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