We start off with a
partial pressure of CO2.
So just like O2, we have three
partial pressures to keep in mind.
One is PaCO2,
PACO2 and PvCO2.
Again, the small A is denoted to be
arterial side of the circulation.
The capital A is for alveolar
carbon dioxide concentration.
And the V is for the venous
side of carbon dioxide.
What are typical numbers?
Usually, you’ll start off with a
PaCO2 of around 40,
A PACO2 of around 40, and
a PvCO2 of around 46.
So you can notice that in this case,
the PaCO2 is less than the PvCO2.
And that should make you
think that you’re giving up
carbon dioxide by the
Now, this is what this
particular number of a PaCO2.
You might think, “Well, what exactly does
this mean in terms of how it is carried?”
And if we think about that,
the first one is that
it involves this carbonic
that we’ve discussed earlier
where carbon dioxide combined
to water form carbonic acid
and then dissociate into a
bicarb ion and a hydrogen ion.
However, for this to really work,
you need to have a red blood cell.
And I’ll explain that in
the next couple slides.
CO2 is also bound to hemoglobin,
but this is related to the amount of
oxygen that’s already bound to hemoglobin.
So hemoglobin has a
higher affinity for O2
and the CO2 binding site is in a
different part of the molecule.
And so it’s going to have a
different binding affinity
based upon the amount
of oxygen that’s bound.
The final thing about CO2 is it has
a higher solubility coefficient
and therefore more of it will be
dissolved in the plasma than O2.
So let’s look at this
a diagrammatic form.
If you look on the left
side of the diagram,
you can see that you have
here a cell in which
it’s going to allow for carbon
dioxide to move out of it.
In this case, a small percent, about 5%
is going to be dissolved in the plasma.
The rest of the CO2 transports
within to the red blood cell.
It undergoes that carbonic
forms bicarb and then
bicarb is kicked out
of the red blood cell in something
called a chloride shift.
And that chloride shift is done
via a chloride bicarb exchanger.
And this particular transporter
allows for an electrogenic,
meaning that there is not a
charge change across the membrane
to allow chloride to come
in versus bicarb to leave.
Some of the carbon dioxide ends
up being bound to hemoglobin.
As it is bound to hemoglobin,
what it does is forms
in terms of about 5% of the carbon
dioxide is bound in that form.
If we look at this in a
total kind of picture,
we see that some of the carbon dioxide
is in the dissolved form, about 5%.
About 5% is bound to hemoglobin and the
rest, the 90%, is in the form of bicarb.
Similar to O2, carbon dioxide also
forms a relationship with hemogblobin.
This allows us to form a carbon dioxide
to hemoglobin dissociation curve.
There are a few things, however, about
these two curves that are different.
So let’s go through each
one of those one by one.
The first portion
that is different
is that you’re going to be travelling
from the venous side of the circulation,
has a higher CO2 than the
arterial side of the circulation.
So in terms of the curve, you’re moving
from a higher level to a lower level.
The other thing that’s different
is that the amount of O2
dramatically affects its
And so as O2 decreases, you
can bind more and more CO2.
This is important in
aspects of thinking about
if you’re in a hypoxic
condition, you could actually
carry more CO2 in the bound
form to hemogblobin.
The last couple item are
just general observations
between an O2 hemoglobin dissociation curve
and a CO2 hemoglobin dissociation curve.
And the first thing to think about
is instead of a sigmoidal shape
that occurs with the
O2 dissociation curve,
the CO2 dissociation
curve is a more linear.
The other aspect to think
about is the CO2 content
is about double that what
occurs with O2 content.
So you can see on the arterial
side of the circulation,
you have nearly 40 milliliters of CO2 per
100 mLs of blood or per deciliter of blood
while on the O2 side, you may have
only had about 20 milliliters of O2.