>> Hello and welcome to the PenguinProf 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.
Please take a second and feed the Penguin.
It feels so good.
Just click those buttons before like share and subscribe.
I also want to thank audible
for sponsoring the PenguinProf channel.
You can click the link below
and download a few audio book of your choice.
So here we go, homeostasis is a central theme for physiology.
And I want to talk about what it is
and what happens when we lose it.
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 disease, 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.
And my little cartoon showing some but not all, obviously,
of the body's 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 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?
So what 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 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 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 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
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 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 is 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 bad,
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 goosebumps,
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 are 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 look,
as the variable get 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 and 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.
Thank you so much for visiting the PenguinProf channel.
Please show your support by clicking those buttons below
like share and subscribe.
Join me on Facebook.
Follow on Twitter.
Good luck.