stimulus? There's the stimulus. What is this making? The stimulus is getting worse or better or more, and it's actually enhancing the stimulus--a positive feedback loop. What takes this away? You pop the baby out and the stimulus is gone. So until the stimulus is taken away, you're in this loop, and some of you know that could take two or three days. I don't see what all the complaining about is. I've had six children and it didn't hurt a bit. I probably just got myself in real big trouble with my wife, but anyway, here we go.
Let's talk about other things. What are some other little cool homeostatic mechanisms? How about temperature? Temperature is another good example. This one is going to be a little bit different. This is not going to be a positive feedback loop. Watch this. I'll tell you the definition again and let's see if you can figure out why it's called this. It's going to be called a negative feedback. Now, why negative feedback? Well, in this case, and this is the most common regulatory mechanism in homeostasis. In this case what's going to happen is the effector, the thing that produces whatever it does, in essence is getting rid of the stimulus. Now, this kind of makes more sense. It eliminates the stimulus. Why? Because the stimulus is messing with you. The stimulus is messing with your stability and you don't like that, so it's going to eliminate the stimulus.
Let's use a widely used analogy and then we'll get into organisms. Let's talk about my house. Now, I like to keep my house at 68 degrees. So I have a thing called a thermostat. What I do with that thermostat is I set it at 68 degrees. We'll do a little graph over here so we can watch what happens. So we'll do 66, 67, 68, and 69 degrees. So my thermostat is set at 68 right here, and that's where I want it, but now we start to cook. What's happening? Heat. Heat is the stimulus. So first I get heat. When heat comes along, what happens next? Well, obviously my environment is changing. So look what's going to start happening. It's going to start going up. And so what happens when I get to this point right here, on comes my air conditioner. There's my response, and once my response happens I start to eliminate the stimulus, and what was the stimulus? The heat, and then I get coolness.
And then, look at this. This is called an oscillation. It's going to oscillate around the set point. It's going to go up and down and up and down. But there are the two things I want you to understand. There's an oscillation around the set point. It's never perfect, number one, and number two, you always are eliminating the original stimulus and maybe the feedback.
Well, that being said, let's take a look at the way some critters do this. Humans--think about it--this is an easy one. Thirty-seven degrees, body temperature--what happens? You start to warm up. Why do you warm up internally? You exercise. So you run upstairs. What do you start to do? Well, think about it. What are the results of running in terms of temperature? You get these temperature sensors right in your brain. There's your control mechanism. There is a temperature control mechanism right there. So as your body organs start to heat up and these thermals receptors all over your body pick up this thing. They're little thermostats. They send a message to the brain, the brain says, "Whoa, we're getting too warm. We're going to mess up our proteins here. What do we need to do? Let's sweat." Why do we sweat? Evaporation cools you down. Why do you turn red? You turn read because your blood vessels open wider and you become a radiator. You radiate heat. Just go stand next to someone who has been exercising. The heat is sucking up your cold is what it's doing. They're hot. They're radiating heat.
Now, all of this being said, I have to ask you a question. That sounds like a horrible life. You and I are what we call "endotherms." We actually heat ourselves up from inside. And you know, if you think back over the course of evolution and where life started, it started in the sea. And you know, think of this whole idea of evolutionary history. Water has a very high specific heat. Now, if water has a high specific heat, why did those foolish creatures ever leave the water? Think about it. It rarely changes its temperature. If you're an ectothermic organism, your body is cooled or warmed by something that barely changes its temperature anyway. Why did creatures leave the water to become endotherms? Well, you know, there are good things to being an endotherm.
Let's talk about why being an endotherm is cool. To be an endotherm you get to burn more energy so you can be faster, you have a higher metabolism, you can escape predators, you can chase things. But there's a trade-off, because an ectotherm is slower, but they have a lower metabolism. Let me show you what I mean by that. You and I, being endotherms, and being human, burn energy all the time. So let's just say it's 20 degrees right now. A human at 20 degrees Celsius burns about 1500 kilocalories every day. That's you and I. An alligator, an ectotherm--things we call kind of cold-blooded. I've got to tell you, I don't like that term that much because a lizard on a rock on a 100-degree day has a higher temperature than you and I do because he's warming himself on the rock. More on that in a second. But let's look at what this alligator does. This alligator uses a whole lot less energy. How much less? About 60 kilocalories per day. Trade-offs. He doesn't have to eat as frequently as you and I. And then he can just kind of like hang out, which sounds like kind of a nice life to me.
Well, all of that being said, let's take a big picture look at the different ways that organisms have of regulating their internal temperature. The first thing you can do--and a lot of these go for both, ectotherms and endotherms--the first thing you can do is you can adjust your rate of heat exchange. That's the first thing you can do. You know, if you've watched a lecture on counter-current exchange, you know how that works. You can adjust the rate of heat exchange. So this is Wolfe's list of how to regulate your internal temperature, or just the rate of heat exchange.
Do you ever wonder why your dog's nose is cold and why is it cooling down? Well, it's evaporating, but the blood passing by that nose, and that nose is alive, must be cold, too. And if that gets cold, how does it get warm again? Did you ever wonder how something like this creature right here--this beaver's arm--you know, a beaver's hand is exposed to really cold water. Beavers live like in Canada. Their hands get cold, and the blood passing through that hand gets cold. Well, counter-current exchange, the same with the dog. The fact is that warm blood coming in cools down, and then as it goes back, look what it's doing, it's passing by--counter-current. How cool is that? Okay, counter current to the warm blood, and it's warming back up so it doesn't chill your organs. That's very important when you're a dog's nose, or a beaver's hand, or related to those structures.
The second thing you can do--you can evaporation. Evaporation is a way to do this. This is obviously something that endotherms do. We sweat. We pant. Well, dogs do. I don't walk around panting. That's not socially acceptable, but it would cool me off if I did.
The third thing is behavior. Hey, I have a question for you. Why don't bees freeze in the winter? There's an obvious answer to that. They hang out together. You see, bees have the ability to actually generate a little heat by muscular action, and so they get in this big old thing like the people in the room, and the ones in the middle are generating all the heat so it radiates out. The ones on the outside, a couple of them may die, but you know, it's a whole hive. And so bees don't freeze in the winter. These ectothermic things sit outdoors all winter long, but they keep each other warm. Lizards--what do lizards do? They go on a rock to warm up, or if it's too warm, they go hide in the shade.
And last, but not least, you can change the rate of production. Not of exchange, but of production. What do you do when you're cold? Well, you put on clothes. But if you're really cold, you shiver. Why? You're moving muscles. And when you move those muscles you're generating heat. Or you get goose bumps. When your great, great, great, great, great, great grandparents were a little bit furrier and their goose bumps actually made hair stand on end, but goose bumps are a way to contract muscles. I could go on and on, but the bottom line is this: Homeostasis rules, and it rules everything in your body. In this case we saw a very cool way of looking at that.
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