Now, we also need to think about what
is happening at the cellular level.
So each individual tissue in the body could
experience low O2s and
we call this hypoxia.
So tissue hypoxia can occur
through many different ways.
Let’s talk through what
the most common are.
One is it’s not
getting enough blood.
So if cardiac output is low or
if tissue perfusion is low,
the tissues could
Another way that tissues
can become hypoxic
is if the blood cells themselves
aren’t carrying enough O2.
Such as if a person has anemia or if
they have a carbon monoxide poisoning.
Because remember carbon
monoxide binds to the
O2 binding sites on hemoglobin
with a greater affinity than O2
and thus replaces them.
So either of those will mean
that the blood that gets
to the tissues is not bringing
as much oxygen with it.
The final way is a hypoxemia.
So hypoxemia is a low blood PO2.
That’s different than hypoxia, which is
occurring at the level of the tissue.
So now let’s talk through the
different hypoxia reasons
and how the body tries
to fix a hypoxia.
So we have hypoxia
and in the kidney, this is sensed and
a hypoxia-induced factor is produced.
What this does is then stimulate
erythropoietin known as EPO,
synthesize its proteins and it
starts to form red blood cells.
So a hypoxia overtime
is sensed by the kidney
and it produces a hormone
which then produces more red
blood cells or erythrocytes.
You may have heard of EPO
or erythropoietin before,
because this is sometimes a drug
that athletes might use to increase
the amount of red blood cells
such as what happened in some Tour
de Frances a number of years back.
You need to go through the
different types of hypoxemias
and there is a way in which you can
talk through these to understand
which particular one might be
prevalent at a certain time.
So low arterial partial
pressures of O2.
Low PaO2 can be induced
by either hypoventilation
or a low PIO2 or a low partial
pressure of inspired oxygen.
It could be caused by a
by a right to left shunt or
ventilation to perfusion inequality.
These are all four main mechanisms
by which a hypoxemia can occur.
If we look at our diagram,
here, you can see that
the blood is going to
be oxygenated as it
goes through the lungs.
So where is the low PaO2?
That’s in the red
portion of the diagram.
So if you don’t have
enough O2 being delivered
to peripheral tissues,
they will become hypoxic.
So let’s talk through first
hypoventilation or low PIO2.
So you can decrease
PIO2, one of two ways.
One is you could be in a low
barometric pressure environment
and that's usually at altitude.
Another way is if the fraction of O2 is
lower in ambient air then you would expect.
We would expect it to be at 21%.
But let’s say you were in an enclosed
room without very good circulation.
Eventually the O2 levels
could decrease in that area.
Hypoventilation is another way in
which you can create a hypoxemia
and this is that you’re not ventilating
or bringing enough air into the lungs
and therefore, you’re not being
able to fully oxygenate your blood.