This is actually just my first course.

For my birthday, the writers wrote me a script

where I just get to eat!

The whole time!

And I can't think of a better way to demonstrate

the workings of the digestive system,

the series of hollow organs that we use to break down

and process the nutrients and energy we need to function.

Though...wait a second, if I remember correctly

digestion is actually pretty freaking disgusting...

so maybe I shouldn't be eating right now.

Oh whatever... Waiter!

The digestive system is so fundamental that it's basically

step number one in the guide: How to Make an Animal.

You probably remember that during the embryonic development of most

animals, the digestive tract is the very first thing that forms.

When the blastula, that little wad of cells that we all used to be,

turns into a little wad of cells with a tube running through it,

that tube is your digestive system.

And pretty much every animal has a digestive system of some kind,

but they're not all alike.

Far from it.

In fact, digestive tracts are specially adapted

to animals' feeding behavior and diet.

For instance, a house fly eats mostly liquid or very finely

granulated food, but before it does that,

it's got to puke its digestive juices all over its lunch

and then let them digest it for a while

before it sucks it up into its mouth.

If we did it like that, first dates would be...less common.

Most vertebrates put food in one end of the tube

and our digestive system processes it,

and then it gets rid of the waste out the other end of the tube.

No muss, no fuss.

Well, actually, there's a little bit of muss, at the end.

You may have noticed.

But the beauty of it is that this whole process is run by our

autonomic nervous system, so we don't have to think about it,

until maybe the very last step when we're in traffic

and just had two cups of coffee and a bran muffin...

then we have to think about it a little bit.

Among vertebrates, the digestive tract might be short or long,

or have organs that do different things

depending on what its feeding habits are.

For instance, dogs are mostly carnivores and also scavengers:

They mostly eat meat,

but sometimes that meat's been dead for a while.

So, the dog's digestive system has developed to take food in,

absorb as many nutrients as possible,

and then deposit it on somebody's lawn,

all in a period of about six hours.

Dogs have an extremely short digestive tract, because,

if you're in the habit of eating rotten meat,

you'd better be able to digest it fast.

If you don't, the bad bacteria that's probably living on that

armadillo carcass is going to take up residence in your gut

and put you in a world of hurt.

Cows, on the other hand, take a very very very long time

to digest their food, around 80 hours,

because they have to process plants, mostly grass.

Grass has a ton of cellulose in it, and evolution has yet

to produce an animal that can manufacture a stomach acid

or enzyme tough enough to break down cellulose.

So, cows have microorganisms in their guts

that break down the cellulose for them.

This process takes a four-chambered stomach

each one with a slightly different microecology

and a lot of cud-chewing,

or regurgitating and re-chewing of grass

before it passes all the way through.

So, nature is full of crazy digestion stories,

and I honestly wish that I had time to tell them all.

But let's focus on human digestion from now on, mostly because:

You're probably a human, we don't assume anything here,

and you'll be wanting to know how YOUR body does all this stuff.

And humans actually have a have a pretty good

all-purpose digestive system:

We're omnivores, after all, we eat plants AND meat

so our systems are generalized to handle all kinds of stuff.

Like most animals, humans have a bunch of different acids

and enzymes in our digestive tracts that break food down

so that it can be absorbed and used by our bodies.

But the secret to successful digestion

is maximizing surface area. In more that one way, actually.

The first way we maximize surface area is on the food itself.

Say I take a bite out of this apple.

Right now there's like, an apple boulder sitting there in my mouth.

I've got enzymes in my saliva that immediately start

breaking it down, like, the outsides of the boulder.

If I swallowed this chunk whole right now,

not only would it hurt like heck,

the rest of my digestive system would have a really hard time

dealing with it, because most of the enzymes and acids

would have the same difficulty working all the way

through this big solid hunk.

But, when I use my awesome teeth to chew up this hunk of apple

suddenly there's double, triple,

quadruple the surface area on the food!

I'm making up apple gravel from the apple boulder.

Maybe even apple sand.

For humans, chewing is key because breaking down our food into

smaller and smaller bits allows enzymes and acids to get at them.

And after our teeth have made the pieces small enough,

the chemicals break them down further until

they're fine enough for our bodies to absorb nutrients from them.

But it's not just the surface area of the food that's important,

the surface area of the digestive system

is key to the whole process as well.

Last time I talked about how we have a whole bunch

of surface area in our lungs to absorb tons of oxygen all at once.

Well, our digestive systems work in much the same way.

Most of the absorption of nutrients happens in our small intestines,

and the length of the average human adult's small intestine

is about 7 meters!

Plus, inside our small intestines

there are a bunch of little folds and little absorbing fibers

with absorbing fibers on them, and no I didn't mis-speak,

the fibers have fibers..

that's how hard our intestines work to increase their surface area.

Last episode I was all impressed that lungs had

a total surface area of 75 square meters...

well the small intestine has a surface area of 250 square meters!

Blegh... It's kind of gross.

I wouldn't want to see it spread out over a tennis court or anything

But I'm getting ahead of myself here.

Digestion does not start at the small intestine, people,

it starts at the mouth.

Now, as you can see, this hot pocket is surrounded

by some kind of bread, if you can call it that.

Bread is a starch, which breaks down into glucose.

When I start gnawing on a piece of bread,

because the outside here is mostly bread.

the glands in my mouth start secreting saliva,

which contains salivary amylase, an enzyme designed to break

down starch into glucose.

The more I chew, the more amylase will get to all the different

sides of the bread, and that's why the more you chew bread,

the sweeter it tastes.

Amylase doesn't really do much to the meat or the cheese

in this thing.

I've got other enzymes and acids that are going to work on them

later on in the system, but I am gonna chew all that stuff up

real good right now so that those other enzymes

can do their jobs later.

I'm gonna swallow all this.

So now the masticated hot pocket has passed down my pharynx,

or throat, and into my esophagus, which leads to my stomach.

There's actually this little cool flap of tissue

called the epiglottis that blocks the trachea when I swallow,

so that the food doesn't end up in my respiratory system.

This ball of food that I just swallowed

actually has a scientific name, it's called a bolus

and it rides a kind of wave of muscle action down the esophagus

into the stomach.

This wave-like contraction of the smooth muscles around the tube

of the esophagus is called peristalsis, and it's basically

how most of the movement in your digestive system is accomplished.

Now my hot pocket bolus is in my stomach now

which is where the food really starts getting manhandled.

The stomach basically takes a scorched earth

approach to digestion. It's not messing around.

It's like a churning cement mixer that can contract

and expand with these big,

accordion-like folds of muscle called rugae.

Your stomach's job is to turn everything over and over,

smooshing and mixing all the pieces up

with its cocktail of acids and enzymes called gastric juice.

Gastric juice is mainly made up of hydrochloric acid,

an enzyme called pepsin, and some mucus and water.

Hydrochloric acid has a Ph of about 1

which is strong enough that, if you got it on your hand,

it would give you a chemical burn.

So the acid breaks things down and hopefully kills

most of the bacteria that you might find on your food.

The pepsin starts breaking down proteins into amino acids.

Now, that mucus is important.

It's there to protect your stomach, so that it doesn't digest itself.

When you don't have enough of that mucus you get peptic ulcers,

which happen when your stomach lining

comes in direct contact with your stomach acid.

And the water's just in there to make everything all soupy,

because what you want by the time your food leaves your stomach

is chyme, which is a kind of liquidy slop that you might

be familiar with from the last time you had a stomach virus.

You knew this conversation was going to have to get

a little bit gross and I didn't want to bring diarrhea into it

too much because, you know, I've been eating.

But when something bad is going on in your digestive tract,

your body doesn't worry too much about absorbing nutrients,

it just wants to get the chyme out of there.

So, chyme is what you see when...

You get the picture.

Anyway, there's a little valve, or sphincter

between the stomach and the small intestine

that regulates how much chyme gets into the small intestine

and when it gets in there.

The very beginning of the small intestine is called the duodenum,

this is where a lot of the small intestine action happens,

by which I mean, lots of things get absorbed and also secreted,

like bicarbonate, which neutralizes the gastric acid

before it goes any further.

Now, the coolness of the small intestine can't be overstated.

It's ground zero for cellular exchange of nutrients

and the breakdown of fats.

And again, the reason it's so good at absorbing

is because of all the surface area it's got going on.

A lot of that surface area comes from the fact that,

despite its name, your small intestine is frickin' long:

in a human, it can range anywhere from 4.5 to 10.5 meters.

But that's not all!

The whole inside is lined with epithelial tissue

that has tons of ridges and folds in it.

Surface area to the max!

And on those ridges and folds are these little hair-like fibers

of flesh called villi.

Each villus has capillaries in it, so that it can absorb nutrients.

And get this: each villus, which is only like half a millimeter long

is covered in teeeny tiny little microvilli,

providing even more surface area!

In fact, apparently, the small intestine has a texture

kind of like velvet, which is...

[DISGUST]

Oh great, now I eat the milkshake? Fantastic. Okay.

So another thing the small intestine does,

with the help of its friend the gallbladder,

is break down fatty stuff, like this milkshake.

Near the top of your small intestine is a little pipe

where bile salts, manufactured by the liver

and stored by the gallbladder, are squirted out