- 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 freakin' disgusting so maybe I shouldn't be eating right now. Oh whatever, waiter. (upbeat 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 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 housefly
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 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 two, 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 the surface area, in more than one way actually. The first way we maximize surface area is on the food itself. So say I take a bite out of this apple. If I swallow 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 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 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. It's kind of gross. I wouldn't wanna 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, we could do the outside
here so it's mostly bread. The glands in my mouth
start creating 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 gonna 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 is 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. So 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 wavelike 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 one, 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 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 that water's 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 liquid-y slop that you might be familiar
with from the last time you had a stomach virus. You knew this conversation
was gonna have to get a little bit gross and
I didn't wanna 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 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 overstated. It's ground zero for cellular exchange of nutrients in 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. A lot of that surface
area comes from the fact that despite its name your small
intestine is frickin' long. In a human, it can range
anywhere from 4.5 to 10.5 meters. But that's not all. The whole inside is lined
with epithelial tissue that has tons of ridges and folds in it. Surface area to the max. And on those ridges and
folds are these little hair-like 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 microvilli, providing
even more surface area. In fact, apparently the small intestine has a texture kind of
like velvet, which is, great, now I eat the
milkshake, fantastic, okay. So another thing the small intestine does with the help of its
friend the gallbladder is break down fatty stuff
like this milkshake. Near the top of your small
intestine is a little pipe where bile salt
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've never had Chunky Monkey before. 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 intestine's
job is to remove most of the water and bile salts from the chyme so you don't have constant diarrhea, so 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 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 cecum there's 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 in modern humans is probably to act as a safehouse for all of 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 sphincters, 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 join us next time for more digustingness as we discuss the details of the excretory system. Until then, bon appetit.