Oxygen content | Human anatomy and physiology | Health & Medicine | Khan Academy
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Oxygen content | Human anatomy and physiology | Health & Medicine | Khan Academy

Let’s talk about oxygen content. And I’m going to actually
spell it out two ways. One is the full word oxygen
content, or the full term. And I’m also going to give
you the shorthand, way you might see. Sometimes it’s written
this way, CaO2. And the C is the content. The little a is arterial. And the O2 is oxygen. So what it means exactly and
the way we think of it as, how much oxygen is there. How much is there? And we measure it
in milliliters, per 100 milliliters of blood. So per 100 ml. and. Sometimes you
might see deciliter instead of milliliter. Let me just quickly
jot that down. That equals 1 deciliter per
100 milliliters of blood. So this is the definition. Now, let’s use this
definition right away. Let’s see if you can
think through this idea. So let’s imagine I
go down and I decide to get 1 pint of blood
taken from my left arm. Let’s, instead of bint,
let me write pint. And this is my left arm. And let’s say I’m in
a huge rush this day. So I decide that
I also want to get another needle stuck
in my right arm. And they also draw blood
out of my right arm, at the same moment,
the same time. So the same kind of blood,
same hemoglobin concentration, and same amount of
oxygen in my lungs when I was getting
the blood drawn. Except for some reason,
maybe this needle, this second one was larger. And they were able to get
more blood out– 2 pints. Now, some smart wise
guy walks by and says, hey, which side, your left
or your right, were you’re able to get a higher
oxygen content from. Now, just looking
at the picture, you might be tempted to
say, well, oxygen content. Sounds like the right
side is the winner. But actually, this is
kind of a trick question because it’s per
100 milliliters. So you got remember,
it’s a certain volume that we’re thinking about. And in this case, since
we know that the blood was drawn at the same
moment from my two arms. And I have no reason to believe
that the left versus right had a higher oxygen saturation. I would say, actually
probably the two had the same oxygen content. That would be my guess
based on this set up. So that’s one important thing to
remember that it’s per 100 ml. So let’s just keep that in mind. And now let me actually
just jot down for you the exact equation,
kind of the formula. If want to mathematically
calculate oxygen content, how would that look? Well, CaO2 is quicker to write. So let me just jot that down. And the units on
this are milliliters of oxygen per, I said,
100 milliliters of blood. So these are the units here. And this is going to
equal– to figure this out, I need to know the
hemoglobin concentration. And there it’s the
grams of hemoglobin per 100 milliliters of blood. And then, I have to
multiply this by a constant. And the constant is 1.34. And what that number
is, is it’s telling me the milliliters
of oxygen that I can expect to bind for
each gram of hemoglobin. So that’s actually quite
a nice little number to have handy
because now you can see that the units
are about to cancel. This will cancel with this. And I end up with
our correct units. But there’s one
more thing I have to add in here which is
the oxygen saturation. Remember, this O2 saturation. And if I know the O2
saturation, remember, there’s this nice little curve. This is O2 saturation. And if I’m looking at
just the arterial side, I could write, S little a O2. And I could compare to
the partial pressure in the arterial side of oxygen. And remember, we have these
little S-shaped curves. These S-shaped curves. And all I want to point out
is that, for any increase in my PaO2, in the partial
pressure of oxygen, I’m going to have an increase
in the O2 saturation. So there’s an actual
relationship there. And we usually measure
this in percentage. Percentage of oxygen that
is bound to hemoglobin. And so this is the same thing
here, as a certain percentage. So this whole top
part of the formula, then, this whole
bit in my brackets really is telling me about
hemoglobin bound to oxygen. Now remember, that’s
not the only way that oxygen actually
travels in the blood. Let me write out this
second way that oxygen likes to get around. And the second way is when
it dissolves in the blood. So this is all going to be plus. And the second part
of the equation is the partial
pressure of oxygen. And this is measured in
millimeters of mercury. So that’s the unit. And this is times, now this
is another constant, 0.003. And then, keep
track of the units here because we have to
end up with these units. So you know everything has to
cancel out to end up with that. So I have milliliters
of oxygen on top. And I’m going to want to cancel
my millimeters of mercury. So take that times 100
milliliters of blood. So these are the
units on the bottom. And they end up the same
as we just worked through. We’ve got this
crosses out with that. And my units are going
to end up perfect. And this bottom
bit, that I’m going to put in purple brackets. This bit tells me
about dissolved oxygen. So I have my oxygen
bound to hemoglobin. And I have my dissolved oxygen. These are the two
parts of my formula. So let me actually just
quickly, before I move on, circle in blue, then,
the important parts that I want you keep
your eyeballs on. There is the total O2 content,
hemoglobin, oxygen saturation, and partial pressure of oxygen. And remember, this guy
influences this guy. And we saw that on the O2
curve that I just drew. Let me just bring
it up again, so I can remind you what
I’m talking about. In this graph, you can see
how the two are related. There’s a very nice
relationship between the two. So this is my formula for
calculating the total oxygen content. So let’s actually
use this formula. Let’s think through this. And when I think
through it, I always go through all of
my four variables. Let me just jot them down here. So we keep track of them. Let’s do PaO2, SaO2, and then
hemoglobin and the total oxygen content. These are my four variables. Now, let’s do a little
problem together. Let me make a
little bit of space. And let’s say I have
two little containers. And the first container, this
first one is full of blood. Here’s a B for blood. And here’s a second
container full of plasma. Remember, plasma is
a part of the blood. But it’s not all of the blood. Plasma specifically does
not have any red blood cells or any hemoglobin. So let me just write that down. No hemoglobin in
the plasma side. Just to make sure we don’t
lose track of that fact. Now, plasma is yellow colored. So let me just make it
yellow colored here. Make sure we clearly
see that that’s plasma. And blood I’m going to
keep as a red color. So now, we have
our two containers full of plasma and blood. So now, let’s say,
I decide to increase the partial pressure
of oxygen in the air. So it’s going to
diffuse in here. And it’s going to
diffuse in here. So I increase the partial
pressure oxygen in the air. And it’s going to diffuse
into those two liquids. It’s going to dissolve
into those liquids. So my question is, as we
go through one by one, each of these four variables,
I want you to think through if they go up, if they go
down, or if they stay the same. So let’s start with
the first one, PaO2. Well, if the oxygen is going
to diffuse into those liquids, then I would say the
partial pressure of oxygen in the liquid would go up. Now, it’s a little
bit confusing to use the words PaO2 in this case, or
even down here, CaO2 or SaO2. Because we’re not really talking
about arterial blood here. We’re just talking about blood. And we’re not talking
about arterial plasma. We’re just talking a
plasma because there’s no artery connected to
these two tanks of fluid. But the concept is the same. So the partial
pressure of oxygen is going to go up in the blood. And it’s going to
go up in the plasma because it just dissolves
into those liquids. Now, what about
saturation of oxygen? Well, O2 saturation
goes up in the blood. Remember, there’s a
relationship, we said, between PaO2 and
oxygen saturation. So it’s going to cause
the SaO2 to go up here. Whereas on the plasma side
there is no hemoglobin. So of course, there’s
going to be no change here. I would say, not applicable
because there is no hemoglobin. So how could you have
an oxygen saturation curve for hemoglobin? Now, what about
the third variable, hemoglobin concentration? Remember, that was grams per
100 milliliters of blood. Well, I’m not talking
about adding or subtracting hemoglobin. So there should
be no change here. I’ll write, no change. And on the other side
of the plasma side, again, there is no hemoglobin. So it’s not going to
affect that at all. It’s not really applicable. Plasma, again, does
not have hemoglobin. So in terms of the total
oxygen content, or the CaO2, what I expected to
go up in the blood, definitely, it’s
definitely going to go up because the dissolved
part of the equation goes up. But even the hemoglobin bound
to oxygen part of the equation goes up because we
said, the SaO2 went up. That’s an interesting point. On the other side on the
plasma side, it also increases. But only a little bit
because here you only have the contribution
from the PaO2. You have no
contribution from any of the oxygen
bound to hemoglobin because, again, there
is no hemoglobin. So this problem illustrates
some of the ideas, specifically around
trying to tie-in an increase in the
partial pressure of oxygen to how that could affect
the saturation of oxygen.

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