Hello and welcome to the Penguin Prof Channel. In today's episode, I'm going to talk about homeostasis and feedback. We're going to get into what homeostasis is, how it's maintained, and the components of feedback loops. Before we get into it, you know the drill. I got to ask for your support.
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Homeostasis is absolutely essential for life. If you lose homeostasis, the result is disease. This is a word a lot of people don't think too much about, but it literally is dis... meaning not to be at ease.
You're going to see in most textbooks a definition of homeostasis something like this, the dynamic constancy of the internal environment despite constant changes in the external environment. So we're going to explore both aspects of this, starting with the internal and external environment idea. So here's your internal and external environment, right?
Inside of you is you and everything else is not you. In my little cartoon showing some but not all obviously of the body systems, the idea is that there are so many variables that our bodies have to control all the time. So there's only a few of them shown here. But the thing is we are living in an environment that is constantly changing. And this really presents a lot of challenges for the body, right, to maintain an internal environment and to maintain a healthy environment.
that's relatively constant despite all of this chaos, you know, that is around us. So that's the internal and external environment. The dynamic constancy might also leave you a little bit confused because if you look at the terms, doesn't it seem kind of like an oxymoron, right? Something that's dynamic is always changing. Something that's constant is always staying the same.
So what the heck is this about? this means is that the variables of the body are maintained within limits. So there is tolerance, but it's not like they're static and completely unchanging over time. In other words, it's not a flat line.
Okay. You know what a flat line is. Okay. That's, that's no good.
You're dead. Try this at home. If you want to experience what I'm talking about, you're going to need only a couple of things, a timer, something to write with.
You're going to need yourself. That's always good to have around. What you're going to be doing is measuring your own.
resting heart rate over time. And easiest way to do that is to use your radial pulse. Now you need to sit quietly, do nothing, try not to have very exciting thoughts.
And you would expect that if you're just, you know, sitting there being calm, your heart rate should be the same, right? I mean, it's not like you're getting up and running up a flight of stairs. So over time, most people would expect that their heart rate is constant. You might be very surprised to find out. that that's not actually what you are going to get.
As you sit very quietly doing absolutely nothing, your heart rate is going to be changing. It is dynamic. It is going up and down and up and down, and you're just sitting there.
And this is surprising to a lot of people. If you were to connect the dots, you're going to get something that looks like this. Now, the thing to notice is, yes, the data are constantly changing, but they are going up and down around.
what we actually call a set point value. This is the idea of dynamic constancy, something that is always changing, but within limits around this set point. And lots of variables in the body oscillate, go back and forth just like this. Even things like secretions, the pancreatic secretion of insulin oscillates every three to six minutes. Body temperature, which we're going to be looking at, goes up.
up and down around a set point value. And hopefully now you're thinking, how does this happen? And that's what feedback is all about. So that's the connection between homeostasis and feedback.
We're going to look at feedback right now. You all know in some general sense what feedback is. Feedback is what you get from your instructor when you submit an exam or an assignment, and it tells you what to do next.
So if you do well... You're going to do the same things, right, to keep doing well. And if you didn't do so hot, you've got to change what you're doing. So in a general sense, any system is going to have inputs and outputs.
And if you take a sample and you measure the outputs, and that's usually done by something called a sensor, the sensor feeds that information into some sort of feedback system. The feedback system analyzes what the variable is doing. and compares it to what it should be and provides feedback, right?
It feeds back into the system, into the inputs. So an easier way to think about this, what I tell my students is feedback is simply this. What happens affects what happens next. So what I'm saying is that something happens. The sensor says.
I saw what happened. I collected that. I know what happened. And the feedback system says, oh, that happened? Was it okay?
Is that what we want? Is it not what we want? If it's not what we want, then we're going to give instructions for change.
Okay? So that's basically the idea. In physiology, it's the same thing. We have slightly different terminology.
But we take our data and the data is collected by a sensor or a receptor. So the variable in this case could be heart rate, body temperature, blood pH, whatever, and you're going to have sensors in the body constantly collecting that data. And that information goes to an integrator.
The integrator for most physiological systems is going to be some component of the nervous system or the endocrine system. And the integrator compares what is happening now, what does the data look like right now, to what should be happening, meaning what is the set point value. And if those two are too far apart, if the data is far away from the set point value, effectors get switched on. And effectors, actually very well named, because they bring about an effect, they are the ones that provide the feedback. And that is how it works.
Now there are only two different types of feedback, positive and negative. And students do get confused about this because we tend to attribute positive and negative with good and. and that is not the case here. So I want to show you what positive and negative feedback actually means. So the variable changed, okay, these are the two options for feedback.
In negative feedback loops, the effectors oppose the change. So the variable gets pushed back toward the set point value. In positive feedback loops, it's the reverse.
Effectors enhance the change. So the variable is pushed even farther from the set point. And of course, we're going to show examples of both. In negative feedback, the goal is to maintain homeostasis, okay, because any deviation of the variable is going to be corrected, and so that will keep things within a narrow range.
And we're going to look at thermoregulation as an example. Thermoregulation, the integrator is the hypothalamus, which is approximately here in my little cartoon. The set. point for body temperature is 37 degrees Celsius for us and we have sensors for body temperature in the skin and also in the hypothalamus itself where we actually sense the temperature of the blood.
The integrator, like I said, is the hypothalamus and check out all these effectors. So we've got smooth muscles in the vessels. They are going to control how constricted or dilated our vessels are.
We've got sweat glands. We've got little erector pili muscles. Those are the little guys in the skin that control goose bumps and actually they make your hair stand up. The skeletal muscles which can contract to shiver if we are really cold and the adrenal and thyroid glands which control metabolic rate.
So let's see what happens if the body temperature falls. So sensors detect this. And that information goes to the hypothalamus.
The hypothalamus will then activate all of these effectors. You get vasoconstriction. Those little erector pili muscles contract.
The skeletal muscles contract. And the adrenal and thyroid glands are stimulated to increase metabolic rate. Now all of these things will act to increase body temperature.
And that's the negative feedback part of this. So. as the body temperature goes up, the sensors will then sense that and the integrator gets that information and then will shut down all of the body warming processes that it had turned on.
So I hope it makes sense now why these variables would oscillate and go back and forth between what we call upper and lower tolerance limits. around a set point value. Every time that upper tolerance limit is reached, the effectors are going to be switched on, and the same thing is true with lower tolerance. Now, by the way, we call this type of control antagonistic negative feedback loops because it's controlled on both the upper and lower limits.
Just so you know, not all variables have that. So negative feedback loops are stabilizing. And one way to think about that is to say the more you have, the less you get. We're going to compare them now with positive feedback loops. So positive feedback loops destabilize the system.
And they are used when we need to do something extreme. And we're going to look at the example of childbirth. Can't get much more extreme than that. So the baby pushes against the cervix causing it to stretch. The stretching of the cervix causes nerve impulses to go to the brain which causes the pituitary to release oxytocin which causes the uterus to contract which increases the baby pushing against the cervix.
And so what you see from this is more is more. So the more you have, the more you get. And the more you have, the more you get. And other examples of physiological systems that work this way. blood clotting, the immune response, and the upsweep of the action potential, these are all really big and very dramatic things.
Here's another way to look at it. So with a negative feedback loop, as the variable gets pushed away from the set point value, you're going to reach an upper limit, and the effectors will kick in and push the variable back down. And that may happen on the lower limit as well, and you're going to go.
within the range, that's fine, and it's going to go up and down and up and down like this. With a positive feedback though, as the variable pushes away from the set point value, those effectors are going to enhance that. So the farther away you get from set point, the farther away you get from set point, right?
So the more you have, the more you get, and the more you have, the more you get. And this is going to give you a very destabilizing effect. And with positive feedback loops, that is the point. So you might be wondering, how's it going to end, right? Because nothing is going to go on like that forever.
Some positive feedback loops are self-terminating. Obviously, that's true with childbirth. Oftentimes, other feedback loops will be activated and will shut them down. So in review, homeostasis is a dynamic constancy of the internal environment. So something is constantly oscillating around a set point value.
Remember, there is never a flat line unless you're dead. And this is maintained by negative feedback loops. When homeostasis is lost, the result is disease.
Feedback simply means what happens affects what happens next. And in negative feedback, the more you have, the less you get. That's what maintains homeostasis. And with positive feedback, the more you have, the more you get.
And that is destabilizing. And it is used for big and extreme body processes. And ladies and gentlemen, That's it. As always, I hope that was helpful.
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Good luck.