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Let me introduce you to one of the bravest
pioneers in the history of life on planet Earth.
An organism that blazed the trail for every
single vertebrate that lives on land today
-- and many that don’t.
It’s one of your most important ancestors.
Meet…well, it doesn’t have a name.
And we don’t know exactly what it looked
But we do know that about 380 million years
ago, this fishy-looking thing with big, fleshy
fins achieved one of the animal kingdom’s
greatest milestones: breathing air.
Sounds simple enough, but believe me it wasn’t.
Because, for billions of years before this
fishy ancestor came around, basically all
of life evolved in water.
From the very beginning, the earliest, simplest
forms of life -- like bacteria -- extracted
oxygen they needed right from the water, through
And they did it through simple diffusion -- when
a material automatically flows from where
it is concentrated, to where it is less concentrated,
so it balances out.
Diffusion works really well, and it requires zero
effort, but it wasn’t gonna cut it in the big leagues.
Anything larger than a small worm is simply too
big and needs too much oxygen for diffusion to work.
So in order to get bigger, early life forms
needed a circulatory system that could move
bulk amounts of oxygen around faster inside
their bodies, and a respiratory system to
bring more oxygen in contact with their wet
So their respiratory surfaces moved from their
outer surfaces to the insides of their bodies.
First, there were gills. But gills, of course,
still only work inside of water.
And a little over 380 million years ago, this
was starting to lose some of its charm.
Earth was getting warmer, the seas were getting
shallower, and much of the planet’s surface
water had lower concentrations of oxygen than
it used to.
Finally, a humble little lobe-finned fish got fed up, swam
up to the water’s surface, and started breathing air.
It could do this because it had evolved a
fancy new interface to move gases between
the air and its cell membranes.
I’m talkin’ about lungs. Wet lungs.
With an efficient new way to take in nearly
limitless amounts of oxygen from air, animals
were eventually able to get bigger and more
diverse over the ages, and now all of us lung-having
vertebrates share that common ancestor.
For lots of animals, including humans, those
lungs come with a bunch of other equipment,
like protective ribs, a stiff trachea, and
in mammals a strong diaphragm. And together,
they form your respiratory system.
Which happens to be best friends and business
partners with your circulatory system.
It’s only by working together and using
both the bulk flow and simple diffusion of
oxygen that they can make possible the process
of cellular respiration.
In other words: life itself.
So, a lot of improvements have been made to
it over the eons, but the respiratory system
that you are using right now is your inheritance
from that ancient, ambitious fish -- leader
of one of the most important anatomical revolutions
of the past half-billion years.
Pretend for a minute that you can’t breathe.
Like, you just don’t have lungs anymore.
You are some bizarre evolutionary oddity -- a
huge, human-shaped organism that doesn’t
have a respiratory system.
Instead, you get all of your oxygen the way
that your oldest, smallest evolutionary ancestors
did -- by simple diffusion.
Or at least, you try to get your oxygen that
How would it work?
And that’s partly because one of the keys to
efficient diffusion of any material is distance.
If you want a molecule to diffuse across a
space quickly, you want it to be as close
to its destination as possible, with the fewest
obstacles in the way.
But, for a single molecule of oxygen to diffuse
from the air through, say, your scalp and
then go to a neuron deep inside your brain,
it would have to move through your skin, and
then your skull, and then your connective
tissue and all sorts of things.
It would eventually get there, like maybe
a month later, but at that point, the cell
that needed the oxygen in the first place
would have, you know, suffocated to death.
Basically, obtaining oxygen through diffusion
alone is like wanting to go to a party at
your friend’s place across town, and then
walking 20 miles to get there. You could do it,
but it would take forever, and by the time you arrived,
you’d be all haggard and the party would be over.
So, diffusion alone isn’t enough to get the job done.
We do use it, but only when a whole bunch of the
materials we need are right up against
the tissues that can absorb them.
So you know what else we need? Bulk flow.
Bulk flow is like public transportation -- it
moves large numbers of molecules, quickly.
Rather than walk the whole way across town,
you can hop on a bus with a bunch of other
people, and get there in twenty minutes.
Every time you take a deep breath, you’re
bringing a hundred quintillion oxygen molecules
into your lungs all at once -- they’re on
a bulk-flow bus ride.
And once those oxygen molecules filter down
into the cells in your lungs, they’re suddenly
very close to the blood they’re trying to
reach. All they have to do is diffuse across
four layers of cell membranes to get from
the lung cell into the blood.
It’s like just hopping off the bus, and then
walking half a block to your friend’s apartment.
That’s why your respiratory system is the
way it is: It’s set up to take full advantage
of both bulk flow and simple diffusion.
The bulk flow part of things is handled by some of
your system’s biggest and most obvious moving parts.
Starting with your lungs, which basically
operate like a pump, or a bellows.
They don’t have any contractible muscle
tissue, because they need to be able to expand,
so they require outside help in order to move.
Enter the diaphragm -- a big, thin set of muscles
that separates your thorax from your abdomen.
When your lungs empty, your diaphragm relaxes and
looks kinda like an arc pushing up to squish your lungs.
You also have the weight of your rib cage,
pushing on your lungs from the top and sides,
and together these forces decrease the volume
of your lungs.
When you breathe in, your diaphragm contracts,
pulling itself flat, and your external intercostal
muscles between your ribs contract. They lift the
ribs up and out, causing the chest cavity to expand.
This makes the pressure inside the lungs lower
than the air outside your body, and -- since
fluids like gases move from areas of high pressure
to low pressure -- the lungs fill up with outside air.
Then the diaphragm relaxes again, and the
weight of the ribs settles in, and the pressure
inside the lungs becomes higher than the outside
air, and the air rushes out.
And that, my friends, is breathing 101.
Now, your respiratory system contains a lot
of parts besides your lungs -- some prominently
displayed on your face, others hidden deep
within your chest. And functionally, all of
these organs fall into one of two physiological
The upper parts that funnel the air in, make
up what’s known as the conducting zone,
and it starts with this thing.
Your nose is supported by bone and cartilage,
and the bristly hairs and mucus inside it
that help filter out dust and other particles.
But it, along with your sinuses, performs
another important function: It warms and moistens
incoming air, so it doesn’t dry out those
sensitive lung cells that must remain wet.
Remember, moisture is key. We evolved from
organisms that lived in water. So, just like
with our aquatic bacterial ancestors, we need
water for oxygen to dissolve into, before
it can diffuse across the phospholipid bilayer
membrane of our cells.
Now, if you’ve ever choked on a poorly timed
sip of water, you’ve noticed that you breathe
through the same tube that you also move foods
and liquids through.
This is yet another leftover from those first
fish lungs, which evolved as a branch off
the esophagus. Looking back, it was not
ideal. But we are stuck with it.
So, the stuff that you swallow soon encounters
the epiglottis -- a little trap door of tissue
-- which covers the larynx, and directs bites
of sandwich and sips of cola toward your esophagus
and keeps them out of your lungs.
And you’ll notice that the esophagus, which
heads to your stomach, is nice and flexible,
while your trachea, or windpipe, is rigid
and has prominent rings.
That’s because your trachea is basically built
like a vacuum hose -- since the lungs create
negative pressure with every breath, the trachea needs
those rings to keep it open. If it were soft and floppy,
it would collapse every time the pressure dropped,
and you wouldn’t be able to breathe.
From there, the trachea splits in two, forming
the right and left main bronchi. You can imagine
these inner lung parts as sort of an upside-down
Now we are in the lung tissue, and have entered
what we call the respiratory zone. This is
where the actual gas exchange occurs, and everything
you find here has a form to suit that function.
So the smaller branches of the upside-down
tree are bronchioles, which taper down into
progressively narrower tubes, until they empty
into the alveolar ducts and then dead end
into tiny alveolar sacs, where the bulk of
the gas exchange finally occurs.
Because that’s because each sac contains
a cluster of alveoli, these tiny cavities
lined with super thin, wet membranes made
of simple squamous epithelium tissue.
It’s here that oxygen molecules dissolve
in the wet mucous, diffuse across the epithelial
cells, and then cross the single layer of endothelial
cells lining the capillaries to enter the bloodstream.
And of course it’s also where carbon dioxide
diffuses out of the blood, and then follows
the same route back up to the nose and mouth,
where it’s exhaled.
So it’s your alveoli where diffusion meets bulk
flow. Because while you’re picking up oxygen
and dispensing with CO2 one molecule at a time, you're
doing it in enormous quantities at any given second.
Both of your lungs contain about 700 million
alveoli, which together provide an amazing
75 square meters of moist membrane surface
So, the principles that make respiration possible
are relatively simple -- diffusion and bulk flow.
And so are the mechanisms in your body that
It just took us about 400 million years to
figure out how to make it all work.
But today you learned how it does work -- including
the mechanics of both simple diffusion and
bulk flow, and the physiology of breathing,
and the anatomy of the conducting zone, and
the respiratory zone, of your respiratory
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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 Nicholas Jenkins; the script supervisor
was Nicole Sweeney; our sound designer is Michael
Aranda, and the graphics team is Thought Cafe.