Let's say this person is lying here in front of me.
And I'm thinking about how the air is
passing through their nose and their mouth
and entering their lungs.
And specifically I'm interested this time in how much
oxygen is actually getting to their alveolar sacs.
So, deep inside their lungs they have these branches,
they're conducting in respiratory bronchials.
But at the end, of course, they have these alveolar
sacs that we've talked about.
And I'm interested in thinking about how much oxygen is really
down there at the very ends.
And you have to excuse this alveolar sac.
It really is that.
It looks a little bit like a three-leaf clover, I guess.
But that's the issue.
How much oxygen is deep down in here where the x is?
So how do we figure this out?
I want to first think about the air
this gentleman is breathing in.
He's breathing in air from the atmosphere.
So this is atmospheric pressure air.
And we say ATM for short.
And we know that atmospheric pressure at sea level
is 760 millimeters of mercury.
It's going to be lower at higher altitudes.
So, if you're at the top of a mountain,
it would be less than that.
And this pressure is made up of many, many different molecules
bouncing around.
So, I've got some molecules of oxygen.
Let's say this is about 21%.
This is my oxygen.
And before I move on, I should mention FiO2.
You might come across this.
And FiO2 stands for the fraction-- which in this case
was 21% or 0.21-- fraction of inspired,
meaning how much oxygen you took in or air you
took in-- fraction of inspired oxygen.
And the fraction happens to be 21%, which is, of course, much,
much lower than the nitrogen.
Now nitrogen-- when I draw it this way--
it's pretty impressive.
All the purple is nitrogen.
This is about 78% of what you're breathing in.
And the last little tiny little bit,
I'm going to draw the green line.
This is mostly argon.
And argon is-- in Greek, it actually
comes from the term lazy.
But it basically reminds me when I think of that,
that argon is not going to do much.
It's not going to react with anything that is in our body.
And of course, you have other.
You have less than 1%.
And this would be things like carbon dioxide.
So, this is a breakdown of the air
that my friend is breathing in.
This is my friend breathing.
And if I want to now think about how much oxygen
they're taking in, all I have to do
is a little tiny bit of math.
I can say OK, pO2-- this is the partial pressure of oxygen--
is just 0.21, or 21%, times 760 millimeters of mercury.
And this turns out to be 160 millimeters of mercury.
Now, that oxygen kind of goes down in his lungs.
And it goes through his trachea and into his-- all
the little bronchials and down into the alveolar sac.
And when it gets there-- on the way over there,
an interesting thing happens.
The body temperature here is 37 degrees Celsius.
He's got a normal body temperature.
And what that does is-- the air is
going through these bronchials and trachea.
And as it does, there's a lot of moisture
in the respiratory tree.
There's moisture there.
And that moisture, when it starts
heating up-- and of course, 37 degrees
is pretty warm-- It's going to start leaving the liquid phase
and going into the gas phase.
So all of a sudden you have now little molecules.
I'm going to draw them as little dots of water.
That's here.
And it's going to start entering and mingling
with the gas that's going through.
So, the gas that got taken in, that he inhaled
is now mingling.
And what happens as a result, is that water
has what we call a vapor pressure.
And that vapor pressure is going to change
depending on the temperature.
But at 37 degrees, that vapor pressure
ends up being 47 millimeters of mercury.
In other words, if the temperature is 37 degrees,
then we can expect that some of those water molecules
will leave the liquid and enter the gas phase.
And it turns out that the amount of molecules--
or the number of molecules-- that leave
are going to generate a pressure that
is 47 millimeters of mercury.
And this is pretty standard.
This is known off of a table.
And in fact, if you think about it,
if you just generated lots of heat-- let's say
you actually were boiling water--
that would be 100 degrees Celsius.
And the vapor pressure there would be very high,
because it's boiling.
And it would be 760.
So boiling is actually 760.
So just keep that in mind.
Boiling water has a vapor pressure of--
And what do you think 760 reminds you of?
That is atmospheric pressure.
So it's interesting.
Vapor pressure is going to equal atmospheric pressure
when you are boiling water.
And that's actually exactly what's happening as you boil.
But I don't want to get too distracted.
We're not boiling water inside of our bodies or our lungs.
We're actually much cooler than that.
But we are warm.
We're at 37 degrees.
And so you do have some of these little water molecules that
have entered the gas phase.
And so if overall it's got to be-- this whole thing
has got to be 760.
So, on average, our lung pressures
are going to be the same as atmospheric pressure.
But now you've got water taking up 47.
So if water's taking up 47, the rest of those little gas
molecules have got to be 713.
So this is the rest.
What was in that rest?
It's going to be the same as before.
It's going to be-- and I'm going to try to sketch it
as best as possible-- this is going
to be my oxygen right here.
This is 21% of 713.
And then we have lots and lots of nitrogen still.
Same kind of break down as before.
And remember this is all air that is being inhaled.
So we're not talking about breathing out.
We're just talking about breathing in.
And this purple right here-- and this is 78%.
Again, this is 78% percent of 713.
And we still have a little bit of that argon,
and those other gases-- I won't write it all out,
but you get the idea.
That basically now because water is taking up
some of the overall pressure, all of the other gases
are going to have a lower partial pressure.
So what is the partial pressure of the air that's
entering into that alveolar sac?
It's going to be basically FiO2, which is 21%.
I'll write that here.
And then we have the atmospheric pressure.
This is atmospheric pressure over here.
And we said that was 760.
We can draw a little arrow so we know what's pointing to what.
760 millimeters of mercury.
And then, from that to account for the partial pressure
of water.
Because now we have some water vapor in there.
We have to subtract out 47.
So, so far, if you've kept up with this math,
you see that we have-- what does that work out to be?
About 150 millimeters of mercury.
Now this is the partial pressure of oxygen, at this spot.
Let me just make it very clear with my arrow,
not at this orange x.
So far, we've figured out that we
have a partial pressure that's a little bit lower than when
we started.
And that was because of the partial pressure of water.
Let's pick up there in our next video.