oh hi! i hope you don'tmind that i'm eating. this is actuallyjust my first course. for my birthday, thewriters wrote me a script where i just get to eat! the whole time!
crash diets that really work, and i can't think of abetter way to demonstrate the workings of thedigestive system, the series of hollow organsthat we use to break down and process the nutrients andenergy we need to function.
though...wait a second,if i remember correctly digestion is actually prettyfreaking disgusting... so maybe i shouldn'tbe eating right now. oh whatever... waiter! the digestive system is sofundamental that it's basically step number one in theguide: how to make an animal. you probably remember that duringthe embryonic development of most animals, the digestive tract isthe very first thing that forms.
when the blastula, that little wadof cells that we all used to be, turns into a little wad of cellswith a tube running through it, that tube is your digestive system. and pretty much every animal hasa digestive system of some kind, but they're not all alike. far from it. in fact, digestive tractsare specially adapted to animals' feedingbehavior and diet. for instance, a house fly eatsmostly liquid or very finely
granulated food, butbefore it does that, it's got to puke its digestivejuices all over its lunch and then let themdigest it for a while before it sucks itup into its mouth. if we did it like that, firstdates would be...less common. most vertebrates put foodin one end of the tube and our digestivesystem processes it, and then it gets rid of the wasteout the other end of the tube. no muss, no fuss.
well, actually, there's alittle bit of muss, at the end. you may have noticed. but the beauty of it is thatthis whole process is run by our autonomic nervous system, so wedon't have to think about it, until maybe the very laststep when we're in traffic and just had two cups ofcoffee and a bran muffin... then we have to thinkabout it a little bit. among vertebrates, the digestivetract might be short or long, or have organs thatdo different things
depending on what itsfeeding habits are. for instance, dogs are mostlycarnivores and also scavengers: they mostly eat meat, but sometimes that meat'sbeen dead for a while. so, the dog's digestive systemhas developed to take food in, absorb as manynutrients as possible, and then deposit iton somebody's lawn, all in a period ofabout six hours. dogs have an extremely shortdigestive tract, because,
if you're in the habitof eating rotten meat, you'd better be ableto digest it fast. if you don't, the bad bacteriathat's probably living on that armadillo carcass is going totake up residence in your gut and put you in a world of hurt. cows, on the other hand, takea very very very long time to digest their food,around 80 hours, because they have to processplants, mostly grass. grass has a ton of cellulosein it, and evolution has yet
to produce an animal that canmanufacture a stomach acid or enzyme tough enoughto break down cellulose. so, cows have microorganismsin their guts that break down thecellulose for them. this process takes afour-chambered stomach each one with a slightlydifferent microecology and a lot of cud-chewing, or regurgitating andre-chewing of grass before it passes allthe way through.
so, nature is full ofcrazy digestion stories, and i honestly wish that ihad time to tell them all. but let's focus on human digestionfrom now on, mostly because: you're probably a human, wedon't assume anything here, and you'll be wanting to knowhow your body does all this stuff. and humans actually havea have a pretty good all-purpose digestive system: we're omnivores, after all,we eat plants and meat so our systems are generalizedto handle all kinds of stuff.
like most animals, humans havea bunch of different acids and enzymes in our digestivetracts that break food down so that it can be absorbedand used by our bodies. but the secret tosuccessful digestion is maximizing surface area.in more that one way, actually. the first way we maximize surfacearea is on the food itself. say i take a biteout of this apple. right now there's like, an appleboulder sitting there in my mouth. i've got enzymes in mysaliva that immediately start
breaking it down, like,the outsides of the boulder. if i swallowed thischunk whole right now, not only would it hurt like heck, the rest of my digestive systemwould have a really hard time dealing with it, becausemost of the enzymes and acids would have the same difficultyworking all the way through this big solid hunk. but, when i use my awesome teethto chew up this hunk of apple suddenly there's double, triple,
quadruple the surfacearea on the food! i'm making up apple gravelfrom the apple boulder. maybe even apple sand. for humans, chewing is key becausebreaking down our food into smaller and smaller bits allowsenzymes and acids to get at them. and after our teeth havemade the pieces small enough, the chemicals break themdown further until they're fine enough for our bodiesto absorb nutrients from them. but it's not just the surface areaof the food that's important,
the surface area ofthe digestive system is key to the wholeprocess as well. last time i talked abouthow we have a whole bunch of surface area in our lungs toabsorb tons of oxygen all at once. well, our digestive systemswork in much the same way. most of the absorption of nutrientshappens in our small intestines, and the length of the averagehuman adult's small intestine is about 7 meters! plus, inside our small intestines
there are a bunch of littlefolds 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 workto increase their surface area. last episode i was allimpressed that lungs had a total surface areaof 75 square meters... well the small intestine has asurface area of 250 square meters! blegh... it's kind of gross. i wouldn't want to see it spreadout over a tennis court or anything
but i'm gettingahead of myself here. digestion does not start atthe small intestine, people, it starts at the mouth. now, as you can see, thishot pocket is surrounded by some kind of bread,if you can call it that. bread is a starch, whichbreaks down into glucose. when i start gnawingon a piece of bread, because the outsidehere is mostly bread. the glands in my mouthstart secreting saliva,
which contains salivary amylase,an enzyme designed to break down starch into glucose. the more i chew, the more amylasewill get to all the different sides of the bread, and that'swhy the more you chew bread, the sweeter it tastes. amylase doesn't really do muchto the meat or the cheese in this thing. i've got other enzymes and acidsthat are going to work on them later on in the system, but i amgonna chew all that stuff up
real good right now sothat those other enzymes can do their jobs later. i'm gonna swallow all this. so now the masticated hot pockethas passed down my pharynx, or throat, and into my esophagus,which leads to my stomach. there's actually thislittle cool flap of tissue called the epiglottis that blocksthe trachea when i swallow, so that the food doesn't endup in my respiratory system. this ball of foodthat i just swallowed
actually has a scientificname, it's called a bolus and it rides a kind of wave ofmuscle action down the esophagus into the stomach. this wave-like contraction of thesmooth muscles around the tube of the esophagus is calledperistalsis, and it's basically how most of the movement in yourdigestive system is accomplished. now my hot pocket bolusis in my stomach now which is where the food reallystarts getting manhandled. the stomach basicallytakes a scorched earth
approach to digestion.it's not messing around. it's like a churning cementmixer that can contract and expand with these big, accordion-like foldsof muscle called rugae. your stomach's job is toturn everything over and over, smooshing and mixingall the pieces up with its cocktail of acids andenzymes called gastric juice. gastric juice is mainly madeup of hydrochloric acid, an enzyme called pepsin,and some mucus and water.
hydrochloric acidhas 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 thingsdown and hopefully kills most of the bacteria thatyou might find on your food. the pepsin starts breaking downproteins 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 thatmucus you get peptic ulcers,
which happen whenyour stomach lining comes in direct contactwith your stomach acid. and the water's just in thereto make everything all soupy, because what you want by thetime your food leaves your stomach is chyme, which is a kind ofliquidy slop that you might be familiar with from the lasttime you had a stomach virus. you knew this conversationwas going to have to get a little bit gross and i didn'twant to bring diarrhea into it too much because, youknow, i've been eating.
but when something bad is goingon in your digestive tract, your body doesn't worry toomuch about absorbing nutrients, it just wants to getthe chyme out of there. so, chyme is what you see when... you get the picture. anyway, there's alittle valve, or sphincter between the stomach andthe small intestine that regulates how much chymegets into the small intestine and when it gets in there.
the very beginning of the smallintestine is called the duodenum, this is where a lot of thesmall intestine action happens, by which i mean, lots of thingsget absorbed and also secreted, like bicarbonate, whichneutralizes the gastric acid before it goes any further. now, the coolness of the smallintestine can't be overstated. it's ground zero forcellular exchange of nutrients and the breakdown of fats. and again, the reasonit's so good at absorbing
is because of all the surfacearea it's got going on. a lot of that surface areacomes from the fact that, despite its name, your smallintestine is frickin' long: in a human, it can range anywherefrom 4.5 to 10.5 meters. but that's not all! the whole inside is linedwith epithelial tissue that has tons of ridgesand folds in it. surface area to the max! and on those ridges and folds arethese 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, whichis only like half a millimeter long is covered in teeenytiny little microvilli, providing even more surface area! in fact, apparently, thesmall intestine has a texture kind of like velvet, which is... [disgust] oh great, now i eat themilkshake? fantastic. okay.
so another thing thesmall intestine does, with the help of itsfriend the gallbladder, is break down fatty stuff,like this milkshake. near the top of your smallintestine is a little pipe where bile salts,manufactured by the liver and stored by thegallbladder, are squirted out into the small intestine. bile works like dishdetergent on a pan you just fried something in:it's an emulsifier.
it takes hydrophobic fatmolecules and breaks them up into fatty acidsand monoglycerides, which can be absorbed by allthat epithelial tissue! i've never had chunky monkey before mmmmm! nuts! after your food passes throughthose yards and yards of small intestine, the chymegoes through another sphincter and enters the cecum, thebeginning of the large intestine.
the large intestine's job isto remove most of the water and bile salts from the chyme soyou don't have constant diarrhea. so, you can thank it for that! it's called "large" because it'swider 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 smallintestine and then it calls it good also, i should mention,at the end of the cecum there is a little tube wherethe the appendix comes in.
for a long time, we thought thatthe appendix was a worthless, vestigial structure that weused to need at some point in our evolution butdidn't need anymore. however, recent studies are findingthat the purpose of the appendix in modern humans is probablyto act as a safe house for all of the good bacteria youneed to help you digest your food. if you get a virus orfood poisoning or something and all your digestive systemssay, "get it all out of me!" the appendix has a littlesample of your gut bacteria
that it spits out to helprecolonize you after your illness. so, i think you're familiarwith the final step in the digestive system. that's the pooping. your food can spend as long as3 days in your digestive tract, and a lot of that time isspent in the large intestine, mostly reabsorbing theexcess water from the chyme and prepping your poo for itsgreat entrance into the world. when it's done, it passes througheverybody's favorite sphincters,
the anal sphincters. there are two of them. and, you know...out in theworld to live its own life. and that's the end ofour little tale here that begins with the hot pocket. i hope you'll join us nexttime for more disgustingness, as we discuss the detailsof the excretory system! until then, bon appetit! thank you for watching thisepisode of crash course biology.
if you want to go backand review anything. table of contentsjust click on it. thanks, of course, to everybody whohelped put this episode together. if you have questions forus, please leave them in the comments below,or on facebook or twitter. and our team of expertswill attempt to answer. goodbye.
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