Breathing is one of the few things you do
continuously, everyday, without thinking about
it.
Except now — now that I pointed it out,
you’re definitely thinking about breathing.
Our lives would be completely different if
we couldn’t breathe air — more technically,
if we couldn’t utilize oxygen.
See, our bodies, like other vertebrates, depend
on oxygen to run our normal aerobic metabolisms.
Some of our bodily processes happen without
oxygen, but to really take advantage of the
food we eat, we need to burn some oxygen.
That means at some point, we need to start
plucking oxygen out of the air and shoving
it down our throats and into our red blood
cells.
But here’s what I love about this topic.
This biological process is really dependent
on physics.
Those ideal gas laws you may have learned
in high school?
Dalton’s law?
We see a beautiful example of them in the
simple act of taking a breath.
So breathe in and breathe out, today we’re
talking about the respiratory system.
We’ve been talking a lot about blood in
this series so far, and with good reason.
Multiple substances need to get to their target
tissues so we can have raw materials to carry
out some key physiology.
Once those materials are in the bloodstream,
the circulatory system delivers them to their
destinations.
And of course, one of the most important of
those materials is oxygen.
It’s everywhere around us, so all we have
to do is pick it out from the air we breathe.
That’s where the respiratory system comes
in, all the hardware involved to breathe in
and breathe out.
You’re already familiar with the big players
here, the lungs.
You have two of these spongy pink air sacs
that span from your stomach to your breastbone,
and while they’re similar to each other,
they’re not identical.
Your right lung — your right, not the right
of your screen — has three lobes, while
your left lung has two.
That’s because your heart rests in between
your lungs, ever so slightly askew to the
left in a little nook called the cardiac notch.
And below all that, you’ll find a weird
shaped, kinda round, kinda dome-shaped muscle
called the diaphragm.
This muscle is a huge deal for breathing.
See, your lungs don’t have any muscles of
their own.
They just go along for the ride with the rib
cage.
So when the diaphragm contracts along with
the external intercostal muscles between the
ribs, they expand the space, or volume, inside
the chest.
What that does is change the pressure inside
the lungs since volume and pressure are inversely
related.
So as the volume of a container increases,
the pressure on all those air molecules decreases.
And vice versa — as the volume decreases,
pressure increases.
And yes, it may seem scary that physics is
coming up in an anatomy video, but the movement
of that vitally important oxygen depends on
pressure differences.
That’s because our lungs can be thought
of as containers for gas.
So when you contract your diaphragm and expand
your chest’s volume, there’s less pressure
on the air inside compared to the air outside
your body, the stuff that you’re breathing
in.
This is where another physics law comes in.
Whenever there’s a difference in pressure
between two gases and they’re connected
somehow, their pressures will tend to equalize.
That means a gas will move from areas of high
pressure to low pressure.
It doesn’t matter if we’re talking about
gas molecules in a weather system or in our
physiology.
Gases tend to flow from high pressure to low
pressure.
So with a reduced pressure inside the chest
and constant pressure in the air around us,
the lungs fill with air.
The opposite happens when you exhale.
Your diaphragm and intercostals relax, which
decreases the space in your lungs, and with
more pressure inside the lungs than outside,
air flows outwards.
Regular breathing really has nothing to do
with sucking air in or squeezing air out.
You’re just letting physics do its thing
to your lungs.
But all that depends on air actually getting
into your lungs, so you have a few organs
in place to get air from outside your body
into your lungs.
Despite starring roles in your ability to
appreciate tacos, your mouth and nose are
the big external interfaces for your respiratory
system.
And they both act as air-treatment centers,
keeping the air warm and humid, and trapping
any dust before it gets too far.
Plus, the airway is lined with mucus membranes
full of immune cells to make sure pathogens
don’t creep in.
Yep, the same mucus membranes that create
boogers.
After air comes in, it flows down cartilaginous
tubes past the larynx, where we can find your
vocal folds that make your beautiful voice.
From here downward, your airway looks an awful
lot like an upside down tree.
In this case, the tree trunk, or trachea,
is a thick tube of epithelial tissue surrounded
by C-shaped cartilage rings.
It traces the path of your sternum, right
about here, where it splits.
Then the trachea branches off into two bronchi.
Those branches keep splitting off further
and further throughout the lung until they
become little twigs, or bronchioles.
These twigs are only about a millimeter thick
and at this point they’re not producing
any mucus.
Each of those tiny bronchial branches have
anywhere from two to eleven leaves, or alveoli.
Not to be confused with ravioli which is a
delicious pasta dish and has nothing to do
with breathing.
These alveoli leaves interact with gases really
similarly to how real leaves interact with
the Carbon Dioxide around them.
These alveoli, hundreds of millions of them
are where air really starts interacting with
our physiology.
See, those alveoli have really thin walls,
and they’re surrounded by extremely tiny
blood vessels called capillaries which also
have really thin walls.
In order for us to get oxygen in and carbon
dioxide out, those gas molecules need to cross
this barrier.
Remember, these aren’t thick concrete walls,
they’re squishy, mobile cells that readily
let certain substances cross.
But these are gas molecules, they’re not
actively swimming through fluid.
So how do they get across the membrane?
It happens thanks to diffusion, a physics
concept you’re familiar with if you use
a perfume or spray deodorant.
At first, the concentration is greatest around
the spray bottle, so the smell is the strongest
around that area.
But then, as the odorant molecule spread throughout
the room, even people far away from the source
can smell it.
And the smell is weaker at the source.
Those molecules spread out evenly throughout
the room.
In the case of diffusing oxygen, it diffuses
through the membrane.
That same principle of diffusion is at work
allowing oxygen into our bloodstream but of
course, that comes with some asterisks.
One of those is because air is transferring
from a gas, the atmosphere, to a liquid, your
blood.
So any given air molecule has to be soluble,
or able to dissolve in your blood, if you
want it to travel through your circulation.
For example, carbon dioxide is very soluble
in liquids, while Nitrogen, literally eighty
percent of the air we breathe, is not very
soluble.
Now, oxygen isn’t very soluble either, but
it takes advantage of another reason that
gases move — pressure differences.
If you had equal levels of oxygen in your
alveoli and in the capillaries around them,
oxygen wouldn’t move across the barrier.
But when we study the movement of dissolved
particles between a liquid and a gas, like
in this instance, we have to compare the pressures
of individual gases.
I’ll explain.
Air pressure itself is a thing because a bunch
of different gas particles collide and bump
into the walls of their container.
When we measure those forces for a given sample
of gas, we call that pressure.
When they collide faster and harder, that’s
a greater pressure — when there are less
and weaker collisions, that’s less pressure.
Now, if you were to take some of the gas particles
away from a sample, you would change the overall
pressure.
After all, those molecules were contributing
to all the bumps and collisions.
And through the beautiful and strange magic
of math, as long as we know the concentration
of the gases in a sample, and the volume doesn’t
change, we can calculate how much each of
those gases contributes to overall pressure.
This is called partial pressure, how much
pressure each gas exerts by itself.
And in the real world, the air in our alveoli
is a mixture of gases — mostly Nitrogen
but also Oxygen, water vapor, and carbon dioxide.
The partial pressure of these gases drives
gas exchange all over the body.
This is why I spent so much of your time talking
about partial pressure.
Oxygen isn’t a person, it can’t move across
membranes just because it feels like it.
It just goes along for the ride.
Speaking of a ride, buckle up, we’re about
to follow an oxygen molecule around the body.
Starting in the alveoli, the partial pressure
of oxygen is higher than in the capillaries
around it, and with that partial pressure
difference, oxygen flows into the blood.
Those oxygen molecules bind to the hemoglobin
in red blood cells and get transported around
the body to oxygen-hungry tissues.
Once it gets to the tissues, we see partial
pressure differences again!
Its partial pressure in the tissues is lower
than the blood, so it flows into the tissues.
The tissues consume that oxygen as part of
their aerobic metabolism and produce carbon
dioxide as a byproduct.
Then to get rid of that CO2, they dump it
back into the bloodstream.
It’s not as crude as it sounds, but either
way your body now has carbon dioxide heading
back to the lungs via blood.
When that blood makes its way back to the
lungs, the partial pressure of carbon dioxide
is higher in the blood than the alveoli, so
it flows out.
And just like that, oxygen comes in, carbon
dioxide goes out and we keep on living.
So at this point in the series we know how
we get oxygen into our bodies and how we deliver
it to different tissues.
Next time, we’ll take a look at one of my
favorite topics, hormones and steroids.
Thanks for watching this episode of Seeker
Human.
I’m Patrick Kelly.
