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  • We all have our reasons for eating nachos at 3 in the afternoon.

  • I happen to have my own. And don't ask -- it's personal.

  • But more generally, we all eat any kind of food to accomplish two simple things: to obtain the energy

  • we need to stay alive and to get the raw materials required for building all of our tissues and stuff.

  • That's because, when it comes down to it, both you and the food you eat contain those two same things:

  • Both you and food are made ofstuff” -- by which I mean, matter, made of certain

  • kinds of atoms -- and both you and food have energy stored in the bonds between those atoms.

  • So all living things need to take in stuff and energy, and convert it into slightly different stuff and energy.

  • And you can get some of the things you need pretty easily. Like, in order to get oxygen

  • for respiration, to unleash the chemical energy in your food, you just have to inhale.

  • But you can't just breathe in the stuff you need to build DNA, or actin, or a phospholipid bilayer.

  • So, how does your body really acquirestuff”?

  • That's where the nachos come in.

  • This cheesy, crunchy dish is made of all different kinds of biological matter -- like carbohydrates

  • and fat and protein -- and it contains a certain, probably shocking, amount of calories, which

  • is how we measure energy stored in the chemical bonds in food.

  • So if I take, like, a 100-calorie bite of nachos -- which probably with this much cheese

  • wouldn't even be a very big bite -- I can convert the chemical energy stored in those

  • carbohydrates and proteins and fats to feed my muscle and heart cells and maybe, like,

  • walk a mile -- an activity that happens to use about 100 calories.

  • But I can't just swallow the nachos and watch the lump of them travel straight to my heart or leg muscles.

  • In order to actually use this food, I have to convert the biological matter into something

  • my body can work with on the cellular level, which as you know, is pretty darn tiny.

  • And, the work of converting the stuff in food, into the stuff that's in my body, is done by my digestive system.

  • Human digestion occurs in six main steps -- some of which you are intimately familiar with. Others less so.

  • But every step of the way, your body is working to reduce all the different kinds of molecules

  • in food into their tiniest and most basic forms.

  • The first step? Is, uh, probably everybody's favorite.

  • When it comes to what your digestive system ultimately does, just think of it as a sort of disassembly line.

  • You could have an order of nachos with The Works -- I'm talking beef and onions and

  • sour cream and slices of jalapeño -- and your digestive system will deconstruct it,

  • both mechanically and chemically, one step at a time.

  • It's gotta do this because your cells work best with materials that are in their most basic form.

  • Your digestive system reduces food to that level in two main ways: by physically smashing

  • it to smithereens, and by bathing them, as much as it can, in enzymes.

  • Enzymes are proteins that living things use as catalysts, to speed up chemical reactions.

  • When used in digestion, enzymes break down the large molecules in your food into the

  • building blocks that your cells can actually absorb.

  • Those large molecules are called biological molecules -- also known as macromolecules

  • -- and everything that you eat, I hope, is at least partially made of them.

  • And there are four main kinds: you got the lipids, the carbohydrates, the proteins, and the nucleic acids.

  • Each possesses its own density of chemical potential energy, or caloric value, like for

  • example, 1 gram of carbohydrate contains about 4 calories, while a gram of fat contains about 9 calories.

  • But many of these biological molecules are polymers -- or sequences of smaller molecules

  • -- and your cells aren't really equipped to take them up whole.

  • What your body trafficks in are those polymers' individual components -- called monomers -- and

  • there are four main kinds of those, too: fatty acids, sugars, amino acids, and nucleotides.

  • The simple idea behind the whole digestive system is to break down the polymers of macromolecules

  • in your food, into the smaller monomers that your cells can use to build their own polymers,

  • while also getting the energy they need.

  • And, what your body needs to build at any given moment is always changing.

  • Maybe you need new fat stores so you can have energy to run a marathon, or new actin and

  • myosin to build bigger muscles, or more DNA so you can replace the skin cells you scraped

  • off your knee when you fell, or more enzymes so you can digest more food to get more building materials.

  • To meet your body's constant, and constantly shifting demands, your digestive system requires a lot of organs

  • that perform a lot of specific tasks to break down and absorb the right nutrient at the right time.

  • Now, I'm quite sure that you're familiar with the key players here -- they're the

  • hollow organs that form the continuous tube that is your alimentary canal, aka the gastrointestinal

  • tract, which runs from your mouth to your anus.

  • It's worth pointing out that these organs are hollow, because you are basically hollow, too.

  • Your digestive tract is really just one unbroken, insulated tunnel of outside that just happens

  • to run through your body, and is open at both ends. You're a donut.

  • So the layer of stratified squamous and columnar epithelial cells that line your tract is actually

  • a barrier between the outside world and your inside world -- but it's a barrier that

  • allows for the selective movement of materials between them.

  • It's these hollow organs that do the actual moving, digesting, and absorbing of food,

  • and they include your mouth, pharynx, esophagus, stomach, and small and large intestines.

  • In your mouth, in your esophagus, and at the other end of things, at your anus, you have

  • stratified squamous epithelial tissue, just like your epidermis, to help resist the abrasive

  • action of like, chewing, like corn chips, maybe.

  • From your stomach on down, though, the inner GI tract is lined with simple columnar epithelial

  • cells, which secrete all sorts of stuff, and which absorb and process various nutrients.

  • Most of those columnar cells secrete mucus, which lubricates everything, and protects

  • your cells from being digested by your own digestive enzymes.

  • So, the innermost epithelial layer of the tube is known as the mucosal layer, and it

  • contains some connective tissue as well, which supplies it with blood.

  • Surrounding the mucosal layer is the submucosal layer, made of loose areolar connective tissue,

  • which helps provide the elasticity that the tube needs when you eat a whole pizza in one

  • sitting, and it contains more blood vessels.

  • And outside that, you have the muscularis externa layer, which as you might guess, is

  • where you find the muscles responsible for moving food through your tube.

  • Beyond these layers, the GI tract gets tons of support from the accessory digestive organs,

  • like your teeth, and your tongue, your gallbladder, salivary glands, liver, and pancreas.

  • They're kind of like a pit crew, and they mostly help by secreting various enzymes that

  • help take apart food as it comes down the tube.

  • Together, these two groups on the digestive disassembly line work in six steps to destroy

  • your food and release and recycle its nutrients.

  • First, of course, you've got to introduce the food to your digestive system. What you

  • know as eating, or ingestion, is basically just creating a bulk flow of nutrients from

  • the outside world into your tissues.

  • This is where the work of disassembly begins: In your face-hole, which scientists call your mouth.

  • Now, we're going to get to the details of what happens here another time, but remember

  • that food disassembly is both mechanical and chemical: So your teeth pulverize the bite

  • of nacho or whatever, while your salivary glands begin that food's hours-long enzyme bath.

  • But the food, at this point, is not nearlymicroenough to be of any use to your

  • cells, so you have to move that mush further down your tube.

  • This stage is called propulsion, and its initial mechanism is swallowing -- which, as you know,

  • is a voluntary action -- but then it's quickly turned over to the involuntary process of peristalsis.

  • In peristalsis, the smooth muscles of the walls of your digestive organs take turns

  • contracting and relaxing to squeeze food through the lumen, or cavity, of your alimentary tract.

  • Waves of peristalsis continue through the esophagus, stomach, and intestines, and they're

  • so strong that even if you were hanging upside down while eating your lunch and drinking

  • your tea, the food would still soldier on, fighting gravity, and eventually make it to its final destination.

  • Don't do that, though. There's other reasons why you shouldn't be upside down.

  • Anyway, all of this shipping and handling mechanically breaks down the food even more,

  • and even after it goes through the stomach and its gastric acid, the mechanical work

  • still continues once it reaches your small intestine, as more smooth muscle segments

  • push the food back and forth to keep crumbling it up.

  • The goal of all this pulverization is to increase the surface area of that bite of food by breaking it down

  • into increasingly tiny pieces, to prepare it to encounter more enzymes in step four: chemical digestion.

  • Really, the actual process of digestion only occurs when the main action becomes

  • more chemical than mechanical.

  • And here, the accessory digestive organs -- namely, the liver, pancreas and gallbladder -- secrete

  • enzymes into the alimentary canal, where they ambush the mush and break it down into its

  • most basic chemical building blocks.

  • Like I said before, our cells prefer to do business in the really basic currency of monomers,

  • like amino acids, fatty acids, and simple sugars. And digestion allows for the absorption of

  • those nutrients as they pass from the small intestine into the blood, by both active and passive transport.

  • Once those nutrients are absorbed by your cells, you can finally use the energy inside

  • of them or use them to build new tissues.

  • The absorption of the nutrients is the goal of the entire process.

  • But, of course, it is not the end of it.

  • Once your body has sucked out all the nutrients it wants, indigestible substances like fiber

  • are escorted out of your body.

  • Yeah, I'm talking about pooping, or defecation.

  • And that is the end of the digestive line -- unless you are a capybara, or one of the

  • other animals who make sure that they get the most out of their lunch, by giving the

  • whole process another round and practicing coprophagia, aka eating their own poop.

  • Now, you should notice here that some of the processes of digestion occur in just one place,

  • and are the job of a single organ -- like hopefully you're only ingesting through

  • your mouth and eliminating from the large intestine.

  • But most of these six steps require cooperation among multiple organs.

  • For example, both mechanical and chemical digestion start in the mouth, and continue

  • through the stomach and small intestines. And some chemical breakdown continues in the

  • large intestine, thanks to our little bacterial farm there.

  • Over the next couple of weeks we're going to take you and your nachos on a stroll through

  • your digestive system and see who's doing what, where, how, and why.

  • But for now, I've got some nachos to finish, so I gotta go.

  • And eating those nachos, as you learned today, will provide me with energy and raw materials,

  • by first ingesting something nutritious, propelling it through my alimentary canal where it will

  • be mechanically broken down, and chemically digested by enzymes until my cells can absorb

  • their monomers and use them to make whatever they need. And eventually, there will be pooping.

  • Thanks to all of our Patreon patrons who help make Crash Course possible through their monthly

  • contributions. And 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 Peter Rapp, Sigmund

  • Leirvåg, Mikael Modin, and Jeremy Bradley 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 by Nicholas Jenkins, edited by Nicole Sweeney; our sound designer is Michael

  • Aranda, and the Graphics team is Thought Cafe.

We all have our reasons for eating nachos at 3 in the afternoon.

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消化器系 その1クラッシュコース A&P #33 (Digestive System, Part 1: Crash Course A&P #33)

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    莊小莊 に公開 2021 年 01 月 14 日
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