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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