Another way to cause a hypoxemia
is via diffusional impairment
and that is seen upwards
on this particular diagram
by not letting O2 get from
the lungs into the blood.
And this means there’s inadequate
amount of gas exchange
that’s occurring across
that blood gas barrier.
So let’s go through this in a little bit more
detail because it’s an important process
and oftentimes pathologies result
because of diffusional impairments.
So normally, you have
diffusion that occurs
because you have to get the O2 from the
lung to the plasma, to the red blood cells.
Diffusion is occurring between
both of these places,
between the lung and the plasma and
the plasma and the red blood cell.
Then you need to of course
get that particular O2 bound
to hemoglobin on your red
blood cell or erythrocyte.
There is a way to calculate this and that
is taking into account these variables.
We need to know what the
partial pressure of O2 is
and we need to know how
far it needs to diffuse.
So this is oftentimes denoted
by this theoretical equation
in which the diffusion of a particular gas
is related to the surface area available
divided by the thickness or the distance
it needs to travel times a coefficient.
And this coefficient is specific
to each gas such as O2.
It is its solubility divided by the
square root of its molecular weight.
So if we’re dealing with any gas,
we can pretty much ignore
that factor because
it’ll be the same no
matter what particular –
If we’re talking about oxygen, it will
always be the same diffusion coefficient.
The other variable that comes very
important is this pressure differential.
You need to know
what P1 minus P2 is.
That would be the partial pressure
in the lung versus the plasma
and the partial pressure between
the plasma and the red blood cell.
To better understand
we need to compare and contrast
a diffusion-limited gas versus
a perfusion-limited gas.
To do that, we’re going to use
these particular figures.
Along the X-axis is going
to be the capillary length
from the beginning of the capillary
to the end of the capillary.
Along the Y-axis is going to be the
partial pressure within the capillary.
The very top dashed line is the partial
pressure in the alveolar space.
So if we take a gas
like carbon monoxide,
you can see that it never quite gets
to the area of the alveolar space.
Let’s contrast that with nitrous
oxide, which is an anesthetic gas.
Here, if we look at the beginning of the
capillary to the end of the capillary,
nitrous oxide is rapidly
diffused across the membrane
so it matches the pulmonary
alveolar gas tension.
This allows for there to be quick diffusion
across the particular lung tissue.
So a perfusion-limited substance
has very fast diffusion
and a diffusion-limited substance
is very slow to diffuse
across the length of the
Now, let’s use our two gasses that we
deal with in physiology, O2 and CO2.
So here is the same type of
graph with the beginning
of the capillary through
the end of the capillary.
We have the partial pressure
of the gas, in this case, O2.
And then we have a dashed line
along the top that is the
partial pressure of O2
in the alveolar space.
Normally, you have a fairly
quick equilibration of the
gas in the pulmonary capillary for
what is in the alveolar capillary
meaning that it diffused
across quite well.
If we take a diffusional-limited person
such as someone that has
you can see that they have
trouble getting their gas
from the lungs into the
They never reach alveolar
This also occurs if you lower
the PO2 in the alveolar gas.
Normally, you’ll take a little bit longer
for you to get to that equilibration phase
so you get all of the gas from the
alveolar space into the capillary.
And a fibrotic person has even lower
amounts of diffusion that occurs.
So if you’re not able to reach up to the
PAO2, a diffusion-limitation has occurred.
The next type of hypoxemia is something
called a right to left shunt.
And for this, it is blood
that’s travelling from the
right side of the heart to the left side of
the heart without undergoing oxygenation.
This can be seen in this diagram
where some of the venous
blood is bypassing the lungs
and going directly to the
And of course, if you don’t get oxygenated,
you’re going to be dumping deoxygenated
blood into the oxygenated blood.
And that’s going to
lower your PaO2.
Now, there is some natural
nonpathological right to left shunt.
And that occurred from
the physiological shunt.
Remember that was the draining of
the blood from the thebesian veins
as well as the bronchial circulation,
but that’s perfectly normal.
But again that should be less
than 15 millimeters of mercury.
Pathological right to left shunts occur
because the severity has increased.
But it’s going to be very dependent upon the
amount of cardiac output that is shunted.
So if you have a small hole
in the septum of your heart
that allowed some blood to pass from the
right ventricle to the left ventricle
without going to the lungs, that is a
great example of a right to left shunt.
Because some blood went from
the right side of the heart
to the left side of the heart
without going to the lungs.
That sometimes does happen
especially for infants
and they will require some surgery to
help fix that particular shunt problem.
The final type of hypoxemia
that we’re going to go through
is one called a ventilation
to perfusion inequality.
And some people will refer to this as
ventilation to perfusion mismatch.
Now, I’m going to go
through this in two forms.
One is in the upright lung
and one is in the pathology.
The upright lung allows us to have a very
good example of this process in physiology.
And then you can simply apply it
to a pathophysiological state.
So in our ventilation to
perfusion inequality diagram,
here I’ve simplified the lungs into
three different alveoli or air sacs.