Oxygen is compulsory. Without oxygen, we die.

But as you know, the byproduct of the process

that keeps us all alive, cellular respiration, is carbon dioxide,

or CO2, and it doesn't do our bodies a bit of good,

so not only do we need to take in the oxygen,

we also have to get rid of the CO2.

And that's why we have the respiratory and circulatory systems

to bring in oxygen from the air with our lungs,

circulate it to all of our cells with our heart and arteries,

collect the CO2 that we don't need with our veins,

and dispose of it with the lungs when we exhale.

Now, when you think of the respiratory system,

the first thing that you probably think of is the lungs.

But some animals can take in oxygen without lungs,

by a process called simple diffusion, which allows gases

to move into and pass through wet membranes.

For instance, arthropods have little pores all over their bodies

that just sort of let oxygen wander into their body,

where it's absorbed by special respiratory structures.

Amphibians can take in oxygen through their skin,

although they also have either lungs or gills

to help them respire,

because getting all your oxygen by way of diffusion

takes freaking forever.

So why do we have to have these stupid lung things

instead of just using simple diffusion?

Well, a couple of reasons.

For starters, the bigger the animal, the more oxygen it needs.

And a lot of mammals are pretty big, so we have to actively

force air into our lungs in order to get enough oxygen

to run our bodies.

Also mammals and birds are warm blooded,

which means they have to regulate their body temperatures,

and that takes many, many calories,

and burning those calories requires lots of oxygen.

Finally, in order for oxygen to pass through a membrane,

the membrane has to be wet, so for a newt to take oxygen

in through its skin, the skin has to be moist all the time,

which, you know, for a newt, isn't a big deal, but, you know,

I don't particularly want to be constantly moist, do you?

Fish need oxygen, too, of course, but they absorb oxygen

that's already dissolved in the water through their gills.

If you've ever seen a fish gill, you'll remember that they're just

sort of a bunch of filaments of tissue layered together.

This gill tissue extracts dissolved oxygen

and excretes the carbon dioxide.

Still, there are some fish that have lungs

like Lungfish, which we call Lungfish because they have lungs.

And that's actually where lungs first appeared

in the animal kingdom.

All animals from reptiles on up respire

with lungs deep in their bodies basically right behind the heart.

So while us more complex animals can't use diffusion

to get oxygen directly, our lungs can.

Lungs are chock full of oxygen-dissolving membranes

that are kept moist with mucus.

Moist with Mucus... another great band name.

The key to these bad boys is that lungs

have a TON of surface area, so they can absorb

a lot of oxygen at once.

You wouldn't know from looking at them, but human lungs

contain about 75 square meters of oxygen-dissolving membrane.

That's bigger than the roof of my house!

And the simple diffusion that your lungs use

is pretty freakin' simple.

You and I breathe oxygen in through our nose and mouth.

It passes down a pipe called your larynx which then

splits off from your esophagus and turns into your trachea,

which then branches to form two bronchi,

one of which goes into each lung.

These bronchi branch off again, forming narrower

and narrower tubes called bronchioles.

These bronchioles eventually end in tiny sacs called alveoli.

Each alveolus is about a fifth of a millimeter in diameter,

but each of us has about 300 million of them,

and this, friends, is where the magic happens.

Alveoli are little bags of thin, moist membranes,

and they're totally covered in

tiny, narrow blood- carrying capillaries.

Oxygen dissolves through the membrane and is absorbed

by the blood in these capillaries, which then goes off

through the circulatory system to make cells

all over your body happy and healthy.

But while the alveoli are handing over the oxygen,

the capillaries are switching it out for carbon dioxide

that the circulatory system just picked up from all over the body.

So the alveoli and capillaries basically just swap

one gas for another.

From there, the alveoli takes that CO2 and squeezes it out

through the bronchioles, the bronchi, the trachea,

and finally out of your nose and/or mouth.

So inhale for me once! Congratulations!

Oxygen is now in your bloodstream!

Now exhale! Wonderful!

The Co2 has now left the building!

And you don't even have to think about it, so you can think

about something more important like how many Cheetos

you could realistically fit into your mouth at the same time!

So, now you're all, "Yeah, that's great Hank,

but how do lungs actually work?

Like how do they do the thing where they do

where they get moved to come in and out and stuff?"

Well, eloquent question! Well asked!

Lungs work like a pump, but they don't actually

have any muscles in them that cause them to contract and expand.

For that we have this big, flat layer of muscles

that sits right underneath the lungs called the thoracic diaphragm

At the end of an exhalation, your diaphragm is relaxed,

so picture an arch pushing up on the bottom of your lungs

and crowding them out so that they don't have very much volume.

But when you breathe in, the diaphragm contracts

and flattens out, allowing the lungs to open up.

And as we know from physics, as the volume

of a container grows larger, the pressure inside it goes down.

And the fluids, including air, always flow down

their pressure gradient, from high pressure to low pressure.

So as the pressure in our lungs goes down, air flows into them.

When the diaphragm relaxes, the pressure inside the lungs

becomes higher than the air outside,

and the deoxygenated air rushes out.

And THAT is breathing!

Now, it just so happens that the circulatory system

works on a pumping mechanism just like the respiratory system.

Except, instead of moving air into and out of the lungs,

it moves blood into and out of the lungs.

The circulatory system moves oxygenated blood out of the lungs

to the places in your body that needs it

and then brings the deoxygenated blood back to your lungs.

And maybe you're thinking, "Whoa, what about the heart?!

Isn't the heart the whole point of the circulatory system?"

Well settle down! I'm going to explain.

We're used to talking about the heart

as the head honcho of the circulatory system.

And yeah, you would be in serious trouble if you didn't have a heart!

But the heart's job is to basically power the circulatory system,

move the blood all around your body and get it back

to the lungs so that it can pick up more oxygen and get rid of the CO2.

As a result, the circulatory system of mammals

essentially makes a figure-8:

Oxygenated blood is pumped from the heart to the rest of the body,

and then when it makes its way back to the heart again,

it's then pumped on a shorter circuit to the lungs

to pick up more oxygen and unload CO2 before it goes back

to the heart and starts the whole cycle over again.

So even though the heart does all the heavy lifting

in the circulatory system, the lungs are the home base

for the red blood cells,

the postal workers that carry the oxygen and CO2.

Now, the way that your circulatory system moves

the blood around is pretty nifty.

Remember when I was talking about air moving

from high pressure to low pressure?

Well, so does blood.

A four chambered heart, which is just one big honkin' beast

of a muscle, is set up so that one chamber,

the left ventricle, has very high pressure.

In fact, the reason it seems like the heart is situated

a little bit to the left of center is because the left ventricle

is so freaking enormous and muscley.

It has to be that way in order to keep the pressure

high enough that the oxygenated blood will get out of there.

From the left ventricle, the blood moves through the aorta,

a giant tube, and then through the arteries,

blood vessels that carry blood away from the heart,

to the rest of the body.

Arteries are muscular and thick- walled to maintain high pressure

as the blood travels along.

As arteries branch off to go to different places,

they form smaller arterioles and finally very fine

little capillary beds, which, through their huge surface area,

facilitate the delivery of oxygen

to all of the cells in the body that need it.

Now the capillary beds are also where blood picks up CO2,

so from there the blood keeps moving down

the pressure gradient through a series of veins.

These do the opposite of what the arteries did:

instead of splitting off from each other to become

smaller and smaller, little ones flow together

to make bigger and bigger veins

to carry the deoxygenated blood back to the heart.

The big difference between most veins and most arteries

is that instead of being thick-walled and squeezy,

veins have thinner walls, and have valves that keep

the blood from flowing backwards.

Which would be bad.

This is necessary because the pressure

in the circulatory system keeps dropping lower and lower,

until the blood flows into two major veins:

The first is the inferior vena cava, which runs pretty much

down the center of the body and handles blood

coming from the lower part of your body.

The second is the superior vena cava, which sits on top

of the heart and collects the blood from the upper body.

Together they run into the right atrium of the heart,

which is the point of the lowest pressure in the circulatory system.

So, all this deoxygenated blood is now back in the heart.

And it needs to sop up some more oxygen,

so it flows into the right ventricle,

and then into the pulmonary artery

now arteries, remember, flow away from the heart,

even though in this case it contains deoxygenated blood,

and pulmonary means "of the lungs,"

so you know this is the path to the lungs.

After the blood makes its way to the alveoli

and picks up some fresh oxygen, it flows to the pulmonary vein,

remember it's a vein because it's flowing to the heart,

even though it contains oxygenated blood

and from there it enters the heart again,

where it flows into the left atrium

and then into the left ventricle,

where it does the whole body circuit again.

And again and again and again. And that is the way that we work!

Our hearts are really efficient and awesome,

and they have to be, because we're endotherms, or warm-blooded,

meaning that we maintain a steady internal temperature.

Having an endothermic metabolism is really great

because you're less vulnerable to fluctuations

in external temperature than ectotherms, or cold-blooded animals

Also, the enzymes that do all the work in our bodies

operate over a very narrow range of temperatures.

In humans that range is between 36 and 37 degrees Celsius.

But the trade-off is that endotherms need to eat

constantly to maintain our high metabolisms and also create heat.

And for that we need a lot of oxygen.

Hence, the amazing, efficient 4-chambered heart

and our gigantic freakin' lungs.

Ectotherms, on the other hand, have slow metabolisms