- With all these tissues we have to also talk
about how they work together, how all tissues interact,
how they make sure they balance each other.
What do they do when they form this multicellular organism?
Well a multicellular organism, like human
or any other animal that has numerous cells,
must maintain homeostatis.
Now homeostatis is pretty important in the body.
If homeostasis is forgotten or neglected,
the body can get into trouble really quickly.
So here's a kind of textbook definition of homeostasis.
Maintenance of relative constancy
of the conditions of the internal environment.
Well it's a whole lot more beneficial
if you put that into your own words,
so think homeostasis, keeping your internal systems in check
or balancing off your internal workings, or whatever.
The idea is put it into your own words so you understand it,
and a lot of homeostasis is maintained
via a negative feedback control system.
Well, negative, doesn't sound very good,
but negative is very important
because a negative feedback system is any system
when homeostasis is going out of balance.
So a deviation from normal is detected.
Those deviations, those abnormalities
will send a negative signal back.
The body says, hey, there's a problem there.
Let's fix it.
So any deviation from normal can be
detected and counteracted.
So, for example, your blood pH.
Now, typically, blood pH falls about a 7.35 to 7.45.
Not a very large range.
Well, if all of sudden, your blood pH starts
hitting about a 7.36
or even a 7.35,
right at that bottom level, you'll start having
this negative feedback cycle kicking in.
The negative feedback will say, hey,
we're at the bottom limit of normal here.
We're having a deviation.
We're having a problem.
Let's counteract and make sure it's working properly.
So there's plenty of negative feedback systems in your body.
So the first part you must have is a control variable.
You can pick something as simple
as a hormone in your system.
Growth hormone, you've probably heard of that,
makes you grow.
Well, the growth hormone should have
a certain amount being produced during childhood,
and for that amount, a little less
during adulthood, but you still have some.
Let's stick with child.
So growth hormone must be controlled.
Otherwise you'll get unusual growth.
Well you must have a sensor, something to realize
if there is growth hormone present.
Control center.
Think of that one as your brain's spinal cord.
Effector.
Some way to change what has gone wrong.
In the case of growth hormone, the effector would be
the gland that secretes it, the pituitary gland.
Have it produce less growth hormone
or have it produce more growth hormone,
whichever is necessary to stay within the homeostatic level.
So there's an example you can see here.
On the top, we have set points in high and low.
If the arrow indicates it's the high level,
the center picks it up, control center,
which could be your brain's spinal cord,
figures out what to do and then caused the effector
to change it back to set point.
But if it's too low, go to that same exact response.
Center picks it up, control center figures what to do,
the effector raises the variable back to set point.
The idea here is doesn't matter if it goes above
or below set point, the negative feedback
will help bring it back into the proper amount.
So in these feedback loops and we're starting
to talk about core body temperature, for instance.
You need to make sure body temperature
stays in a certain range.
So what would you say your body temperature
typically is around?
Mmm, 97.8, 97.6, 97.9,
somewhere in the upper 97s
to maybe a 98 even and some people
do have higher or lower than that.
It all depends on the person.
But that body temp is pretty much kept
very, very close to that normal number.
You have sensors throughout your organs,
throughout your skin that can pick up
if you're too warm or too cool.
The control center listed here is the hypothalamus.
That's a very special area right in the,
pretty much middle of the brain, so it's just
at the very base or bottom of the brain,
right in the dead center.
Well, if that's tripped up or that's causing a problem,
it means, okay, body temp is off.
What do we do?
We can change blood vessels.
We can change sweat glands.
We can change skeletal muscle.
Let's do the skeletal muscle for a second.
If you're working out, let's say you're at the gym,
working out, half hour in, a good workout.
Are you warmer or cooler than you were when you came in?
Well if you were really working out hard,
I'm willing to bet you're probably warmer.
One of the reasons why
is your skeletal muscles give off heat.
The more you work out, the more muscles
you're using, the more heat is generated.
Sweat glands.
Well, when you sweat, the idea is to cool your body off.
The idea of evaporative cooling.
The liquid evaporates, taking some heat with it.
Blood vessels.
Well, when you're outside in a really cold,
let's say, zero degrees,
you're out there for an hour without gloves on.
What do you fingers look like?
Well, they're probably pretty numb.
They're, wouldn't be doubt they're turning,
like, bluish-purple, the reason being
the blood vessels have gotten smaller,
temporarily, in your hands and your fingers.
Your body's trying to keep all the core temperature,
all the blood in the center to keep you as warm as possible.
By being out in your fingers,
that's the furthest from your core.
It allows you to cool off too much.
So all these effectors can be controlled by the hypothalamus
which in turn will be activated based on the temperature
sensors in your skin organs, which in turn
are going to be triggered if your body temp
goes too high or it goes too low.
To continue using this temperature example,
if your core temperature drops, it goes below normal,
sensors pick it up, the control center,
remember hypothalamus in the brain, is triggered.
What it can do is it'll increase your nerve activity.
By increasing nerve activity to blood vessels,
it constricts the blood vessels in the skin.
Less blood goes in the skin, therefore,
less heat reaches the skin.
You stay warmer.
But at the same time, you can use
your skeletal muscles to start shivering.
They generate heat, also helping
to raise the core body temperature.
But the core body temperature is too high.
The sensors pick it up again, the hypothalamus
figures out, hey, we'd better change some things.
It will decrease nerve activity to blood vessels.
That makes the blood vessels open up, or they dilate.
More blood reaches the skin.
More heat is lost the skin.
But at the same time, they can increase
nerve activity to sweat glands.
More activity in the sweat glands means more sweat.
More sweat means more heat is being lost.
So blood vessel dilation and sweating helps cool you down.
Blood vessel constriction and shivering helps warm you up.