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In the summer of 1822, a French-Canadian fur-trapper
named Alexis St. Martin was going about his
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business near Lake Michigan, when he was shot
by a hunter, right in the stomach.
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The wound was severe, and everyone expected
St. Martin to die that night.
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But…he didn’t.
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A local army doctor named William Beaumont
kept him alive.
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In fact, Beaumont performed so many surgeries
on the injury over the next several months,
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that he decided, somewhat questionably, to
just keep St. Martin’s stomach wound open.
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St. Martin was left with a hole, or fistula,
in his abdominal wall, which allowed anyone
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to see right into his stomach.
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Now, it’s probably hard to work as a fur
trapper with a hole in your guts, but Beaumont
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saw -- or possibly created -- an opportunity.
He hired St. Martin -- technically as a handyman,
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but really as a guinea pig.
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Over several years and some 238 experiments,
Beaumont recorded what St. Martin ate, and
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what his stomach did to his meals.
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Sometimes they just skipped the eating part
all together and just shoved some food, tied
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to a string, right into the guy’s gut-hole.
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Beaumont took samples of gastric juices and
had them analyzed by chemists -- something
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no one else had done before -- and he also
noticed that St. Martin’s digestion slowed
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at certain times, like when he was sick or
stressed.
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I mean, like, beyond the stress of having
a gaping hole in your abdomen.
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Through his somewhat questionable research,
Beaumont discovered some major secrets of
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the digestive system, like that the stomach’s
extremely strong acids and muscular contractions
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break down food, and that some foods are more
digestible or less digestible than others,
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and that the brain can affect the stomach.
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Beaumont’s findings -- as well as his methods of clinical
observation -- revolutionized the field of physiology.
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And St. Martin? Don’t worry about him. He
lived to be 83 years old, in great health.
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And a hole in his guts...
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Now, I sincerely hope that you can’t actually
see what’s going on in your stomach, but
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lemme tell you, the story there is epic.
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In your digestive system’s mission to disassemble
food into its tiniest, most basic molecular
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forms, the stretch that runs from your mouth
to your stomach unleashes all of the mechanical
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and chemical powers at its disposal.
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It physically roughs up food; douses it in
protein-loving, acid-triggered enzymes; reduces
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it all into a creamy paste -- and as a bonus,
because it likes you, it also kills a whole
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host of harmful invaders that, for whatever reason,
found their way through your face and into your tube.
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But your stomach’s not the end of the line
for your food.
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Unless…it is.
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I mean, most of the time, everything from
your mouth to your stomach prepares food to
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be absorbed by your tissues. But sometimes...food
finds its way back up.
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Yeah, in case the story of Alexis St. Martin
didn’t make you wanna do this already,
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now I’m talking directly about vomiting.
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Let’s begin with the beginning: with your
mouth, aka your oral, or buccal, cavity.
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Now we don’t usually think of it this way,
but that is where digestion starts -- the
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mechanical and chemical breakdown of food
through chewing and enzyme-action.
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The inside of your mouth is lined with a tough,
thick layer of stratified squamous epithelium
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that can stand up to lots of friction, like
getting scraped by tortilla chips and, like,
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grilled cheese sandwiches that maybe were
cooked a little too much on the top.
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Your anterior hard palate and the flexible posterior
soft palate form the roof of your mouth.
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The hard palate provides, like, a hard surface
for the tongue to mash food against, while
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the soft palate forms a movable fold of flesh
that reflexively closes off the nasopharynx
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when you swallow, so food gets directed down
your esophagus and not up into your nasal cavity.
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We all know what teeth are for, and you have
roughly 32 of them in your basic types that
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help you masticate, or chew your food.
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The tongue lives on the floor of your mouth,
and is basically just a big muscle that grips
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and constantly repositions your food as you
chew.
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The resulting ball of mush actually has its
own special name -- it’s a bolus -- and
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the tongue rolls it back to the pharynx, in
preparation for swallowing.
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But that’s just the physical action that
goes on in your mouth. Just as much destruction
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is taking place through chemistry.
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The bolus is broken down with the help of
three major pairs of salivary glands that
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churn out an average of 1.5 liters of slightly
acidic saliva every day.
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More than four soda cans worth of spit. Per
day.
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And all that saliva delivers enzymes like
salivary amylase, a digestive enzyme that
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breaks down starches into glucose monomers.
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Now, once the food enters the pharynx, it’s
propelled by peristalsis into the esophagus,
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which, except for the little sphincter at
the end that keeps food moving in the right
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direction, is really just a glorified laundry
chute lined with smooth muscle.
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The only time you probably even remember that
you have an esophagus is when something’s
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stuck in there, or if you’re feeling intense
heartburn, or if you just puked.
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But, moving on.
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Assuming you have not puked yet, then the
bolus moves on to Dr. Beaumont’s ticket
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to fame: The stomach.
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The stomach is the stretchiest part of your
digestive tube, capable of holding 2 to 4
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liters of material at any given time.
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TWO TO FOUR LITERS! That’s a lot of nachos.
Mixed with spit.
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But of course it’s much more than just a
storage tank -- it’s lined with the same
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four main layers found through most of the
GI tract -- the mucosa, submucosa, muscularis
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externa, and serosa -- but it’s got a few
special modifications.
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For one thing, the muscularis includes an
additional layer of smooth muscle that gives
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it extra strength, allowing the stomach not just to
hold materials, but to actively smush them around.
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And the inner mucosa is made up almost entirely
of mucous cells, which produce a protective
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coat that keeps the stomach tissues from getting
digested along with your lunch.
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This inner lining is dotted with millions
of tiny, deep gastric pits which lead down
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to tubular gastric glands. These glands, in
turn, contain various types of secretory cells
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that brew up some of the most potent chemicals
in your body.
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For example, your stomach has parietal cells
that release hydrochloric acid -- a substance
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more acidic than battery acid -- which lays
waste to most of the bacteria, viruses, and
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other stuff that could make you sick.
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It also helps denature, or change the shape of, proteins
to make it easier for enzymes to digest them.
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And maybe more importantly, when the hydrochloric
acid is combined with pepsinogen, an inactive
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enzyme that’s secreted by another kind of
stomach cell called chief cells, the mixture
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creates the protein-digesting enzyme pepsin.
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Together, this super-powered acid and protein-hungry
enzyme can annihilate nearly anything they encounter.
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This was apparently something that Beaumont
observed first-hand, by dropping hunks of
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meat into a cup filled with St. Martin’s
personal gastric fluids.
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He watched the gobbits of food dissolve over
time, which is partly how he discovered the
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stomach’s role in digestion was as much
chemical as mechanical.
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But with so much mind-blowingly powerful stuff
at your stomach’s disposal, somebody down
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there has to be in charge -- so your gastric
glands also contain enteroendocrine cells.
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These cells release regulatory hormones, like
serotonin and histamine, which act locally
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to trigger other cells, to, say, release more
acid, or contract muscle tissue. And when
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the time comes to tamp the action down, they secrete
other hormones like somatostatin, to inhibit secretions.
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And then there are G-cells, which produce
the most important hormone for stimulating
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gastric activity: gastrin.
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Most signals that increase stomach activity
get the job done by increasing the secretion
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of gastrin, which then stimulates the release of other
gastric fluids, as well as stomach-muscle activity.
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Now, if the smell of baking cookies has ever
made your mouth water and your belly grumble,
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then it might not surprise you to learn that
these stomach secretions are ruled by neural
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mechanisms as well hormonal ones.
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In fact, stomach regulation occurs in three
phases, based on where the food is sensed
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-- the brain, the stomach, and the small intestine.
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The cephalic phase is the one ruled by your
brain, and it kicks in when you first see,
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smell, taste, or even think about food.
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That sensory input gets relayed to the hypothalamus,
which stimulates the medulla oblongata, which
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then taps the parasympathetic fibers in the vagus nerve.
From there, the signals are sent to the stomach
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with the word that, “Hey, we think that maybe cookies
are on the way, so you might want to prepare yourself.”
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Now this is a conditioned reflex, so it only works
if you want to eat the food in question. If
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I happen to be super-full, or not feeling
well, or somebody puts a pile of squid eyeballs
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in front of me, the cephalic phase isn’t
gonna happen.
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And no offense if squid eyeballs are totally
your thing, they’re just not my thing.
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But say I eat the plate of squid eyeballs
anyway because, you know, I’m trying to
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be polite. Well, even without the cephalic
warm up, when that food hits my stomach, local
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mechanisms, both neural and hormonal jump
start the gastric phase.
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For the next few hours, as my stomach grows
distended from the food, it activates stretch
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receptors that again stimulate my medulla and get my
vagus nerves to tell my stomach to turn up the juice.
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At the same time, the secretion of gastrin
is activated by other signals, like the rise
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in alkalinity caused by the stomach acid getting
neutralized as it does its job.
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Conversely, as stomach acidity increases,
it inhibits the release of gastrin.
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Now, the third phase of gastric regulation
-- the intestinal phase -- speeds or slows
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the rate in which your stomach empties, so
that the small intestine doesn’t get too
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overloaded with too much acid -- or with the
creamy paste that your stomach turns your
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food into, known as chyme.
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Now remember, not a lot of absorption actually
occurs in the stomach.
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The stomach is more like a decontamination
tank. Sure, it pummels your food down to a
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paste, but it’s also where your body tries to
obliterate any nasties that could make you sick.
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As long as food is still in there, your body
has a chance to kind of size it up, and feel
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it out, and it reserves the right to eject
anything that it feels is potentially dangerous.
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Lots of factors can trigger the stomach’s
urge to purge, or vomit, but the most common
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are simply ingesting too much food, or eating
some kind of irritant or toxin, like those
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produced by bad bacteria, too much alcohol,
certain drugs, or unappealing foods.
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Of course if you’ve ever puked in a moment
of trauma or stress, you know how emotions
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and anxiety can also trigger your stomach
to launch its lunch.
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That’s the brain influencing the cephalic
phase of gastric regulation again, by sending
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extra fight or flight signals to the stomach.
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Beaumont noticed this mind-stomach connection
whenever St. Martin’s digestion was affected
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by illness or stress -- something you’d
think he’d have felt every time that doctor
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came at him with some meat on a string.
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If you were able to keep down your lunch down
today, you learned how mechanical and chemical
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digestion start in the mouth and continue
in the stomach, where food is pummeled by acids
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and enzymes and turned into chyme. We also
looked at the stomach’s cephalic, gastric,
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and intestinal phases of digestive regulation.
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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
00:10:15
and want to help us keep making videos like
this one, you can go to patreon.com/crashcourse.
00:10:20
Also, a big thank you to Sigmund Leirvåg,
Alexis & Brian Carpenter, and Luke Peterson
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for co-sponsoring this episode of Crash Course
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 and edited by Nicole Sweeney,
our script supervisor was Valerie Barr, Michael
00:10:40
Aranda was our sound designer, and the graphics
team is Thought Cafe.
