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We all have our reasons for eating nachos
at 3 in the afternoon.
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I happen to have my own. And don’t ask -- it’s
personal.
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But more generally, we all eat any kind of food to
accomplish two simple things: to obtain the energy
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we need to stay alive and to get the raw materials
required for building all of our tissues and stuff.
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That’s because, when it comes down to it, both you
and the food you eat contain those two same things:
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Both you and food are made of “stuff”
-- by which I mean, matter, made of certain
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kinds of atoms -- and both you and food have
energy stored in the bonds between those atoms.
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So all living things need to take in stuff and energy,
and convert it into slightly different stuff and energy.
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And you can get some of the things you need
pretty easily. Like, in order to get oxygen
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for respiration, to unleash the chemical energy
in your food, you just have to inhale.
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But you can’t just breathe in the stuff you need
to build DNA, or actin, or a phospholipid bilayer.
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So, how does your body really acquire “stuff”?
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That’s where the nachos come in.
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This cheesy, crunchy dish is made of all different
kinds of biological matter -- like carbohydrates
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and fat and protein -- and it contains a certain,
probably shocking, amount of calories, which
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is how we measure energy stored in the chemical
bonds in food.
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So if I take, like, a 100-calorie bite of
nachos -- which probably with this much cheese
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wouldn’t even be a very big bite -- I can
convert the chemical energy stored in those
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carbohydrates and proteins and fats to feed
my muscle and heart cells and maybe, like,
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walk a mile -- an activity that happens to
use about 100 calories.
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But I can’t just swallow the nachos and watch the
lump of them travel straight to my heart or leg muscles.
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In order to actually use this food, I have
to convert the biological matter into something
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my body can work with on the cellular level,
which as you know, is pretty darn tiny.
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And, the work of converting the stuff in food, into the
stuff that’s in my body, is done by my digestive system.
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Human digestion occurs in six main steps -- some of
which you are intimately familiar with. Others less so.
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But every step of the way, your body is working
to reduce all the different kinds of molecules
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in food into their tiniest and most basic
forms.
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The first step? Is, uh, probably everybody’s favorite.
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When it comes to what your digestive system ultimately
does, just think of it as a sort of disassembly line.
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You could have an order of nachos with The
Works -- I’m talking beef and onions and
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sour cream and slices of jalapeño -- and
your digestive system will deconstruct it,
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both mechanically and chemically, one step
at a time.
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It’s gotta do this because your cells work best
with materials that are in their most basic form.
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Your digestive system reduces food to that
level in two main ways: by physically smashing
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it to smithereens, and by bathing them, as
much as it can, in enzymes.
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Enzymes are proteins that living things use
as catalysts, to speed up chemical reactions.
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When used in digestion, enzymes break down
the large molecules in your food into the
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building blocks that your cells can actually
absorb.
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Those large molecules are called biological
molecules -- also known as macromolecules
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-- and everything that you eat, I hope, is
at least partially made of them.
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And there are four main kinds: you got the lipids, the
carbohydrates, the proteins, and the nucleic acids.
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Each possesses its own density of chemical
potential energy, or caloric value, like for
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example, 1 gram of carbohydrate contains about 4
calories, while a gram of fat contains about 9 calories.
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But many of these biological molecules are
polymers -- or sequences of smaller molecules
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-- and your cells aren’t really equipped
to take them up whole.
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What your body trafficks in are those polymers’
individual components -- called monomers -- and
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there are four main kinds of those, too: fatty
acids, sugars, amino acids, and nucleotides.
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The simple idea behind the whole digestive system
is to break down the polymers of macromolecules
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in your food, into the smaller monomers that
your cells can use to build their own polymers,
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while also getting the energy they need.
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And, what your body needs to build at any
given moment is always changing.
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Maybe you need new fat stores so you can have
energy to run a marathon, or new actin and
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myosin to build bigger muscles, or more DNA
so you can replace the skin cells you scraped
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off your knee when you fell, or more enzymes so you
can digest more food to get more building materials.
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To meet your body’s constant, and constantly shifting
demands, your digestive system requires a lot of organs
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that perform a lot of specific tasks to break down
and absorb the right nutrient at the right time.
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Now, I’m quite sure that you’re familiar
with the key players here -- they’re the
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hollow organs that form the continuous tube
that is your alimentary canal, aka the gastrointestinal
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tract, which runs from your mouth to your
anus.
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It’s worth pointing out that these organs are
hollow, because you are basically hollow, too.
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Your digestive tract is really just one unbroken,
insulated tunnel of outside that just happens
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to run through your body, and is open at both
ends. You’re a donut.
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So the layer of stratified squamous and columnar
epithelial cells that line your tract is actually
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a barrier between the outside world and your
inside world -- but it’s a barrier that
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allows for the selective movement of materials
between them.
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It’s these hollow organs that do the actual
moving, digesting, and absorbing of food,
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and they include your mouth, pharynx, esophagus,
stomach, and small and large intestines.
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In your mouth, in your esophagus, and at the
other end of things, at your anus, you have
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stratified squamous epithelial tissue, just
like your epidermis, to help resist the abrasive
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action of like, chewing, like corn chips, maybe.
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From your stomach on down, though, the inner
GI tract is lined with simple columnar epithelial
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cells, which secrete all sorts of stuff, and
which absorb and process various nutrients.
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Most of those columnar cells secrete mucus,
which lubricates everything, and protects
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your cells from being digested by your own
digestive enzymes.
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So, the innermost epithelial layer of the
tube is known as the mucosal layer, and it
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contains some connective tissue as well, which
supplies it with blood.
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Surrounding the mucosal layer is the submucosal
layer, made of loose areolar connective tissue,
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which helps provide the elasticity that the
tube needs when you eat a whole pizza in one
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sitting, and it contains more blood vessels.
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And outside that, you have the muscularis
externa layer, which as you might guess, is
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where you find the muscles responsible for
moving food through your tube.
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Beyond these layers, the GI tract gets tons
of support from the accessory digestive organs,
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like your teeth, and your tongue, your gallbladder,
salivary glands, liver, and pancreas.
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They’re kind of like a pit crew, and they
mostly help by secreting various enzymes that
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help take apart food as it comes down the
tube.
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Together, these two groups on the digestive
disassembly line work in six steps to destroy
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your food and release and recycle its nutrients.
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First, of course, you’ve got to introduce
the food to your digestive system. What you
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know as eating, or ingestion, is basically
just creating a bulk flow of nutrients from
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the outside world into your tissues.
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This is where the work of disassembly begins:
In your face-hole, which scientists call your mouth.
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Now, we’re going to get to the details of
what happens here another time, but remember
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that food disassembly is both mechanical and
chemical: So your teeth pulverize the bite
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of nacho or whatever, while your salivary glands
begin that food’s hours-long enzyme bath.
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But the food, at this point, is not nearly
“micro” enough to be of any use to your
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cells, so you have to move that mush further
down your tube.
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This stage is called propulsion, and its initial
mechanism is swallowing -- which, as you know,
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is a voluntary action -- but then it’s quickly turned
over to the involuntary process of peristalsis.
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In peristalsis, the smooth muscles of the
walls of your digestive organs take turns
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contracting and relaxing to squeeze food through
the lumen, or cavity, of your alimentary tract.
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Waves of peristalsis continue through the
esophagus, stomach, and intestines, and they’re
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so strong that even if you were hanging upside
down while eating your lunch and drinking
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your tea, the food would still soldier on, fighting
gravity, and eventually make it to its final destination.
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Don’t do that, though. There’s other reasons
why you shouldn’t be upside down.
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Anyway, all of this shipping and handling
mechanically breaks down the food even more,
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and even after it goes through the stomach
and its gastric acid, the mechanical work
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still continues once it reaches your small
intestine, as more smooth muscle segments
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push the food back and forth to keep crumbling
it up.
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The goal of all this pulverization is to increase the
surface area of that bite of food by breaking it down
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into increasingly tiny pieces, to prepare it to encounter
more enzymes in step four: chemical digestion.
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Really, the actual process of digestion only
occurs when the main action becomes
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more chemical than mechanical.
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And here, the accessory digestive organs -- namely,
the liver, pancreas and gallbladder -- secrete
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enzymes into the alimentary canal, where they
ambush the mush and break it down into its
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most basic chemical building blocks.
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Like I said before, our cells prefer to do
business in the really basic currency of monomers,
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like amino acids, fatty acids, and simple sugars.
And digestion allows for the absorption of
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those nutrients as they pass from the small intestine
into the blood, by both active and passive transport.
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Once those nutrients are absorbed by your
cells, you can finally use the energy inside
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of them or use them to build new tissues.
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The absorption of the nutrients is the
goal of the entire process.
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But, of course, it is not the end of it.
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Once your body has sucked out all the nutrients
it wants, indigestible substances like fiber
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are escorted out of your body.
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Yeah, I’m talking about pooping, or defecation.
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And that is the end of the digestive line
-- unless you are a capybara, or one of the
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other animals who make sure that they get
the most out of their lunch, by giving the
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whole process another round and practicing
coprophagia, aka eating their own poop.
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Now, you should notice here that some of the
processes of digestion occur in just one place,
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and are the job of a single organ -- like
hopefully you’re only ingesting through
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your mouth and eliminating from the large
intestine.
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But most of these six steps require cooperation
among multiple organs.
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For example, both mechanical and chemical
digestion start in the mouth, and continue
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through the stomach and small intestines.
And some chemical breakdown continues in the
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large intestine, thanks to our little bacterial
farm there.
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Over the next couple of weeks we’re going
to take you and your nachos on a stroll through
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your digestive system and see who’s doing
what, where, how, and why.
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But for now, I’ve got some nachos to finish,
so I gotta go.
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And eating those nachos, as you learned today,
will provide me with energy and raw materials,
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by first ingesting something nutritious, propelling
it through my alimentary canal where it will
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be mechanically broken down, and chemically
digested by enzymes until my cells can absorb
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their monomers and use them to make whatever
they need. And eventually, there will be pooping.
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Thanks to all of our Patreon patrons who help
make Crash Course possible through their monthly
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contributions. And if you like Crash Course
and want to help us keep making videos like
00:10:16
this one, you can go to patreon.com/crashcourse.
Also, a big thank you to Peter Rapp, Sigmund
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Leirvåg, Mikael Modin, and Jeremy Bradley
for co-sponsoring this episode of Crash Course
00:10:25
Anatomy and Physiology.
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This episode was filmed in the Doctor Cheryl
C. Kinney Crash Course Studio, it was written
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by Kathleen Yale, edited by Blake de Pastino,
and our consultant is Dr. Brandon Jackson.
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It was directed by Nicholas Jenkins, edited
by Nicole Sweeney; our sound designer is Michael
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Aranda, and the Graphics team is Thought Cafe.
