And then we have three
pulmonary capillaries that one
each is going to go by each
of those particular air sacs.
So let’s start with
our first example.
Here, we have a mixed
venous blood coming back
at an O2 sat of around 72%, which
is an O2 concentration of 14.6.
That blood is going to go
around this particular alveoli.
This alveoli though has an impairment
in the ability to get ventilation.
So let’s say there’s some sort of
constriction in one of the airways.
And this will yield a lesser amount of
O2 being oxygenated by the time that
that blood reaches from the beginning
of the pulmonary capillary to the end.
So maybe you’ll only get 79%
saturated or an O2 content of 16.
Let’s contrast that
to a normal condition
in which ventilation to perfusion
is matched really well.
Here, the mixed venous blood
still comes in at 72%,
but now it’s saturated to 96%
because there is good matching between
the ventilation and perfusion.
Our final example is a
condition where you still
have the same mixed venous
blood coming in at 72%.
However, you have a time when there
is a low amount of blood flow
going to through that
This allows for a condition in which this
alveoli or air sac is overventilated.
Meaning that it’s getting too much
O2 or gas per amount of blood flow.
This will allow for a more
complete O2 saturation, 99%.
But the problem is there’s very little
blood flow that occurs in this condition.
If we look at this across
all three of our alveoli,
you can see that even though
you had some capillary blood
that gotten more oxygen,
the amount of blood flow that went through
that pulmonary capillary was so low
that it didn’t contribute
very much to the arteriole.
If we look at our normal condition
of 19.5 or 96% saturated,
you see that that still
wasn’t able to overcome
some areas that had a ventilation
to perfusion mismatch.
So anytime someone has a
ventilation to perfusion mismatch
somewhere in the lungs,
the overall result
will be a lower PaO2.