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- [Voiceover] As you're
breathing and as the gas

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is moving in and out,
it's one of these things

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where we have to understand how it moves.

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This all comes back to partial pressure.

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Now partial pressure is
the pressure exerted by

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one particular gas in a mixture of gases.

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In the air we breathe, one particular gas

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is going to be oxygen.

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Another particular gas is going to be

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carbon dioxide. Then you end up with some

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nitrogen, carbon monoxide,
you have some water vapor.

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All these different
things are going to have

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their own individual partial pressure,

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and the amount of pressure a gas exerts

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is proportional to how
much total gas there is.

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So for example, nitrogen
is the most common

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gas in our environment.
That means nitrogen

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will have the highest partial pressure.

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The one rule to remember:

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gases always diffuse down
the pressure gradient.

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They always go from a
point of high pressure

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to a point of low pressure,
just like a balloon.

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What happens to any balloon any time

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you open it up, make a little slit,

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make a little cut in it?

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Where would the gases inside, whether it's

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compressed air from the
environment, helium,

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or anything else, always
comes rushing out.

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It's a higher pressure state inside.

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It's rushing out to a
lower pressure state.

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So we'll look at oxygen.

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Now, please don't memorize these numbers.

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They're just meant to give you

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an idea of what we're looking at.

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Oxygen diffuses from
the alveoli, so that's

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inside the lungs, into the blood.

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You can see in the lungs it's 104 mmHg.

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In the blood it's 40.
Goes from high to low.

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High in the lungs, low in the blood.

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While carbon dioxide goes from the blood

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and diffuses to the alveoli.

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That's because in the blood,

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carbon dioxide is 46 mmHg

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and in the alveoli, in the lungs,

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it's only 40. It's going from high to low.

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It's going down its pressure gradient.

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But now it's not just
happening in the lungs.

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It's happening in the
tissues all around your body.

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Oxygen is diffusing
down its gradient in the

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capillaries into the interstitial fluid.

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Well, interstitial fluid is the fluid

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around the cells of your body.

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That means oxygen
diffuses out of the blood

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into the liquid around the cells.

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It then diffuses from the liquid around

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the cells into the cells themselves.

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Carbon dioxide, it also diffuses down

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its pressure gradients, but it's going

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from the cells to the interstitial fluid,

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or the fluid around the cells,

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and then into the capillaries,

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because the ultimate
goal of carbon dioxide

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is to get it out of the system.

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So here's what we have.

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Up top in figure A, you
see dry inhaled air.

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How much oxygen compared
to carbon dioxide?

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But moist exhaled air, you notice there's

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less oxygen, more carbon dioxide.

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That makes sense. You're taking the oxygen

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and it's diffusing into the blood,

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so there's less of it there,

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and you're adding more carbon dioxide

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as it diffuses out of the blood.

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So if you look down at
the pulmonary circulation,

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part B, you can see how
the carbon dioxide is

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leaving the capillary while the oxygen

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is going into the capillary.

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But once you transport things around and

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get down to systemic
circulation down in D,

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you're going to notice
that the carbon dioxide

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is leaving the cell and
going into the blood,

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while oxygen is leaving the blood

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and going to the cell.

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So the constant process of diffusion,

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always high pressure to low pressure.

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Now the vast majority
of oxygen in the blood

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is bound by hemoglobin.

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Hemoglobin is a protein that's found

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in your blood cells.

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So you take hemoglobin, you bind oxygen,

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you end up with oxyhemoglobin.

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But that remaining 2% just simply

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dissolved right in the blood plasma.

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Now carbon dioxide isn't
quite that straightforward.

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About 70% of your carbon dioxide is

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converted into a molecule
known as bicarbonate.

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Bicarbonate is part of a buffering system.

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It helps the blood reduce
the risk of large pH changes,

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while another 10% is dissolved
straight off in the plasma.

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So you get straight CO₂ dissolved,

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which leaves us with 20%.

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That 20% of carbon dioxide can actually

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bind to hemoglobin, but it can only bind

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to hemoglobin if there
are no oxygen molecules.

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So those spaces where the oxygen molecules

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were released, carbon
dioxide can come in and bind.

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So the majority of it,
70% or so, is as bicarb.

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The other 20% hemoglobin, 10% plasma.

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So here you can see
just a little schematic.

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In A you can see from the lungs

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the O₂ is heading into the
red blood cell, majority,

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but a little bit goes
into right in the plasma,

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while in figure B on
the right, you can see

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how carbon dioxide has
three different pathways.

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The majority, about 70%, is going

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into the red blood cell and is being

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converted into bicarbonate.

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10% of CO₂ goes right in
and dissolves in plasma,

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and the last 20% actually
binds up with hemoglobin,

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but by far the majority is being converted

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into bicarbonate, HCO₃-.