The digestive system

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 nutrients and energy we need to function though wait a second if I remember correctly digestion is actually pretty frickin disgusting so maybe I shouldn't be eating right now but whatever waiter the digestive system is so fundamental that it's basically step number one in the guide to how to make an animal you probably remember that during the embryonic development of most animals the digestive tract is 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 housefly eats mostly liquid or very finely granulated food but before it does that it's got a 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 into 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 meats 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 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 Al's 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 micro ecology and a lot of cud-chewing her giorgia tating and reaching 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 to humans actually 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 down food so that it can be absorbed and used by our bodies but the secret to successful digestion is maximizing surface area in more than one way actually the first way we maximize surface area is on the food it sells to say I take a bite out of this Apple right now like a Apple Boulder sitting there in my mouth I got enzymes in myself I would that immediately start breaking it down like the outsides of the boulder as I follow this chunk hole 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 hump but when I use my awesome teeth to chew up theirs hunka Apple Sutton where 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 system works 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 they're a bunch of little folds and little absorbing fibers with absorbing fibers on them and Noah didn't misspeak the fibers have fibers that's how hard our intestines work to increase their surface area last episode I was all impress that lungs have a total surface area of 75 square meters well the small intestine has a surface area of 250 square meters yeah 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 record ooh the outside here so it's most bread the glands in the mouth start secreting saliva which contains salivary amylase an enzyme designed to break down starch into glucose the more I chew the more analysts will get to all the different sides of the bread that's why the more you chew bread the sweeter it tastes analyst doesn't really do much to the meat of 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 going to choose all that stuff up real good right now so that those other enzymes can do their jobs later amount of swallow Aldus so now the masticated hot pocket has passed down my pharynx or throat in 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 so this ball of food that I just followed actually has a scientific name it's called a bolus and it rides a kind of wave a 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 ruiji your stomach's job is to turn everything over and over smashing 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 and the pepsin starts breaking down proteins into amino acids now that mucus is important it's there to protect to your stomach so that it doesn't like 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 waters 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 see where 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 kind 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 time 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 all of the surface area it's got going on although that service area comes from the fact that despite its name your small intestine is freaking long in a human it can range anywhere from four point five to ten point five meters but that's not all the hole inside is lined with epithelial tissue and has tons of ridges and folds in it surface area to the max and on those ridges and folds are these little hairlike fibers of flesh called villi each villus has capillaries on it so that it can absorb nutrients and get this each villus which is only like half a millimeter long is covered in teeny tiny little micro villi providing even more surface area in fact apparently the small intestine has a texture kind of like velvet which is ah oh great now I eat the milkshake fantastic okay so another thing the small intestine does with the help of its friend that gall bladder is break down fatty stuff like this milkshake near the top of your small intestine it's a little pipe where bile salts manufactured by the liver and stored by the gall bladder are squirted out into the small intestine bile works like dish detergent on a pan you just fried something and 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 remember how chunky monkey before mm-hmm not after your food passes through those yards and yards of small intestine the chyme goes through another sphincter and enters the sea 'some the beginning of the large intestine the large intestines job is to remove most of the water and bile salts from the chyme so you don't have constant diarrhea so you can think 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 the outside of the small intestine and then it calls it good also should mention at the end of the season there is a little tube where 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 and modern humans is probably to act as a safe house for all 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 you recolonize after your illness so I think you're probably familiar with the final step in the digestive system that's the pooping your food can spend as long as three 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 sinkers 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've drawn it next time for more disgustingness as we discuss the details of the excretory system until then Bon Appetit