Respiratory System, part 2: Crash Course A&P #32

Can a paper bag really help you when you are hyperventilating? It turns out that it can. In part 2 of our look at your respiratory system Hank explains how your blood cells exchange oxygen and CO2 to maintain homeostasis. We'll dive into partial pressure gradients, and how they, along with changes in blood temperature, acidity, and CO2 concentrations, change how hemoglobin binds to gases in your blood. (And yes, we'll explain the paper bag thing too!)

Table of Contents
How Blood Cells Exchange Oxygen and CO2 2:23
Partial Pressure Gradients 2:41
How Hemoglobin Binds to Gases in the Blood 4:40
The Thing With The Bag 9:04

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Picture this: You’re getting ready to give
a big presentation in front of, like, a lot

of important people. You’re practicing in
front of your mirror, and then just for a

second you forget how to speak.

Suddenly, you feel that familiar sting of anxiety,
like an icy hand on the back of your neck.

You look at yourself in that mirror and you
start imagining some of the worst, worst-case

scenarios. Like, what if you totally lose
your train of thought up there? What if you

barf? What if everybody gets up and leaves?
Now you’re really nervous. I’m getting

freaked out just talking about it.

So ou start taking quick, shallow breaths,
and you’re feeling light-headed, and seeing

stars, and now you, my friend, are hyperventilating.

When we talk about respiration, we tend to
focus on oxygen -- and who could blame us?

It’s easy to forget the equally important
role that carbon dioxide plays in maintaining

homeostasis. Your internal balance between
oxygen and carbon dioxide factors heavily

into all sorts of stuff -- especially in your
blood, where it can affect your blood’s

pressure, its pH level, even its temperature.

And now -- at, like, T-minus 5 minutes to
your presentation -- all of those things are

out of whack, because you’re exhaling more
CO2 than you should.

You’re just about to faint, when a friend
suddenly hands you a paper bag to breathe

into. And you’ve never been so grateful
for a lunch bag in your life, because, somehow,

it does the trick.

Within seconds, you’re back to normal.

The drop in CO2 that occurs in your blood
when you hyperventilate is called hypocapnia,

and it signals a breakdown in one of the most
complex and important functions that your

respiratory system performs.

That is: the exchange of gases inside your
blood cells, where the stuff your body doesn’t

want is swapped out for what it desperately
needs.

This exchange -- between carbon dioxide and
oxygen -- is regulated by a whole series of

biological signals that your blood cells use
to communicate with your tissues, about what

they have, what they want, and what they need
to get rid of.

It’s almost like a code, one that’s written
into your blood’s chemistry, in the folding

of its proteins -- even in its temperature
and acidity.

It’s what allows you to perform strenuous
physical tasks, like climbing a mountain.

It’s also what lets you reboot your whole respiratory
system, with nothing more than a paper bag.

I’ll admit it: when we’ve talked about the
chemistry of your blood so far, we’ve

tended to keep things pretty simple.

Like, hemoglobin contains four protein chains,
each of which contains an iron atom; since

iron binds readily with oxygen, that’s how
hemoglobin transports oxygen around your body.

Ba-da-bing.

But the fact is, hemoglobin’s affinity for
oxygen isn’t always the same.

In some places, we want our hemoglobin to
have a high affinity for oxygen, so it can

easily grab it out of the air.
And in others, we want it to have

a low affinity for oxygen oxygen, so it can
dump those molecules to feed our cells.

So how does your hemoglobin know when to collect
its precious cargo and when to let it go?

Well, a lot of it has to do with a principle
of chemistry known as partial pressure.

One of the things that fluids always do is
move from areas of high pressure to low pressure.

And molecules also diffuse from areas of high
concentration to areas of low concentration.

But when we talk about gases in a mixture, we need
to combine the ideas of pressure and concentration.

See, air is a mixture of molecules. And when
you’re studying the respiratory system,

you often need to focus on the oxygen, which
makes up about 21% of it.

But that doesn’t tell us how many oxygen
molecules there are. For that, we need to

know the overall air pressure, since more
molecules in a certain volume means more pressure.

So, partial pressure gives us a way of
understanding how much oxygen there is,

based on the pressure that it’s creating.

Example: The pressure of air at sea level
is about 760 millimeters of mercury. But since

only about 21 percent of that air is oxygen,
oxygen’s part of that pressure -- or partial

pressure of oxygen -- is 21% of 760, or about
160 millimeters of mercury.

Now, that’s just outside, at sea level.

When that air mixes with the air deep in your
lungs -- including a lot of air that you haven’t

exhaled yet -- the partial pressure of oxygen
drops to about 104 millimeters of mercury.

And in the blood that’s entering your lungs
-- after most of its oxygen has been stripped

away by your hungry muscles and neurons -- the
oxygen partial pressure is only about 40 millimeters.

This big differences in pressure make it easy for
oxygen molecules to travel from the outside air into

your blood plasma, because, as a rule dissolved gases
always diffuse down their partial pressure gradients.

This is why it’s so much harder to breathe
at higher altitudes. When you climb a mountain,

the concentration of oxygen stays at about 21%.
But the pressure gets lower, which means the

partial pressure of oxygen also decreases to about
45 millimeters of mercury at the top of Mt. Everest.

So the partial pressure of oxygen at the top
of the highest peak in the world, is almost

the same as the de-oxygenated blood that’s
entering your lungs.

So basically there is no partial pressure
gradient, which makes it really hard to get

oxygen into your blood. But, let’s get back
to the red blood cells.

Remember that the globin in your hemoglobin
is a protein -- and when proteins bind to

stuff, they tend to change shape. And that
shape-change can make the protein more or

less likely to bind to other stuff.

When an empty hemoglobin runs into an oxygen
molecule, things are a little awkward.

It’s like a first date -- bonding isn’t
so easy.

But once they finally bind, hemoglobin suddenly
changes shape, which makes it easier for other

oxygen molecules to attach, like friends gathering
around the lunch table.

That affinity for joining in -- or cooperativity,
as it’s known -- continues until all four

binding sites are taken, and the molecule
is fully saturated.

Now your hemoglobin is known as oxyhemoglobin,
or HbO2. It is not...not why the cable network

is called that. That’s the “Home Box Office.”
Anyway.

By the time the blood leaves the lungs, each
hemoglobin is fully saturated, the oxygen

partial pressure in your plasma is about 100
millimeters, and now it is ready to be delivered

to where it is needed most.

Active tissues, like the brain, heart, and
muscles, are always hungry for oxygen. They

burn through it quickly, lowering the oxygen
partial pressure around them to about 40 millimeters.

So when the blood arrives on the scene, oxygen
moves down the gradient from the plasma to

the tissues, to feed those hungry cells.

That makes the oxygen partial pressure in
your plasma drop, so your hemoglobin starts

to give up more of its oxygen to the plasma.

BUT! Partial pressures are only part of what’s
prodding your hemoglobin to give up the goods.

All of that metabolic activity going on in
your tissues is also producing other triggers,

in the form of waste products -- specifically
heat and CO2.

Both of those things activate the release of more
oxygen, by lowering hemoglobin’s affinity for it.

Say you’re climbing that mountain again,
and your thighs are feeling the burn. Red

blood cells saturated with oxygen are going
to the muscle tissue in your quads, where

the hemoglobin can dump a bunch of O2, because of
the lower partial pressures of oxygen in your muscles.

But a hard-working quad will also heat up
the surrounding tissues, and that rise in

temperature changes the shape of hemoglobin -- and
it does it in such a way that lowers its affinity for O2.

So when those red blood cells hit that warm
active tissue, they release even more oxygen

-- like 20 percent more -- beyond what partial
pressures would trigger.

But wait! There’s more!

Carbon dioxide triggers the release of oxygen,
too, because it also binds to the hemoglobin,

changing its shape again, lowering its affinity
for oxygen still more. And as oxygen jumps

ship, the hemoglobin can pick up more CO2.

Finally, JUST IN CASE the hemoglobin isn’t
getting the message at this point, there’s

one more trigger that your respiratory system
has up its sleeve. The spike in CO2 that’s

released by your active muscle tissues
actually makes your blood more acidic.

Since your blood is mostly water, when CO2
dissolves in it, it forms carbonic acid, which

breaks down into bicarbonate and hydrogen
ions. Those ions bind to the hemoglobin, changing

its shape yet again, further lowering its
affinity for oxygen.

So now, at last, your tissues have the oxygen
they need, and your red blood cells are stuck

with all this CO2 that they need to get rid of.

Your red blood cells ride the vein-train
back to the lungs,

where they encounter a new wave
of freshly inhaled oxygen.

And when that O2 binds to the hemoglobin -- which,
again, is hard at first -- it eventually changes

its shape back to the way it was when we started,
which decreases its affinity for CO2.

So the hemoglobin drops its carbon dioxide,
which moves down its partial pressure gradient

into the air of your lungs, so you can exhale it,
and the whole thing can start all over again.

Now if that isn’t enough to make you hyperventilate,
I’m not sure what is.

But this brings us back to that unfortunate
episode you had before your big presentation.

This whole complex code of chemical signals
that I just described? Well, it assumes that

what your cells and tissues are telling each
other is actually true.

But as we all know, sometimes our bodies don’t
mean what they say. Thanks, body.

Like, when you’re freaking out about your
presentation, your sympathetic nervous system

makes your heart race and your breathing increase,
to prepare you to fight or flee.

The problem is: there’s nothing to actually
fight or flee from, so your muscles aren’t

actually doing anything, so they’re not using
all the extra oxygen you’re breathing in.

And they also aren't producing the extra CO2 that
you're suddenly exhaling all over the place.

So when you start to exhale CO2 faster than
your cells release it, its concentration in

your blood drops. And with less carbonic acid
around, your blood’s pH starts to rise.

And you know what else? While low blood pH
does things like change the shape of your

hemoglobin to deliver oxygen, high pH causes
vasoconstriction.

Normally, this is supposed to divert blood
from the parts you’re not using during times

of stress, like your digestive organs, to
the parts that you are using.

But when you hyperventilate, this constriction
happens everywhere, which means less blood

is delivered to your brain, which makes you
light-headed.

Luckily, that trick with the breathing into
the paper bag -- it really does work.

It works because it lets you breathe back
in all of the CO2 you just breathed out. So

the partial pressure of carbon dioxide in
the bag is higher, which forces that CO2 into

your blood, which lowers its pH, and you get
back to homeostasis.

And of course, homeostasis is the key to life...and
you know, also to a successful presentation.

If you were able to remain calm today, you
learned how your blood cells exchange oxygen

and CO2 to maintain homeostasis. We talked
about partial pressure gradients, and how

they, along with changes in blood temperature,
acidity, and CO2 concentrations, change how

hemoglobin binds to gases in your blood. And
you learned how the thing with the bag works.

Of course, we must say thank you to our patrons on
Patreon who help make Crash Course possible through

their monthly contributions, not just for themselves,
but for everyone. If you like Crash Course and want to

help us keep making videos like this one,
you can go to patreon.com/crashcourse.

This episode was filmed in the Doctor Cheryl
C. Kinney Crash Course Studio, it was written

by Kathleen Yale, the script was edited by
Blake de Pastino, and our consultant is Dr.

Brandon Jackson. It was directed and edited
by Nicole Sweeney; our sound designer is Michael

Aranda, and the graphics team is Thought Cafe.