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You know, we’ve been talking about a lot
of serious stuff here lately.
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Heart failure. Respiratory gas exchange. People
with holes in their stomachs.
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Nachos.
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Some might say I’ve even been flaunting
my ability to eat, digest, and enjoy a plate
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of chips and melted cheese.
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And I wouldn't blame them if they did, because
sadly, nachos aren’t for everyone.
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In fact, I can safely say nachos are really
only a good idea for about a third of humans.
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For the rest, what may start as a party in
your mouth will surely end in gastric distress.
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Such is the fate of the lactose intolerant.
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Lactose is basically milk-sugar that can only
be digested with the help of a special intestinal
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enzyme -- lactase -- which many adults do
not produce enough of.
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In fact, way back in the day, none of us did,
until about 7500 years ago, when a particularly
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handy genetic mutation popped up in central
Europe.
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This so-called lactase persistence trait probably
spread as Neolithic groups trekked north and
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west through Europe. Today nearly 90 percent
of adult Britons and Scandinavians can chug
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all the milk they want, whereas down toward
the Mediterranean, probably less than 40 percent
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have lactase persistence, and fewer than ten
percent in Africa and Asia.
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Now technically, a lactose intolerant person
can still consume dairy at their own risk,
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but since their own bodies can’t break down
lactose, the job is left to the three-pound
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bacteria farm living in their large intestines
-- bacteria that try their hardest to make
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something of those milk sugars, the results
of which are gas, and bloating, and diarrhea.
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So, it turns out, nachos aren’t just a good
way to talk about how the digestive system
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works, they’re also a good way to talk about
when it doesn’t.
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Remember how the stomach is great at obliterating
matter, but not so hot when it comes to actually
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chemically digesting stuff, or really absorbing
much of anything?
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You might say the stomach lacks subtlety.
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But luckily, it’s got friends in low places,
and the small intestine is more than happy
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to pick up the slack and provide a cozy environment
where your food is at long last disassembled
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and absorbed by your cells.
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Now there’s a lot of mechanical action and
peristalsis going on here, but there’s also
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a ton of chemical digesting too.
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And while homebrewed intestinal juices help
digest the chyme that your stomach turns food
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into, the real power actually comes from the outside
helpers -- the liver, gallbladder, and pancreas.
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Now, the small intestine is called “small”
not because it’s short but because it’s
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about half the diameter of the large intestine
-- the thing is actually like 6 or 7 meters long.
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Not only that, but whole deal is lined with epithelial
tissue that has more folds than an origami octopus.
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These folds are lined with tiny hair-like
villi and even tinier microvilli, which create
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a truly impressive surface area -- large enough that,
if it were unfolded, it would cover a tennis court.
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It’s this massive surface area, and the
countless capillaries just beneath it, that
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make the small intestine such a champion absorber
of nutrients.
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It shares the same four tissue layers seen throughout
the GI tract, and has three main subdivisions:
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Straight outta the stomach and snuggled around
the pancreas, you’ve got the relatively
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short and mostly immovable duodenum, which
is where most of the chemical digestion occurs.
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The middle section is the jejunum, where most
of the absorption takes place.
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And finally at the end, running into the large
intestine is the ileum, where important vitamins
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like A, B12, E, D, and K are absorbed.
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But the duodenum is what you might call the
business end of the small intestine. It receives
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chyme and gastric juices from the stomach
through the pyloric sphincter, but it also
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imports bile from the liver and gallbladder,
enzymes from the pancreas, and creates its
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own homegrown mix of enzymes.
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Some of the imported enzymes eventually pass
through your system on the wave of gooey chyme.
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But other enzymes are actually bound to cell
membranes in the intestinal mucosal layer,
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and they’re reusable.
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Enzymes are proteins, and proteins are expensive.
So these compounds -- known as brush border
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enzymes -- can just sit around and process
food as it passes by, without your body having
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to make new ones.
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And the lactase that so many of us don’t
have, is one of these.
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Now, the duodenum communicates with the stomach
in the last phase of gastric regulation that
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we talked about in the last episode -- the
intestinal phase.
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This is where the duodenum lets the stomach
know, with hormones and nerve signals, when
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and how much chyme to release so it doesn’t
get overwhelmed all at once.
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It’s also where stuff like bicarbonate from
the pancreas gets dumped, to help neutralize
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the stomach acid before it burns a hole in
your guts.
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And this brings me to your crucial accessory
organs -- the things apart from the alimentary
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canal that never come in contact with ingested
material, but still play an essential role
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in digestion.
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First up: the liver.
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The liver is a massive, fatty, four-lobed,
and very important organ. It lives directly
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under your diaphragm and -- fun fact -- it
can actually fully regenerate itself after
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an injury or surgery, with as little 25 percent
of its original tissue.
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The liver serves tons of critical metabolic
and regulatory roles that we don’t have
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time to get into right now, but its main role
in the digestive system is to make bile.
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Bile is the missing ingredient your body needs
to attack fatty foods, which is a tricky business.
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In part, that’s because fat isn’t water
soluble, and since your insides are mostly
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water, fats will clump together, becoming
hard to digest.
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To keep fat from clumping, you need an emulsifier,
so bile comes in to keep big, hydrophobic
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fat molecules from sticking together, which
allows lipid-hungry enzymes to move in and
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break them down into fatty acids and monoglycerides
that you can then digest and absorb.
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But while your liver creates the bile, it
gets stored and concentrated in the neighboring
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gallbladder, the thin, green sac cozied up
to the liver.
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It gets the signal when chyme slides into
the duodenum, which activates the enteroendocrine
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cells to release a pair of hormones. Those
hormones in turn tell the gallbladder to contract
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and squirt bile through the cystic and bile
ducts into the duodenum.
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Another crucial accessory organ is the pancreas,
a gland that looks like a fistful of cottage
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cheese stuffed in a plastic bag.
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The pancreas also does lots of important things
for your body, especially related to your
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endocrine system, but for our purposes today,
just know that it brews up a powerful enzyme
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cocktail that is also triggered by those same
two hormones.
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Pancreatic juice is kinda like the Neapolitan
Ice Cream of bodily secretions -- it’s like
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everybody’s favorite ingredients all put
together, and when you mix them, the result
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is especially powerful.
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You’ve got trypsin and peptidase in there,
which break proteins down into amino acids,
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and you have lipases that turn triglycerides
into fatty acids and glycerol.
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Amylase, meanwhile, reduces carbs to glucose
and fructose, and nuclease busts the nucleic
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acids that are in DNA and RNA into nucleotides.
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Once all of those macromolecules have been
dissembled into their monomers, the small
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intestine’s epithelial cells can finally
absorb and transport them through your capillaries
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and into the bloodstream, where they can travel
to pretty much to any cell in your body, and
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be used to build collagen, or store fat, or
replace dying cells.
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The true purpose behind all the eating that
you do.
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So! Once the chyme has worked through your
small intestine, it passes through the ileocecal
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valve and hits the cecum, the first part of
the large intestine, where, congratulations,
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your food is now officially feces!
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The large intestine -- consisting of the colon,
rectum, and anus -- is relatively short, at
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about one and half meters, and it provides
a nice little frame for the small intestine,
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here at the end of the alimentary canal.
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By now, your body has sucked up almost all
of the nutrients it can, and is basically
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just pushing indigestible goo around, so the
large intestine doesn’t have a lot of hard
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work to do.
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Its main functions are to absorb any remaining
water so you don’t have constant diarrhea,
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and to store the rest until it’s ready to
exit the body.
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It also plays host to hundreds of species
and trillions of individual gut bacteria,
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which digest whatever chyme your body couldn’t,
releasing essential B and K vitamins, and
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some short fatty acids, which the large intestine
can still absorb.
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In doing so, they also produce gases like
carbon dioxide and methane, sulfurous compounds
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called mercaptans, and hydrogen sulfide, which
eventually...pass.
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OK, I know what you’re thinking now, you’re
like, “Hank, what’s up with the nachos?”
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“Surely you’re not just gonna to bring
up nachos at the beginning of the episode
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without explaining how they can turn on you?!”
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Well, I’ve never disappointed you before,
have I?
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Those of us who can’t produce the enzyme
lactase in our small intestine simply let
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milk and cheese pass through the organ untouched,
leaving the digestion to these bacteria in
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the large intestine.
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And those bacteria possess about 1000 different
kinds of enzymes of their own, including lactase.
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But their digestion process produces a whole
lotta extra gas, which is why nachos may leave
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me feeling cheesy and satisfied, but leave
you bloated, and crampy, and malodorous.
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But enough farting around, let’s wrap up
this fantastic journey.
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Fecal matter keeps moving through in a couple
of different ways. Slow, segmenting haustral
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contractions keep mixing and chopping it in
the large intestine, occurring every 30 minutes
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or so and lasting about a minute.
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But most people also experience a few mass
peristalsis movements a day -- big, intense
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contractions that clear out a large swath
of intestine at once, pushing feces into the
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rectum. These often occur just after eating.
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Once in the rectum, your poop stimulates stretch
receptors that tap the parasympathetic defecation
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reflex, which signals the colon and rectum
to contract, and the internal anal sphincter
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to relax.
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This forces the poop into the anal canal,
sending more messages to the brain that allow
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us to decide whether to voluntarily open the
external anal sphincter, or just hold it for
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a minute while we find a bathroom.
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And when that moment arrives, what was once
food says farewell to the alimentary canal
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that temporarily held it, and passes back
into the light of day.
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And that, my friends, is the end of your digestive
system. Pretty cool, right?
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And so it’s all over, but that doesn’t
mean you should forget about what we learned
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today, which is that the small intestine performs
most of your chemical digestion in the duodenum,
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while accessory organs including the liver,
gallbladder, and pancreas contribute enzymes
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that all but finish the job. Then your large
intestine, which is actually shorter than
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the small intestine, tries to extract the
last bit of nutrition, including the occasional
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attempt to turn nachos into energy, which
for most humans, ends in gassy failure.
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Thank you to all of our Patreon patrons who
help make Crash Course possible, not only
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for themselves, but for everyone through their
monthly contributions. If you like Crash Course
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and want to help us keep making videos like
this one, go to patreon.com/crashcourse.
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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.
