- [Voiceover] As you're breathing and as the gas
is moving in and out, it's one of these things
where we have to understand how it moves.
This all comes back to partial pressure.
Now partial pressure is the pressure exerted by
one particular gas in a mixture of gases.
In the air we breathe, one particular gas
is going to be oxygen.
Another particular gas is going to be
carbon dioxide. Then you end up with some
nitrogen, carbon monoxide, you have some water vapor.
All these different things are going to have
their own individual partial pressure,
and the amount of pressure a gas exerts
is proportional to how much total gas there is.
So for example, nitrogen is the most common
gas in our environment. That means nitrogen
will have the highest partial pressure.
The one rule to remember:
gases always diffuse down the pressure gradient.
They always go from a point of high pressure
to a point of low pressure, just like a balloon.
What happens to any balloon any time
you open it up, make a little slit,
make a little cut in it?
Where would the gases inside, whether it's
compressed air from the environment, helium,
or anything else, always comes rushing out.
It's a higher pressure state inside.
It's rushing out to a lower pressure state.
So we'll look at oxygen.
Now, please don't memorize these numbers.
They're just meant to give you
an idea of what we're looking at.
Oxygen diffuses from the alveoli, so that's
inside the lungs, into the blood.
You can see in the lungs it's 104 mmHg.
In the blood it's 40. Goes from high to low.
High in the lungs, low in the blood.
While carbon dioxide goes from the blood
and diffuses to the alveoli.
That's because in the blood,
carbon dioxide is 46 mmHg
and in the alveoli, in the lungs,
it's only 40. It's going from high to low.
It's going down its pressure gradient.
But now it's not just happening in the lungs.
It's happening in the tissues all around your body.
Oxygen is diffusing down its gradient in the
capillaries into the interstitial fluid.
Well, interstitial fluid is the fluid
around the cells of your body.
That means oxygen diffuses out of the blood
into the liquid around the cells.
It then diffuses from the liquid around
the cells into the cells themselves.
Carbon dioxide, it also diffuses down
its pressure gradients, but it's going
from the cells to the interstitial fluid,
or the fluid around the cells,
and then into the capillaries,
because the ultimate goal of carbon dioxide
is to get it out of the system.
So here's what we have.
Up top in figure A, you see dry inhaled air.
How much oxygen compared to carbon dioxide?
But moist exhaled air, you notice there's
less oxygen, more carbon dioxide.
That makes sense. You're taking the oxygen
and it's diffusing into the blood,
so there's less of it there,
and you're adding more carbon dioxide
as it diffuses out of the blood.
So if you look down at the pulmonary circulation,
part B, you can see how the carbon dioxide is
leaving the capillary while the oxygen
is going into the capillary.
But once you transport things around and
get down to systemic circulation down in D,
you're going to notice that the carbon dioxide
is leaving the cell and going into the blood,
while oxygen is leaving the blood
and going to the cell.
So the constant process of diffusion,
always high pressure to low pressure.
Now the vast majority of oxygen in the blood
is bound by hemoglobin.
Hemoglobin is a protein that's found
in your blood cells.
So you take hemoglobin, you bind oxygen,
you end up with oxyhemoglobin.
But that remaining 2% just simply
dissolved right in the blood plasma.
Now carbon dioxide isn't quite that straightforward.
About 70% of your carbon dioxide is
converted into a molecule known as bicarbonate.
Bicarbonate is part of a buffering system.
It helps the blood reduce the risk of large pH changes,
while another 10% is dissolved straight off in the plasma.
So you get straight CO₂ dissolved,
which leaves us with 20%.
That 20% of carbon dioxide can actually
bind to hemoglobin, but it can only bind
to hemoglobin if there are no oxygen molecules.
So those spaces where the oxygen molecules
were released, carbon dioxide can come in and bind.
So the majority of it, 70% or so, is as bicarb.
The other 20% hemoglobin, 10% plasma.
So here you can see just a little schematic.
In A you can see from the lungs
the O₂ is heading into the red blood cell, majority,
but a little bit goes into right in the plasma,
while in figure B on the right, you can see
how carbon dioxide has three different pathways.
The majority, about 70%, is going
into the red blood cell and is being
converted into bicarbonate.
10% of CO₂ goes right in and dissolves in plasma,
and the last 20% actually binds up with hemoglobin,
but by far the majority is being converted
into bicarbonate, HCO₃-.