>> 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,
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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
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