Digestive System, Part 2: Crash Course Anatomy & Physiology #34

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.