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Let's say this person is
lying here in front of me.

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And I'm thinking
about how the air is

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passing through their
nose and their mouth

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and entering their lungs.

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And specifically I'm interested
this time in how much

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oxygen is actually getting
to their alveolar sacs.

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So, deep inside their lungs
they have these branches,

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they're conducting in
respiratory bronchials.

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But at the end, of course,
they have these alveolar

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sacs that we've talked about.

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And I'm interested in thinking
about how much oxygen is really

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down there at the very ends.

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And you have to excuse
this alveolar sac.

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It really is that.

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It looks a little bit like a
three-leaf clover, I guess.

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But that's the issue.

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How much oxygen is deep
down in here where the x is?

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So how do we figure this out?

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I want to first
think about the air

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this gentleman is breathing in.

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He's breathing in air
from the atmosphere.

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So this is atmospheric
pressure air.

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And we say ATM for short.

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And we know that atmospheric
pressure at sea level

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is 760 millimeters of mercury.

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It's going to be lower
at higher altitudes.

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So, if you're at the
top of a mountain,

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it would be less than that.

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And this pressure is made up of
many, many different molecules

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bouncing around.

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So, I've got some
molecules of oxygen.

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Let's say this is about 21%.

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This is my oxygen.

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And before I move on,
I should mention FiO2.

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You might come across this.

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And FiO2 stands for the
fraction-- which in this case

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was 21% or 0.21--
fraction of inspired,

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meaning how much oxygen
you took in or air you

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took in-- fraction
of inspired oxygen.

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And the fraction happens to be
21%, which is, of course, much,

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much lower than the nitrogen.

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Now nitrogen-- when
I draw it this way--

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it's pretty impressive.

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All the purple is nitrogen.

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This is about 78% of
what you're breathing in.

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And the last little
tiny little bit,

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I'm going to draw
the green line.

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This is mostly argon.

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And argon is-- in
Greek, it actually

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comes from the term lazy.

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But it basically reminds
me when I think of that,

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that argon is not
going to do much.

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It's not going to react with
anything that is in our body.

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And of course, you have other.

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You have less than 1%.

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And this would be things
like carbon dioxide.

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So, this is a
breakdown of the air

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that my friend is breathing in.

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This is my friend breathing.

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And if I want to now think
about how much oxygen

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they're taking in,
all I have to do

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is a little tiny bit of math.

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I can say OK, pO2-- this is the
partial pressure of oxygen--

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is just 0.21, or 21%, times
760 millimeters of mercury.

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And this turns out to be
160 millimeters of mercury.

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Now, that oxygen kind of
goes down in his lungs.

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And it goes through his
trachea and into his-- all

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the little bronchials and
down into the alveolar sac.

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And when it gets there--
on the way over there,

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an interesting thing happens.

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The body temperature here
is 37 degrees Celsius.

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He's got a normal
body temperature.

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And what that does
is-- the air is

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going through these
bronchials and trachea.

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And as it does, there's
a lot of moisture

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in the respiratory tree.

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There's moisture there.

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And that moisture,
when it starts

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heating up-- and of
course, 37 degrees

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is pretty warm-- It's going to
start leaving the liquid phase

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and going into the gas phase.

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So all of a sudden you
have now little molecules.

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I'm going to draw them
as little dots of water.

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That's here.

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And it's going to start
entering and mingling

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with the gas that's
going through.

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So, the gas that got
taken in, that he inhaled

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is now mingling.

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And what happens as a
result, is that water

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has what we call
a vapor pressure.

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And that vapor pressure
is going to change

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depending on the temperature.

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But at 37 degrees,
that vapor pressure

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ends up being 47
millimeters of mercury.

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In other words, if the
temperature is 37 degrees,

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then we can expect that some
of those water molecules

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will leave the liquid
and enter the gas phase.

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And it turns out that
the amount of molecules--

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or the number of
molecules-- that leave

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are going to generate
a pressure that

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is 47 millimeters of mercury.

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And this is pretty standard.

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This is known off of a table.

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And in fact, if
you think about it,

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if you just generated
lots of heat-- let's say

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you actually were
boiling water--

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that would be 100
degrees Celsius.

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And the vapor pressure
there would be very high,

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because it's boiling.

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And it would be 760.

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So boiling is actually 760.

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So just keep that in mind.

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Boiling water has a
vapor pressure of--

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And what do you think
760 reminds you of?

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That is atmospheric pressure.

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So it's interesting.

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Vapor pressure is going to
equal atmospheric pressure

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when you are boiling water.

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And that's actually exactly
what's happening as you boil.

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But I don't want to
get too distracted.

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We're not boiling water inside
of our bodies or our lungs.

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We're actually much
cooler than that.

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But we are warm.

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We're at 37 degrees.

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And so you do have some of these
little water molecules that

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have entered the gas phase.

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And so if overall it's got
to be-- this whole thing

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has got to be 760.

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So, on average,
our lung pressures

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are going to be the same
as atmospheric pressure.

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But now you've got
water taking up 47.

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So if water's taking up 47,
the rest of those little gas

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molecules have got to be 713.

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So this is the rest.

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What was in that rest?

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It's going to be
the same as before.

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It's going to be-- and I'm
going to try to sketch it

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as best as possible--
this is going

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to be my oxygen right here.

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This is 21% of 713.

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And then we have lots and
lots of nitrogen still.

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Same kind of break
down as before.

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And remember this is all
air that is being inhaled.

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So we're not talking
about breathing out.

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We're just talking
about breathing in.

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And this purple right
here-- and this is 78%.

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Again, this is 78%
percent of 713.

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And we still have a
little bit of that argon,

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and those other gases--
I won't write it all out,

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but you get the idea.

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That basically now
because water is taking up

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some of the overall pressure,
all of the other gases

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are going to have a
lower partial pressure.

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So what is the partial
pressure of the air that's

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entering into that alveolar sac?

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It's going to be basically
FiO2, which is 21%.

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I'll write that here.

158
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And then we have the
atmospheric pressure.

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This is atmospheric
pressure over here.

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And we said that was 760.

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We can draw a little arrow so
we know what's pointing to what.

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760 millimeters of mercury.

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And then, from that to account
for the partial pressure

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of water.

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Because now we have some
water vapor in there.

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We have to subtract out 47.

167
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So, so far, if you've
kept up with this math,

168
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you see that we have-- what
does that work out to be?

169
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About 150 millimeters
of mercury.

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Now this is the partial pressure
of oxygen, at this spot.

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Let me just make it very
clear with my arrow,

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not at this orange x.

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So far, we've
figured out that we

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have a partial pressure that's
a little bit lower than when

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we started.

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And that was because of the
partial pressure of water.

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Let's pick up there
in our next video.