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Nearly 200 years ago, Alexis St. Martin was shot in the stomach. He was saved by local army doctor William Beaumont, but had to live out his remaining years with a gaping hole in the stomach -- allowing Beaumont to learn a lot about how human digestion works. So today we're going to walk you through that process. We'll cover how mechanical and chemical digestion start in the mouth and continues in the stomach, where it’s pummeled by acids and enzymes and turned into chyme. We will also go over the stomach’s cephalic, gastric, and intestinal phases of digestive regulation.

Table of Contents
Mechanical and Chemical Digestion Starts in the Mouth 2:45
Continues in the Stomach 4:37
Food is Pummeled by Acids and Enzymes and Turned Into Chyme 5:31
Cephalic, Gastric and Intestinal Phases of Digestive Regulation 7:17

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Hank: 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 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 possible created an opportunity. He hired St. Martin, technically as a handyman, but really as a guinea pig.

Over the next 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 had ever 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, have 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 let me tell you; the story there is epic. And your digestive system's mission to disassemble food into its tiniest, most molecular forms. The stretch that runs from your mouth to your stomach unleashes all of the mechanical and chemical powers at its disposal. It basically roughs up food, douses it in protein-loving, acid-triggered enzymes, reduces it all to a creamy paste, and as a bonus, because it like you, it kills a whole hose 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 want to do this already, now I'm talking directly about vomiting.

(Intro)

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 epithelial that can stand up to lots of friction, like getting scraped by tortilla chips and 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 a hard surface for the tongue to mash food against, while the soft palate forms a moveable 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 we have roughly have 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 everyday. 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 you have an esophagus is when something is stuck in there or I you're feeling intense heart burn, or if you've just puked. But, moving on.

Assuming you have not just puked, then the bolus moves onto Dr. Beaumont's ticket to fame, the stomach. The stomach is the stretchiest part of your digestive tube, capable of holding 2-4 liters of material at any given time. Two to four liters; that is 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 layers found though most of the G.I. 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 smoosh them around. And the inner mucosa is made up almost entirely of mucus cells, which provide a protective coat that keep 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 containing St. Martin's personal gastric fluids. He watched the gobbets of food dissolve over time, which is partly how he discovered the stomachs 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 regularly hormones like serotonin and histamine which act locally to trigger other cells to 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 secretion.

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 secretion 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 as hormonal ones. In fact, stomach secretion occurs in 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 get relayed into the hypothalamus, which stimulates the medulla oblongata, which then taps the parasympathetic fibers in the vagus nerve. From there, the signals are then sent to the stomach with the word that "Hey, we think, that cookies are maybe 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 going to 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 anyways, because 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 v 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 getting neutralized as it does its job. Conversely, as stomach acidity increases, it increases 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 your small intestine doesn't get too overloaded with too much acid, or with the creamy paste that your stomach turns your food into, also know as chyme.

Now, remember not a lot of absorption 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 eject anything 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, 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 that mind-stomach connection whenever St. Martin 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 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 available 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 Dr. Cheryl C. Kinney Crash Course Studio and 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 is Valerie Barr. Michael Aranda is our sound designer and the graphics team is Thought Cafe.