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Studying for a test? Prepare with these 2 lessons on Crash Course: Biology and Ecology.
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Male narrator: 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. 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. Wait a second, if I remember correctly digestion is actually pretty freaking disgusting. Maybe I shouldn't be eating right now or whatever. Waiter! (lively music) 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 watt of cells that we all used to be turns into a little watt of cells with a tube running through it. That tube is your digestive system. Pretty much every animal has a digestive system of some kind, but their not all alike, far from it. In fact, digestive tracts are especially 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 it's digestive juices all over it's lunch and then let them digest it for awhile before it sucks it up into it's 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. 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 it's feeding habits are. For instance, dogs are mostly carnivores and also scavengers. They mostly eat meat but sometimes that meats been dead for awhile. The dog's digestive system has developed to take food in, absorb as many nutrients as possible and then to pass it onto 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. [Tels] 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 an enzyme tough enough to break down cellulose. Calves have microorganisms in their guts that breakdown 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 or regurgitating and re-chewing of grass before it passes all the way through. 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 these stuff. Two, humans actually have a pretty good all purpose digestive system we're omnivores after all, we eat plants and meat. 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. 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 it's like an apple boulder sitting their in my mouth. I got enzymes in my saliva that immediately start breaking it down. Like the outsides of the boulder. As I swallow this chunk, whole right now, not only would it hurt like heck. The rest of my digestive system will 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 chunk. When I use my awesome teeth to chew up, there's chunk of apple suddenly there's double, triple, quadruple to 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 it done. After our teeth have made the pieces small enough, the chemicals break them down further until their fine enough for our bodies to absorb nutrients from them. 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 inetstines and the length of the average human adult small intestine is about seven 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 misspeak the fibers have fibers. That's how hard our intestines work to increase their surface area. Last episode I was all impressed that lungs have a total surface area of 75 square meters. Well the small intestine has a surface area of 250 square meters. Weh, 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 could 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. I used to gnawing on a piece of bread. Look at it, the outside here is most bread. The glands in my mouth start secreting saliva which contains salivary amylase, an enzyme designed to breakdown 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 with the meat or the cheese in this thing, I've got other enzyme and acids that are going to work on them later on in the system. I am going to chew all that stuff up real good right now so that those other enzymes can do their jobs later. I'm going to swallow all this. Now the masticated hot pocket is passed down in my pharynx or throat and into my esophagus which leads to my stomach. Actually there's tittle 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, 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 the peristalsis. 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 man handled. The stomach basically takes a scorch dirt approach to digestion, it's not messing around. It's like a churning cement mixture that can contract and expand with this big accordion like folds of muscles called rugae. The stomachs job it to turn everything over and over, smooching and mixing all the pieces up with it's cocktail of acids and enzymes called gastric juice. Gastric juices mainly made up of hydrochloric acid and enzyme called pepsin, and some mucus and water. Hydrochloric acid has a PH of about one which is strong enough that if you got in on your hand it would give you a chemical burn. 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 stomachs 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. The water is 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. Even this conversation's 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. 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 over stayed and it's ground zero for cellular exchange of nutrients in the breakdown of fats. Again the reason it's so good at absorbing is because all of the surface area has got going on. About that surface area, it comes from the fact that despite it's name your small intestine is freaking long. In a human it can range anywhere form 4.5 to 10.5 meters, but that's not all. The whole inside is lined with epithelial tissue and has tons of ridges and folds in it, surface area to the max. 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. 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 uhh ... Great now I eat the milkshake, fantastic, okay. Another thing the small intestine does with the help of it's friend the gallbladder is break down fatty stuff, like this milkshake. In 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 never had chunky [mango] before. Hmm-hmm. 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 intestines job is to remove most of the water and bile salts from the chyme, so you don't have constant diarrhea. 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 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 cecum there's a little tube where the appendix comes in. For a long time we thought that the appendix was a worthless vestibule 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 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 system say "Get 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. 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 it's 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 it's own life. That's the end of our little tale here that begins with the hot pocket. I hope you try this next time for more disgusting as we discussed the details of the excretory system. Until then bon appetite.