Name: Digestive System, Part 1: Crash Course A&P #33
Uploaded: 2018-12-19T11:20:25.000Z
Duration: 11 min 5 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

We all have our reasons for eating nachos
at 3 in the afternoon.

I happen to have my own. And don't ask -- it's
personal.

But more generally, we all eat any kind of food to
accomplish two simple things: to obtain the energy

we need to stay alive and to get the raw materials
required for building all of our tissues and stuff.

That's because, when it comes down to it, both you
and the food you eat contain those two same things:

Both you and food are made of “stuff”
-- by which I mean, matter, made of certain

kinds of atoms -- and both you and food have
energy stored in the bonds between those atoms.

So all living things need to take in stuff and energy,
and convert it into slightly different stuff and energy.

And you can get some of the things you need
pretty easily. Like, in order to get oxygen

for respiration, to unleash the chemical energy
in your food, you just have to inhale.

But you can't just breathe in the stuff you need
to build DNA, or actin, or a phospholipid bilayer.

So, how does your body really acquire “stuff”?

This cheesy, crunchy dish is made of all different
kinds of biological matter -- like carbohydrates

and fat and protein -- and it contains a certain,
probably shocking, amount of calories, which

is how we measure energy stored in the chemical
bonds in food.

So if I take, like, a 100-calorie bite of
nachos -- which probably with this much cheese

wouldn't even be a very big bite -- I can
convert the chemical energy stored in those

carbohydrates and proteins and fats to feed
my muscle and heart cells and maybe, like,

walk a mile -- an activity that happens to
use about 100 calories.

But I can't just swallow the nachos and watch the
lump of them travel straight to my heart or leg muscles.

In order to actually use this food, I have
to convert the biological matter into something

my body can work with on the cellular level,
which as you know, is pretty darn tiny.

And, the work of converting the stuff in food, into the
stuff that's in my body, is done by my digestive system.

Human digestion occurs in six main steps -- some of
which you are intimately familiar with. Others less so.

But every step of the way, your body is working
to reduce all the different kinds of molecules

in food into their tiniest and most basic
forms.

The first step? Is, uh, probably everybody's favorite.

When it comes to what your digestive system ultimately
does, just think of it as a sort of disassembly line.

You could have an order of nachos with The
Works -- I'm talking beef and onions and

sour cream and slices of jalapeño -- and
your digestive system will deconstruct it,

both mechanically and chemically, one step
at a time.

It's gotta do this because your cells work best
with materials that are in their most basic form.

Your digestive system reduces food to that
level in two main ways: by physically smashing

it to smithereens, and by bathing them, as
much as it can, in enzymes.

Enzymes are proteins that living things use
as catalysts, to speed up chemical reactions.

When used in digestion, enzymes break down
the large molecules in your food into the

building blocks that your cells can actually
absorb.

Those large molecules are called biological
molecules -- also known as macromolecules

-- and everything that you eat, I hope, is
at least partially made of them.

And there are four main kinds: you got the lipids, the
carbohydrates, the proteins, and the nucleic acids.

Each possesses its own density of chemical
potential energy, or caloric value, like for

example, 1 gram of carbohydrate contains about 4
calories, while a gram of fat contains about 9 calories.

But many of these biological molecules are
polymers -- or sequences of smaller molecules

-- and your cells aren't really equipped
to take them up whole.

What your body trafficks in are those polymers'
individual components -- called monomers -- and

there are four main kinds of those, too: fatty
acids, sugars, amino acids, and nucleotides.

The simple idea behind the whole digestive system
is to break down the polymers of macromolecules

in your food, into the smaller monomers that
your cells can use to build their own polymers,

And, what your body needs to build at any
given moment is always changing.

Maybe you need new fat stores so you can have
energy to run a marathon, or new actin and

myosin to build bigger muscles, or more DNA
so you can replace the skin cells you scraped

off your knee when you fell, or more enzymes so you
can digest more food to get more building materials.

To meet your body's constant, and constantly shifting
demands, your digestive system requires a lot of organs

that perform a lot of specific tasks to break down
and absorb the right nutrient at the right time.

Now, I'm quite sure that you're familiar
with the key players here -- they're the

hollow organs that form the continuous tube
that is your alimentary canal, aka the gastrointestinal

tract, which runs from your mouth to your
anus.

It's worth pointing out that these organs are
hollow, because you are basically hollow, too.

Your digestive tract is really just one unbroken,
insulated tunnel of outside that just happens

to run through your body, and is open at both
ends. You're a donut.

So the layer of stratified squamous and columnar
epithelial cells that line your tract is actually

a barrier between the outside world and your
inside world -- but it's a barrier that

allows for the selective movement of materials
between them.

It's these hollow organs that do the actual
moving, digesting, and absorbing of food,

and they include your mouth, pharynx, esophagus,
stomach, and small and large intestines.

In your mouth, in your esophagus, and at the
other end of things, at your anus, you have

stratified squamous epithelial tissue, just
like your epidermis, to help resist the abrasive

action of like, chewing, like corn chips, maybe.

From your stomach on down, though, the inner
GI tract is lined with simple columnar epithelial

cells, which secrete all sorts of stuff, and
which absorb and process various nutrients.

Most of those columnar cells secrete mucus,
which lubricates everything, and protects

your cells from being digested by your own
digestive enzymes.

So, the innermost epithelial layer of the
tube is known as the mucosal layer, and it

contains some connective tissue as well, which
supplies it with blood.

Surrounding the mucosal layer is the submucosal
layer, made of loose areolar connective tissue,

which helps provide the elasticity that the
tube needs when you eat a whole pizza in one

sitting, and it contains more blood vessels.

And outside that, you have the muscularis
externa layer, which as you might guess, is

where you find the muscles responsible for
moving food through your tube.

Beyond these layers, the GI tract gets tons
of support from the accessory digestive organs,

like your teeth, and your tongue, your gallbladder,
salivary glands, liver, and pancreas.

They're kind of like a pit crew, and they
mostly help by secreting various enzymes that

help take apart food as it comes down the
tube.

Together, these two groups on the digestive
disassembly line work in six steps to destroy

your food and release and recycle its nutrients.

First, of course, you've got to introduce
the food to your digestive system. What you

know as eating, or ingestion, is basically
just creating a bulk flow of nutrients from

This is where the work of disassembly begins:
In your face-hole, which scientists call your mouth.

Now, we're going to get to the details of
what happens here another time, but remember

that food disassembly is both mechanical and
chemical: So your teeth pulverize the bite

of nacho or whatever, while your salivary glands
begin that food's hours-long enzyme bath.

But the food, at this point, is not nearly
“micro” enough to be of any use to your

cells, so you have to move that mush further
down your tube.

This stage is called propulsion, and its initial
mechanism is swallowing -- which, as you know,

is a voluntary action -- but then it's quickly turned
over to the involuntary process of peristalsis.

In peristalsis, the smooth muscles of the
walls of your digestive organs take turns

contracting and relaxing to squeeze food through
the lumen, or cavity, of your alimentary tract.

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Digestive System, Part 1: Crash Course A&P #33

stuff

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process

matter