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  • Oh hi! I hope you don't mind that I'm eating.

  • This is actually just my first course.

  • For my birthday, the writers wrote me a script

  • where I just get to eat!

  • The whole time!

  • And I can't think of a better way to demonstrate

  • the workings of the digestive system,

  • the series of hollow organs that we use to break down

  • and process the nutrients and energy we need to function.

  • Though...wait a second, if I remember correctly

  • digestion is actually pretty freaking disgusting...

  • so maybe I shouldn't be eating right now.

  • Oh whatever... Waiter!

  • The digestive system is so fundamental that it's basically

  • step number one in the guide: How to Make an Animal.

  • You probably remember that during the embryonic development of most

  • animals, the digestive tract is the very first thing that forms.

  • When the blastula, that little wad of cells that we all used to be,

  • turns into a little wad of cells with a tube running through it,

  • that tube is your digestive system.

  • And pretty much every animal has a digestive system of some kind,

  • but they're not all alike.

  • Far from it.

  • In fact, digestive tracts are specially adapted

  • to animals' feeding behavior and diet.

  • For instance, a house fly eats mostly liquid or very finely

  • granulated food, but before it does that,

  • it's got to puke its digestive juices all over its lunch

  • and then let them digest it for a while

  • before it sucks it up into its mouth.

  • If we did it like that, first dates would be...less common.

  • Most vertebrates put food in one end of the tube

  • and our digestive system processes it,

  • and then it gets rid of the waste out the other end of the tube.

  • No muss, no fuss.

  • Well, actually, there's a little bit of muss, at the end.

  • You may have noticed.

  • But the beauty of it is that this whole process is run by our

  • autonomic nervous system, so we don't have to think about it,

  • until maybe the very last step when we're in traffic

  • and just had two cups of coffee and a bran muffin...

  • then we have to think about it a little bit.

  • Among vertebrates, the digestive tract might be short or long,

  • or have organs that do different things

  • depending on what its feeding habits are.

  • For instance, dogs are mostly carnivores and also scavengers:

  • They mostly eat meat,

  • but sometimes that meat's been dead for a while.

  • So, the dog's digestive system has developed to take food in,

  • absorb as many nutrients as possible,

  • and then deposit it on somebody's lawn,

  • all in a period of about six hours.

  • Dogs have an extremely short digestive tract, because,

  • if you're in the habit of eating rotten meat,

  • you'd better be able to digest it fast.

  • If you don't, the bad bacteria that's probably living on that

  • armadillo carcass is going to take up residence in your gut

  • and put you in a world of hurt.

  • Cows, on the other hand, take a very very very long time

  • to digest their food, around 80 hours,

  • because they have to process plants, mostly grass.

  • Grass has a ton of cellulose in it, and evolution has yet

  • to produce an animal that can manufacture a stomach acid

  • or enzyme tough enough to break down cellulose.

  • So, cows have microorganisms in their guts

  • that break down the cellulose for them.

  • This process takes a four-chambered stomach

  • each one with a slightly different microecology

  • and a lot of cud-chewing,

  • or regurgitating and re-chewing of grass

  • before it passes all the way through.

  • So, nature is full of crazy digestion stories,

  • and I honestly wish that I had time to tell them all.

  • But let's focus on human digestion from now on, mostly because:

  • You're probably a human, we don't assume anything here,

  • and you'll be wanting to know how YOUR body does all this stuff.

  • And humans actually have a have a pretty good

  • all-purpose digestive system:

  • We're omnivores, after all, we eat plants AND meat

  • so our systems are generalized to handle all kinds of stuff.

  • Like most animals, humans have a bunch of different acids

  • and enzymes in our digestive tracts that break food down

  • so that it can be absorbed and used by our bodies.

  • But the secret to successful digestion

  • is maximizing surface area. In more that one way, actually.

  • The first way we maximize surface area is on the food itself.

  • Say I take a bite out of this apple.

  • Right now there's like, an apple boulder sitting there in my mouth.

  • I've got enzymes in my saliva that immediately start

  • breaking it down, like, the outsides of the boulder.

  • If I swallowed this chunk whole right now,

  • not only would it hurt like heck,

  • the rest of my digestive system would have a really hard time

  • dealing with it, because most of the enzymes and acids

  • would have the same difficulty working all the way

  • through this big solid hunk.

  • But, when I use my awesome teeth to chew up this hunk of apple

  • suddenly there's double, triple,

  • quadruple the surface area on the food!

  • I'm making up apple gravel from the apple boulder.

  • Maybe even apple sand.

  • For humans, chewing is key because breaking down our food into

  • smaller and smaller bits allows enzymes and acids to get at them.

  • And after our teeth have made the pieces small enough,

  • the chemicals break them down further until

  • they're fine enough for our bodies to absorb nutrients from them.

  • But it's not just the surface area of the food that's important,

  • the surface area of the digestive system

  • is key to the whole process as well.

  • Last time I talked about how we have a whole bunch

  • of surface area in our lungs to absorb tons of oxygen all at once.

  • Well, our digestive systems work in much the same way.

  • Most of the absorption of nutrients happens in our small intestines,

  • and the length of the average human adult's small intestine

  • is about 7 meters!

  • Plus, inside our small intestines

  • there are a bunch of little folds and little absorbing fibers

  • with absorbing fibers on them, and no I didn't mis-speak,

  • the fibers have fibers..

  • that's how hard our intestines work to increase their surface area.

  • Last episode I was all impressed that lungs had

  • a total surface area of 75 square meters...

  • well the small intestine has a surface area of 250 square meters!

  • Blegh... It's kind of gross.

  • I wouldn't want to see it spread out over a tennis court or anything

  • But I'm getting ahead of myself here.

  • Digestion does not start at the small intestine, people,

  • it starts at the mouth.

  • Now, as you can see, this hot pocket is surrounded

  • by some kind of bread, if you can call it that.

  • Bread is a starch, which breaks down into glucose.

  • When I start gnawing on a piece of bread,

  • because the outside here is mostly bread.

  • the glands in my mouth start secreting saliva,

  • which contains salivary amylase, an enzyme designed to break

  • down starch into glucose.

  • The more I chew, the more amylase will get to all the different

  • sides of the bread, and that's why the more you chew bread,

  • the sweeter it tastes.

  • Amylase doesn't really do much to the meat or the cheese

  • in this thing.

  • I've got other enzymes and acids that are going to work on them

  • later on in the system, but I am gonna chew all that stuff up

  • real good right now so that those other enzymes

  • can do their jobs later.

  • I'm gonna swallow all this.

  • So now the masticated hot pocket has passed down my pharynx,

  • or throat, and into my esophagus, which leads to my stomach.

  • There's actually this little cool flap of tissue

  • called the epiglottis that blocks the trachea when I swallow,

  • so that the food doesn't end up in my respiratory system.

  • This ball of food that I just swallowed

  • actually has a scientific name, it's called a bolus

  • and it rides a kind of wave of muscle action down the esophagus

  • into the stomach.

  • This wave-like contraction of the smooth muscles around the tube

  • of the esophagus is called peristalsis, and it's basically

  • how most of the movement in your digestive system is accomplished.

  • Now my hot pocket bolus is in my stomach now

  • which is where the food really starts getting manhandled.

  • The stomach basically takes a scorched earth

  • approach to digestion. It's not messing around.

  • It's like a churning cement mixer that can contract

  • and expand with these big,

  • accordion-like folds of muscle called rugae.

  • Your stomach's job is to turn everything over and over,

  • smooshing and mixing all the pieces up

  • with its cocktail of acids and enzymes called gastric juice.

  • Gastric juice is mainly made up of hydrochloric acid,

  • an enzyme called pepsin, and some mucus and water.

  • Hydrochloric acid has a Ph of about 1

  • which is strong enough that, if you got it on your hand,

  • it would give you a chemical burn.

  • So the acid breaks things down and hopefully kills

  • most of the bacteria that you might find on your food.

  • The pepsin starts breaking down proteins into amino acids.

  • Now, that mucus is important.

  • It's there to protect your stomach, so that it doesn't digest itself.

  • When you don't have enough of that mucus you get peptic ulcers,

  • which happen when your stomach lining

  • comes in direct contact with your stomach acid.

  • And the water's just in there to make everything all soupy,

  • because what you want by the time your food leaves your stomach

  • is chyme, which is a kind of liquidy slop that you might

  • be familiar with from the last time you had a stomach virus.

  • You knew this conversation was going to have to get

  • a little bit gross and I didn't want to bring diarrhea into it

  • too much because, you know, I've been eating.

  • But when something bad is going on in your digestive tract,

  • your body doesn't worry too much about absorbing nutrients,

  • it just wants to get the chyme out of there.

  • So, chyme is what you see when...

  • You get the picture.

  • Anyway, there's a little valve, or sphincter

  • between the stomach and the small intestine

  • that regulates how much chyme gets into the small intestine

  • and when it gets in there.

  • The very beginning of the small intestine is called the duodenum,

  • this is where a lot of the small intestine action happens,

  • by which I mean, lots of things get absorbed and also secreted,

  • like bicarbonate, which neutralizes the gastric acid

  • before it goes any further.

  • Now, the coolness of the small intestine can't be overstated.

  • It's ground zero for cellular exchange of nutrients

  • and the breakdown of fats.

  • And again, the reason it's so good at absorbing

  • is because of all the surface area it's got going on.

  • A lot of that surface area comes from the fact that,

  • despite its name, your small intestine is frickin' long:

  • in a human, it can range anywhere from 4.5 to 10.5 meters.

  • But that's not all!

  • The whole inside is lined with epithelial tissue

  • that has tons of ridges and folds in it.

  • Surface area to the max!

  • And on those ridges and folds are these little hair-like fibers

  • of flesh called villi.

  • Each villus has capillaries in it, so that it can absorb nutrients.

  • And get this: each villus, which is only like half a millimeter long

  • is covered in teeeny tiny little microvilli,

  • providing even more surface area!

  • In fact, apparently, the small intestine has a texture

  • kind of like velvet, which is...

  • [DISGUST]

  • Oh great, now I eat the milkshake? Fantastic. Okay.

  • So another thing the small intestine does,

  • with the help of its friend the gallbladder,

  • is break down fatty stuff, like this milkshake.

  • Near the top of your small intestine is a little pipe

  • where bile salts, manufactured by the liver

  • and stored by the gallbladder, are squirted out

  • into the small intestine.

  • Bile works like dish detergent on a pan

  • you just fried something in: it's an emulsifier.

  • It takes hydrophobic fat molecules and breaks them up

  • into fatty acids and monoglycerides,

  • which can be absorbed by all that epithelial tissue!

  • I've never had Chunky Monkey before

  • Mmmmm!

  • Nuts!

  • After your food passes through those yards and yards

  • of small intestine, the chyme goes through another sphincter

  • and enters the cecum, the beginning of the large intestine.

  • The large intestine's job is to remove most of the water

  • and bile salts from the chyme so you don't have constant diarrhea.

  • So, you can thank it for that!

  • It's called "large" because it's wider than the small intestine,

  • but it's not nearly as long:

  • it's basically just a one- and-a-half meter victory lap

  • around outside of the small intestine and then it calls it good

  • Also, I should mention, at the end of the cecum

  • there is a little tube where the the appendix comes in.

  • For a long time, we thought that the appendix was a worthless,

  • vestigial structure that we used to need at some point

  • in our evolution but didn't need anymore.

  • However, recent studies are finding that the purpose of the appendix

  • in modern humans is probably to act as a safe house

  • for all of the good bacteria you need to help you digest your food.

  • If you get a virus or food poisoning or something

  • and all your digestive systems say, "GET IT ALL OUT OF ME!"

  • The appendix has a little sample of your gut bacteria

  • that it spits out to help recolonize you after your illness.

  • So, I think you're familiar with the final step

  • in the digestive system.

  • That's the pooping.

  • Your food can spend as long as 3 days in your digestive tract,

  • and a lot of that time is spent in the large intestine,

  • mostly reabsorbing the excess water from the chyme

  • and prepping your poo for its great entrance into the world.

  • When it's done, it passes through everybody's favorite sphincters,

  • the anal sphincters.

  • There are two of them.

  • And, you know...out in the world to live its own life.

  • And that's the end of our little tale here

  • that begins with the hot pocket.

  • I hope you'll join us next time for more disgustingness,

  • as we discuss the details of the Excretory System!

  • Until then, bon appetit!

  • Thank you for watching this episode of Crash Course Biology.

  • If you want to go back and review anything.

  • Table of contents just click on it.

  • Thanks, of course, to everybody who helped put this episode together.

  • If you have questions for us, please leave them

  • in the comments below, or on Facebook or Twitter.

  • And our team of experts will attempt to answer.

  • Goodbye.

Oh hi! I hope you don't mind that I'm eating.

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