Henry's Law (Respiratory System Physiology) Video Lecture - MBBS

Let's say you're taking
a look at the interface
between a gas-- I'm going to
do in yellow-- and a liquid
down here in blue.
And the liquid I'm going
to use is H2O, or water.
And you actually
want to kind of keep
your eye on exactly what's
happening right here.
So this is your
eyeball, and you're
watching exactly
what's happening right
at that surface layer.
In fact, let me write that
down because it ends up
being kind of an important idea.
You're just watching the
surface layer of water.
And you really want to make sure
that you keep your eye on how
the molecules are moving around.
So let's say you've got
some molecules in purple,
and you've got some green
molecules here as well.
And four of each, so overall
it's 50% purple and 50% green.
And down below, you've
got some water molecules.
Let's draw some oxygens here.
And I'm going to draw
some hydrogens as well.
So these are little hydrogens
on my water molecules.
So these are H2Os,
and all this is
happening in a
giant cup of water.
So this is a big cup of water.
And the purple and
green molecules
represent some sort of molecule.
Who knows what kind of gas that
is, but some hypothetical gas.
And to think
through this, I want
to kind of get to the
idea of partial pressure.
So we know total pressure
is one atmosphere,
or you could write it as
760 millimeters of mercury.
But if I'm only interested
in the green molecules,
then I would really rephrase
that as partial pressure.
And if I wanted
to calculate what
that would be, I
could say, I know
that there are 4 green
molecules out of a total of 8,
and that is 50% green molecules.
And I know that the overall
pressure is 760-- actually
let me leave it in
the same color--
760 millimeters of mercury.
And I've got 50%, I
said, that are green.
So that means that the
green partial pressure
is going to be half
of 760, which is 380.
So this is the partial pressure
of the green molecules.
I figured it out.
And I could actually
complicate this a little bit.
I could say, well, what
if I got rid of those two
and replaced them
with green molecules?
So now the gas is
looking different.
I've got 6 out of 8
molecules that are green.
So what is the new partial
pressure looking like?
Well, 6 out of 8 means
that the percentage
is going to be different.
So I've got a new
number here and here.
So I'd say 75% is
the new number.
And I've got 75% times 760 is
570 millimeters of mercury.
This is my new partial pressure.
And the reason I actually
went through that
is because I wanted to
show you a way of thinking
about partial pressure, which is
that if the number of molecules
in a group of molecules--
if the proportion goes up--
then really that's another way
of saying the partial pressure
has gone up.
And if you have more molecules,
what does that mean exactly?
Well, from this
person's standpoint,
this person that's watching
this surface layer,
they're going to see,
of course, molecules
going every which way.
Every once in a while,
these green molecules
are going to go down
and into the liquid.
They're going to bounce
in different ways,
and just by random chance, a
couple of these green molecules
might end up down here
in the surface layer.
So that's something
that you would observe.
And you'd probably observe
it more often if you actually
have more green molecules.
In other words, having a
higher partial pressure
will cause more of the
molecules to actually switch
from the gas part of this
cup into the liquid part
of the cup.
So I don't want to
be too redundant,
but I want to point out that
as the partial pressure rises,
we're going to have
more molecules, more
green molecules,
going into the liquid.
So now let me actually
ask you to try
to focus on this little green
molecule, this little fella
right here, this guy.
Now imagine, he's just entered
our world of H2O's, and he's
trying to figure
out what to do next.
And one thing he might
do is pop right back out.
You'd agree that that's
something he could do, right?
If he entered the
liquid phase, he
could also just
re-enter the gas phase.
He could leave.
And a lot of molecules
want to do that.
They want to actually
get out of the liquid
because the liquid
is a little stifling.
It's kind of crammed
in there, a lot
of H2O molecules around in
this case may not like that.
So it turns out you
can actually look up,
in a table, this value
called K with a little h.
And this H with a little
h is just a constant.
So this is just a
constant value that's
listed on a table somewhere.
And this K sub h
actually is going
to take into account
things like which
solute are we talking about.
When I say solute,
you basically can
think of these green molecules.
So which is it?
Is it a green molecule or
a purple one or a blue one?
What exact solute
are we talking about?
And what solvent are
we talking about?
Are we talking about water?
Or is it dish soap or
ethanol or some other liquid
that we're worried
about in this case?
And finally, what temperature
are we talking about?
Because we know that molecules
are going to want to leave.
Especially molecules that
prefer to be in a gas phase,
they're going to want
to leave the liquid,
and they're going to
do it much, much more
if the temperature is high.
Because when the temperature
is high, remember,
the little H2O molecules are
dancing around and shaking
around, And that allows
them to free up and leave.
So these are three
important issues.
What is the solute?
What is the solvent?
And what is the temperature?
And if you know
these three things,
you can actually--
like I said, you
could look up in a
table what the Kh is.
And that tells you a little
bit about that red arrow.
What is the likelihood of
leaving the surface layer?
So just as before,
where we talked
about going into a liquid, this
is now going out of liquid.
So Kh, these values that I
said you can find in a table,
tell you about the likelihood
of going out of a liquid.
And the partial
pressure tells you
the likelihood of
going into a liquid.
So if you are
looking now-- let's
go back to this person that's
been very patiently observing.
If you're looking at
this surface layer,
you can actually do a good job
of checking how many molecules
are entering, how many
molecules are exiting,
and you can now
calculate a concentration
of the molecule in
the surface layer.
You could actually say
something like this-- pressure,
or partial pressure, divided by
K over h equals concentration.
So let me write all this out.
Concentration is here.
And the other two are what we've
already been talking about.
The p just partial pressure,
and that is right there.
And the K with a little
h is the constant,
and that is right there.
So that's this guy.
So if you just divide the
two, you can figure out
the concentration,
and specifically, I
mean the concentration of green
molecules in the surface layer.
And what does that
really tell you?
OK, so now you figure
out the concentration
of green molecules
in the surface layer.
What the heck does that mean?
Well this, my friends,
this formula--
actually, I don't know
if you recognize it,
but this is Henry's law.
So a guy named William
Henry-- and actually Henry
was his last name-- came up
with this fantastic formula.
And sometimes you
see it rewritten.
You might see p equals
concentration times K
with the little h.
It depends on how you're
going to present it,
but it's the same formula.
And basically what
it says-- and it's
a very clever way
of saying it-- is
that you can take a look at
the molecules that are going
into a liquid and
the molecules that
are going to want
to leave a liquid.
And basically it
gives you a sense
for the concentration of
molecules in the surface layer.
In fact, another
way of saying is
that there's a relationship
between partial pressure
and concentration
within the liquid.
So it's actually a
pretty powerful way
of thinking about it.
And I hope that by describing
K with a little h in this way
you get a more intuitive
feel for what it stands for.