Now let’s go through a couple of examples
of partial pressures of inspired O2.
If you think of your
sea level calculation,
if we are counting for
humidity in the air,
that value ends up being around 150
millimeters of mercury for PIO2.
If you look at Colorado Springs,
with the barometric pressure of somewhere
just over 600 millimeters of mercury,
the pressure is only 118
millimeters of mercury.
If we look at the
summit of Mount Everest
in which barometric pressure is
only 250 millimeters of mercury,
barometric pressure in inspired air
is only 43 millimeters of mercury.
That is less or about
the same amount
as normal sea level venous
oxygen partial pressures are.
This is a very, very
And in fact, without supplemental
oxygen, this is very difficult,
particular barometric pressure
to be able to overcome.
So now what we have the PIO2s which is
the partial pressure of inspired O2,
that doesn’t even account yet for what
is in your alveolar or in your air sacs.
So at the summit of Mount Everest, the
PIO2 is only 42 millimeters of mercury.
But that’s just the first
part of this equation.
So you can understand how the
alveolar partial pressure
might be a lot more decreased
than simply the inspired O2.
Now, what are the effects
of high altitude?
And we’re going to usually call
high altitude, hypobaric hypoxia.
Hypo- meaning low
pressure and low oxygen.
Alveolar ventilation and pulmonary blood
flow are going to be the first topics.
Ventilation increases during a
hypobaric, hypoxic exposure.
In terms of pulmonary blood flow, there
are increases in cardiovascular responses
and these usually are due to the
mechanical effects of the lung.
The last areas that changes
in terms of blood pressure
is there is there is a hypoxic
And what this does is increases
mean pulmonary artery pressure
and I will emphasize that right now
because this may have something
to do with high altitude pulmonary
edema that we’ll talk about later.
There are also changes that happen
in diffusion and gas transport.
So in terms of diffusion,
there is a less of a gradient for O2
because there is a lower PAO2.
There’s also more surface area
involved and this allows for –
And this occurs because
of recruitment issues
that were previously
unventilated or underperfused
So the first aspect of diffusion
increased your ability to diffuse.
The second aspect might increase it.
So diffusion is hard to know how much is
going to change in hypobaric hypoxia.
In terms of gas transport,
there could be issues
associated with the loading,
the O2 on hemoglobin.
Cardiac output though increases and
that helps to maintain O2 delivery.
And finally, there usually is an
increase in hemoglobin concentration
and this occurs through
one of two mechanisms.
One is either the person dehydrates
themselves by urinating out more fluid
or, chronically across time, you might
increase red blood cell production.
What are the effects on
the control of breathing?
There’s usually a
decrease in arterial O2.
And this will of course stimulate peripheral
chemoreceptors to increase ventilation.
This ventilation though
does have a downside.
And that is as
there is a decrease in the partial
pressure of carbon dioxide
both in the arterial as well
as the alveolar spaces.
Now, this arterial
hypocapnea or low PCO2
can diffuse out some CO2 in
the cerebrospinal fluid.
Why this is bad is that
respond to this particular
And this creates an alkalosis
in the cerebrospinal fluid.
And thus, can suppress
the drive to breathe.
So what do you want to do if you’re
exposed to hypobaric hypoxia?
The first thing is you
want to ventilate more.
So you want to hyperventilate,
The second thing you’d want to
do is increase O2 transport
and you can do that by pumping
more blood through the system.
Finally, you want to
hemoconcentrate the blood,
meaning that you want to have more red
blood cells per 100 mLs of blood.
You can either do it acutely
by decreasing plasma volume
or chronically by increasing
the number of red blood cells.
And finally, with these
acid-base changes that happen,
the body must adapt to that and so it
can get rid of bicarbonate easily.
Now, if we can’t adapt to an
environment, what happens to you?
Well, it usually is an
illness can result.
Acute mountain sickness and high
altitude cerebral edema are two.
High altitude pulmonary edema is another
as well as there are other ones
that we’re not going to go through
dealing with high altitude illnesses
such as chronic
there can some renal
hemorrhages that occur
and a lot of times, this can also
exacerbate some chronic conditions
whether it be cardiovascular
or pulmonary in nature.