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- With all these tissues
we have to also talk

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about how they work together,
how all tissues interact,

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how they make sure they
balance each other.

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What do they do when they form
this multicellular organism?

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Well a multicellular organism, like human

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or any other animal
that has numerous cells,

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must maintain homeostatis.

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Now homeostatis is pretty
important in the body.

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If homeostasis is forgotten or neglected,

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the body can get into
trouble really quickly.

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So here's a kind of textbook
definition of homeostasis.

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Maintenance of relative constancy

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of the conditions of the
internal environment.

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Well it's a whole lot more beneficial

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if you put that into your own words,

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so think homeostasis, keeping
your internal systems in check

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or balancing off your internal
workings, or whatever.

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The idea is put it into your
own words so you understand it,

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and a lot of homeostasis is maintained

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via a negative feedback control system.

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Well, negative, doesn't sound very good,

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but negative is very important

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because a negative feedback
system is any system

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when homeostasis is going out of balance.

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So a deviation from normal is detected.

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Those deviations, those abnormalities

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will send a negative signal back.

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The body says, hey,
there's a problem there.

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Let's fix it.

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So any deviation from normal can be

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detected and counteracted.

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So, for example, your blood pH.

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Now, typically, blood pH
falls about a 7.35 to 7.45.

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Not a very large range.

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Well, if all of sudden,
your blood pH starts

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hitting about a 7.36

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or even a 7.35,

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right at that bottom
level, you'll start having

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this negative feedback cycle kicking in.

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The negative feedback will say, hey,

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we're at the bottom limit of normal here.

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We're having a deviation.

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We're having a problem.

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Let's counteract and make
sure it's working properly.

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So there's plenty of negative
feedback systems in your body.

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So the first part you must
have is a control variable.

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You can pick something as simple

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as a hormone in your system.

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Growth hormone, you've
probably heard of that,

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makes you grow.

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Well, the growth hormone should have

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a certain amount being
produced during childhood,

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and for that amount, a little less

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during adulthood, but you still have some.

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Let's stick with child.

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So growth hormone must be controlled.

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Otherwise you'll get unusual growth.

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Well you must have a
sensor, something to realize

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if there is growth hormone present.

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Control center.

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Think of that one as
your brain's spinal cord.

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Effector.

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Some way to change what has gone wrong.

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In the case of growth
hormone, the effector would be

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the gland that secretes
it, the pituitary gland.

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Have it produce less growth hormone

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or have it produce more growth hormone,

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whichever is necessary to stay
within the homeostatic level.

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So there's an example you can see here.

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On the top, we have set
points in high and low.

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If the arrow indicates
it's the high level,

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the center picks it up, control center,

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which could be your brain's spinal cord,

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figures out what to do and
then caused the effector

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to change it back to set point.

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But if it's too low, go to
that same exact response.

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Center picks it up, control
center figures what to do,

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the effector raises the
variable back to set point.

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The idea here is doesn't
matter if it goes above

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or below set point, the negative feedback

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will help bring it back
into the proper amount.

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So in these feedback
loops and we're starting

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to talk about core body
temperature, for instance.

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You need to make sure body temperature

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stays in a certain range.

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So what would you say
your body temperature

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typically is around?

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Mmm, 97.8, 97.6, 97.9,

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somewhere in the upper 97s

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to maybe a 98 even and some people

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do have higher or lower than that.

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It all depends on the person.

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But that body temp is pretty much kept

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very, very close to that normal number.

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You have sensors throughout your organs,

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throughout your skin that can pick up

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if you're too warm or too cool.

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The control center listed
here is the hypothalamus.

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That's a very special area right in the,

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pretty much middle of
the brain, so it's just

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at the very base or bottom of the brain,

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right in the dead center.

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Well, if that's tripped up
or that's causing a problem,

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it means, okay, body temp is off.

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What do we do?

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We can change blood vessels.

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We can change sweat glands.

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We can change skeletal muscle.

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Let's do the skeletal muscle for a second.

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If you're working out,
let's say you're at the gym,

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working out, half hour in, a good workout.

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Are you warmer or cooler than
you were when you came in?

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Well if you were really working out hard,

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I'm willing to bet you're probably warmer.

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One of the reasons why

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is your skeletal muscles give off heat.

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The more you work out, the more muscles

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you're using, the more heat is generated.

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Sweat glands.

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Well, when you sweat, the
idea is to cool your body off.

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The idea of evaporative cooling.

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The liquid evaporates,
taking some heat with it.

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Blood vessels.

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Well, when you're
outside in a really cold,

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let's say, zero degrees,

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you're out there for an
hour without gloves on.

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What do you fingers look like?

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Well, they're probably pretty numb.

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They're, wouldn't be
doubt they're turning,

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like, bluish-purple, the reason being

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the blood vessels have gotten smaller,

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temporarily, in your
hands and your fingers.

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Your body's trying to keep
all the core temperature,

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all the blood in the center to
keep you as warm as possible.

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By being out in your fingers,

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that's the furthest from your core.

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It allows you to cool off too much.

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So all these effectors can be
controlled by the hypothalamus

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which in turn will be activated
based on the temperature

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sensors in your skin organs, which in turn

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are going to be triggered
if your body temp

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goes too high or it goes too low.

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To continue using this
temperature example,

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if your core temperature
drops, it goes below normal,

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sensors pick it up, the control center,

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remember hypothalamus in
the brain, is triggered.

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What it can do is it'll
increase your nerve activity.

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By increasing nerve
activity to blood vessels,

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it constricts the blood
vessels in the skin.

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Less blood goes in the skin, therefore,

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less heat reaches the skin.

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You stay warmer.

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But at the same time, you can use

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your skeletal muscles to start shivering.

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They generate heat, also helping

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to raise the core body temperature.

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But the core body temperature is too high.

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The sensors pick it up
again, the hypothalamus

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figures out, hey, we'd
better change some things.

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It will decrease nerve
activity to blood vessels.

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That makes the blood vessels
open up, or they dilate.

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More blood reaches the skin.

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More heat is lost the skin.

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But at the same time, they can increase

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nerve activity to sweat glands.

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More activity in the sweat
glands means more sweat.

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More sweat means more heat is being lost.

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So blood vessel dilation and
sweating helps cool you down.

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Blood vessel constriction and
shivering helps warm you up.