Positive and Negative Feedback Loops


Hi. It’s Mr. Andersen and welcome
to Biology Essentials Video number 18. This is on positive and negative feedback loops.
If you’ve ever driven you’ve probably seen a sign like this. This is a guilt sign. So
they put the speed limit up here and then they show you how fast you’re going down at
the bottom. But you constantly get feedback as far as your speed goes. So you will see
that you’re going a little fast and so you’ll slow down. But then you’ll realize you’re
slowing down way too much and so then you’ll speed up. And if it works out well you’ll
hit that speed limit or that target set point. Now what I’m just showing you here is called
a negative feedback loop. In other words, you’ll dance around that point. You’ll speed
up, you’ll slow down and eventually you hit that point. So negative feedback loop brings
you closer to the target set point. A positive feedback loop you would experience if you
were to see this sign as a challenge. So you see you’re going 30 miles per hour. So you
realize I could go 39 or maybe 51. And eventually get a ticket or break the sign. So that’s
a positive feedback loop. It’s when you’re amplifying and moving away from that target
set point. So in this podcast I’m going to start by talking about homeostasis. And that’s our internal
environment. And to maintain a stable internal environment we use feedback loops. With each
of those you have a target set point. In a negative feedback loop you’re going to move
above or below that but try to stay as stabilized and so you’re as close to that as you possibly
can be. An example I’ll give you is temperature regulation in mammals. In a positive feedback
loop you move away from that target set point. So you amplify that. An example I’ll give
you is fruit ripening. Why all the apples on a tree seem to ripen at the same exact
time. Now with a feedback loop you can always have mistakes. You can always have alterations
in that feedback loop. And that can lead to tragedy. An example would be diabetes. It’s
a problem in creating insulin or sensing that insulin. And so that’s just a problem in a
feedback loop. And it can lead to really bad things. And so let’s start with homeostasis. Homeostasis,
if we were to define what it is, it’s an internal stable environment. And so if you live in
a pond, if you’re a paramecium like this and you live in a pond you let a lot of your environment
just go and you maintain that environment by just maintaining the same as the pond around
you. So you have this external pond, and whatever the temperature of that is, that’s the temperature
of the paramecium. Now they’ll regulate a few things like water concentration using
contractile vacuoles, but mostly they have a pond. Now as we move out of the pond or
as we become more sophisticated, we kind of bring that pond with us. That internal pond
now is what homeostasis is. And so this is a hairless cat. A hairless cat is going to
maintain it’s body temperature. And so it does that through feedback loops. It’s going
to maintain it’s blood glucose level. It does that through feedback loops. It’s going to
maintain the osmolarity of it’s blood. It does that through feedback loops. Now hairless
cat doesn’t have hair and so it’s actually hard to maintain it’s internal body temperature
and so you don’t want to let them go outside on their own. They’ll lots of times put a
coat on so they can maintain that. Now there are kind of two life strategies. In this picture it will take you a second
to figure out what it is. This is a snake eating a rat and it’s taken with a thermal
image camera. And so the snake itself is what’s called an ectotherm. And what that means is
that their internal temperature is going to be the same as their external temperature.
So the snake is about the same as whatever the counter that it’s sitting on. But the
mouse is going to be an endotherm. And so that is going to keep an internal temperature
that’s going to be constant. Now there’s some advantages if you keep an internal constant
temperature. Then all of the metabolism will work at the same exact rate. The problem with
that is that the rat right here is probably going to eat a lot more than that snake just
to maintain that body temperature. So let’s see how that works in a mammal. And so in humans we use a negative feedback loop
to maintain our constant body temperature. And so our constant body temperature is around
37 degrees or about 98.6 degrees fahrenheit. And so we use a negative feedback loop to
maintain that. So let’s say we put a thermometer in this kid’s mouth and the temperature all
of a sudden increases. Now you actually sense that, the area where we sense that is on the
roof or our mouth in an area apart of, an extension of the brain called the hypothalamus.
And so we’re going to sense that temperature change and so what’s the first thing that
your body does when you start to get too hot, is that you’re going to sweat. And so what
does that do? Well as these water molecule evaporate, that’s going to create evaporative
cooling. In other words they’re going to carry a little bit of heat with them. You’ll also
start to vasodilate. In other words the blood is going to be carried towards the surface
of the skin and so then just through convection, we’re going to start to lose more of that
heat to our environment. Now another thing that would happen if we had fur, which we
don’t really have is that that fur is going to lay really flat and the reason why is that
then we’re going to have more of that heat being lost through convection. So what does
that do to our temperature? Well, it’s going to drop. But it might drop a little bit too
far so then we’re going to turn those things off. We’re going to quit sweating and we’re
going to quit vasodilating. But now we’re too cold so what do we do? How do we keep
ourselves warm? Well we’re going to start to maybe get maybe goose bumps. And these
are kind of crazy goose bumps. But we’re going to get goose bumps right here and what goose
bumps do, is if we had hair it’d actually stand the hair up on end but it also kind
of pulls your skin in. It’s kind of like taking a coat and pulling it in. It’s conserving
that heat. We’re also going to vasoconstrict. In other words we’re going to shut off those
capillary shunts and we’re going to hold that body, that blood in towards the inside of
our body to maintain that. And as a result of that we’re going to have less convection
and our body heat is going to increase until we hit that target set point. And so right
now in fact, throughout the whole day your whole body is regulating your body temperature
and it’s doing that through a negative feedback loop. Again, trying to maintain that target
set point. What happens if we want to go in the opposite direction? Well let’s say we have this. So
we’ve got a fruit on a tree and we call that the target set point. But let’s say we want
to move from fruit on a tree to fruit that is ripe. Now why do trees make ripe fruit,
I could talk about that for a long time. What they’re essentially doing is making it attractive
so birds and humans are going to come and eat the apple and spread the seeds somewhere
else. But how do they maintain that? Well they communicate. And it’s kind of hard to
understand how a plant could communication but they communicate through a plant hormone
called ethylene. Ethylene is just C2H4 and it’s given off by ripe fruit. In other words
this gas is going to be given off by ripe fruit. It’s going to be picked up by apples
next to it. And then it’s going to cause them to ripen as well. And so if you take one nasty
over ripened apple and put it in a barrel of apples that aren’t ripened at all, they
will all ripen as a result of that ethylene. And so one bad apple can really spoil the
whole lot. But how is this an example of positive feedback? What goes on is that that first
apple will become ripe. And so it’s going to start giving off ethylene. And that ethylene
is going to be picked up by apples right next to it. And they’re going to create more ethylene
which is going to create more ethylene which is going to create more ethylene and so through
this positive feedback loop or amplification all of a sudden all of the fruit on the tree
are ripe at the same exact time. Another famous example of a positive feedback would be loop
would be in childbirth. The pressure of the head on the cervix of the mother actually
causes contractions which pushes more pressure on the cervix which causes more contractions
and so eventually the baby is born. And so usually we see positive feedbacks when we
want something to happen really really quickly. It’s not something that we maintain for a
long period of time. So what happens when something goes wrong? Or what happens when
there is a mistake? A great example of a feedback loop in us, so this is a negative feedback
loop, would be blood glucose levels. And so the blood glucose levels, the amount of glucose
that is moving around the blood in your body. Where else could it be? That glucose could
also be taken in by the cells so they can do respiration, we can get ATP from it, or
we could also store it in glycogen which is mostly going to be found in the liver. And
so we use two different hormones, insulin and glucagon to do that. And so this is your
pancreas. Pancreas is going to, its major job is to empty digestive enzymes into the
small intestine when there’s food there. And so we can break it, break it down or absorb
that, digestive absorption. But the also have a dual purpose. And one is to regulate the
blood glucose level. And so they have two types of cells in here. They have beta cells
and alpha cells. And so the beta cells and the alpha cells are just maintaining, they’re just
sensing the blood glucose level. And so this is an example of a beta cell right here. And
so this is our glucose transport. It takes glucose in, this would be cellular respiration
here and so if we have a lot of blood glucose outside of the cell, in a beta cell in the
pancreas that’s going to trigger an influx of calcium. But more importantly it’s going
to increase the amount of insulin that it’s giving off. And so what that means is that
when the blood glucose level is high, insulin which I’ll represent with this red kind of
a dot is going to be secreted from the pancreas. And that’s going to move throughout your whole
body and it’s going to trigger cells in your body to take in that blood glucose. And it’s
also going to tell your liver to store that as glycogen. And so when your blood glucose
level goes too high, insulin is secreted and that’s going to cause your blood glucose level
to go down. What happens when your blood glucose goes down, so dangerously down? Well then
your body is going to quit producing that insulin and it’s going to start producing
glucagon. Glucagon’s going to be created by the alpha cells in the pancreas. And so now
when the blood glucose level goes too far down, glucagon in added and that’s going to
increase the blood glucose level. In other words it’s going to free up glucose from the,
from glycogen in your liver. It’s going to increase the blood glucose so you can actually
use that throughout your body. Now let’s actually see how this works throughout the day. So
this would be in a typical person. What we’ve got here is the glucose levels in red and
so the glucose is going to increase. But it’s increasing three times a day and that’s because
you have breakfast, lunch and dinner. But if you look at this it dances like right next
to it. It’s a dance between the blood glucose level which is red. As it goes up the insulin
level is going to increase as well. What does that insulin do? It causes the blood glucose
to drop and then we’re going to increase insulin again and so we get this cycle throughout the
day of an increase in blood glucose, increase in insulin and then we keep that at that level.
And so if you look at it, our blood glucose levels throughout the day are going to maintain
a fairly statistic kind of position. Now if we were to eat smaller meals throughout the
day that would probably help us keep that together. But what happens if we have a mistake.
What if we have an alteration in this feedback loop? Let’s say for example you are a Type
I diabetic. Type I diabetic, the problem with type I diabetics is that they have beta cells
that don’t work. So remember the beta cells inside your pancreas are secreting that insulin.
But if you have type I diabetes, or sometimes called child onset diabetes, you have destroyed
these cells. It’s usually a genetic component to it, but it’s mostly an autoimmune disease
where you destroy the beta cells. Well what happens if you destroy the beta cells? Now
we’re going to have a feedback loop where the blood glucose comes up, there’s no insulin
to secrete and so blood glucose keeps going up and going up and going up. And so your
cells aren’t going to take in that blood glucose and so we have some nasty stuff that comes
as a result of that. So here are some things that happen. It increases the blood pressure.
It can eventually effect the eyes, you have nausea, vomiting. A lot of the ones in blue
here are for type I diabetics. But it eventually can lead to putting you in a coma or actually
death. So if you’re a type I diabetic that’s just a mistake in the beta cells in the pancreas
so we can’t make insulin. If you’re a type II diabetic that essentially means that you’ve
had too much glucose throughout your whole life. It usually is tied to lack of exercise,
obesity, things like that. Type II diabetics, the cells in your body just stop recognizing
that insulin and as a result they quit taking it in. And so how could we solve that problem
knowing what you now know? Well, if you were to get insulin shots like an insulin shot
right here or an insulin shot right here or an insulin shot right here, and insulin I
think was first synthesized or created in the 1920s but now we make it through molecular
biology, but if you could get insulin shots throughout the day, then you can regulate
that blood glucose. Or now they use an insulin pump which is going to administer different
amounts of insulin throughout the day. So it works almost as a feedback loop. Now this
is a map of where diabetes is on our planet. About 3% of the people in the US or worldwide
have diabetes. But 90-95% of that is not a mistake in the beta cells, it’s actually tied
to our diet, type II diabetes. And this is a chronic disease. Once you get diabetes you
have it the rest of your life. And if you look here, where are we seeing the greatest
incidence of diabetes? It’s just tied to our diet. And so we’re eating a high fat diet
and as a result of that the diabetes, a high sugar diet, corn syrup diet, and as a result
of that we’re getting huge diabetic increases. But the whole thing is tied to a mistake in
a feedback loop. And so that’s feedback loops, positive and negative and so I hope that’s
helpful.

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