So, in our ventilation to perfusion inequality diagram,
here I've simplified the lungs into three different alveoli or air sacs.
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 gonna 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 will only get 79% saturated or an O2 content of 16.
Let’s contrast that to the normal condition in which ventilation to perfusion is matched really well.
Here the mixed venous blood still comes in at 72% but now is saturated to 96%
because there’s 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 low amount of blood flow going through that pulmonary capillary.
This allows for a condition in which this alveoli or air sac is over ventilated,
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 got 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 arterial.
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 or perfusion mismatch somewhere in the lungs,
the overall result will be a lower p small a O2.