The Digestive System: CrashCourse Biology #28

Summary

In this entertaining video, the host explores the intricacies of the human digestive system while humorously eating a meal. Starting with the embryonic development of the digestive tract, the script covers various animals' digestive adaptations, then delves into the detailed process of human digestion—from chewing food to nutrient absorption in the small intestine, and finally, to waste elimination. The video emphasizes how enzymes, acids, and surface area play critical roles in digestion, while keeping the tone light and engaging throughout.

Takeaways

• 🍽️ Digestion is crucial for breaking down food and processing nutrients in the body.
• 👶 The digestive system is one of the first things to develop during embryonic growth.
• 🦠 Different animals have unique digestive systems tailored to their diets (e.g., dogs digest meat quickly, cows take longer with grass).
• 😬 Digestion starts in the mouth, where chewing increases surface area, making it easier for enzymes to break down food.
• 🧪 Saliva contains amylase, an enzyme that breaks down starch into glucose, making food like bread taste sweeter as you chew.
• 🍲 In the stomach, strong acids and enzymes, like hydrochloric acid and pepsin, break down food into a soupy mixture called chyme.
• 📏 The small intestine is where most nutrient absorption happens, and it maximizes surface area with folds and villi to absorb nutrients efficiently.
• 🌿 The gallbladder and liver help break down fats using bile, which emulsifies fat for easier absorption.
• 🚰 The large intestine's main job is to absorb water from the chyme and prepare waste for elimination.
• 🦠 The appendix may play a role in protecting good gut bacteria and recolonizing it after illness.

Q & A

What is the primary function of the digestive system?

The primary function of the digestive system is to break down food into nutrients and energy that the body can use, as well as to eliminate waste.

Why is the digestive tract one of the first structures to form during embryonic development?

The digestive tract is one of the first structures to form during embryonic development because it is fundamental to an animal's survival, providing the means to process nutrients from food.

How does the digestive system of a house fly differ from that of a human?

A house fly's digestive system involves external digestion, where it regurgitates digestive juices onto its food to break it down before sucking it up, unlike humans who have an internal digestive system.

What adaptation does a dog's digestive system have that allows it to handle rotten meat?

Dogs have a short digestive tract to quickly process and eliminate rotten meat to prevent harmful bacteria from proliferating in their gut.

Why does it take cows around 80 hours to digest their food?

Cows take a long time to digest their food because they are herbivores that consume cellulose-rich grass, which requires a complex process involving microorganisms in their four-chambered stomach.

How does the human digestive system maximize the surface area for nutrient absorption?

The human digestive system maximizes surface area for nutrient absorption by having a long small intestine with folds, villi, and microvilli that increase the area for enzymes and acids to act upon.

What is the role of saliva in the human digestive process?

Saliva contains enzymes like salivary amylase that begin the breakdown of starches into glucose, starting the digestion process in the mouth.

What is the function of the stomach in the digestive system?

The stomach's function is to mechanically and chemically break down food with the help of gastric juices containing hydrochloric acid and pepsin, turning it into a semi-liquid called chyme.

How does the small intestine contribute to the digestion of fats?

The small intestine contributes to fat digestion by secreting bile, which emulsifies fats, breaking them down into smaller components that can be absorbed.

What is the purpose of the appendix in the modern human body?

Recent studies suggest that the appendix may serve as a reservoir for beneficial gut bacteria, which can help repopulate the gut after an illness.

What is the final step in the human digestive process?

The final step in the human digestive process is the elimination of waste through the large intestine and rectum as feces, which is then expelled through the anus.

Outlines

00:00

🍽 Introduction to the Digestive System

The paragraph introduces the concept of the digestive system by humorously setting the scene with the speaker eating throughout the script. It explains the digestive system as a series of organs that break down food into nutrients and energy. The speaker acknowledges the unappetizing nature of digestion but dives into the topic by discussing how the digestive tract is the first structure to form during embryonic development. It highlights the diversity in digestive tracts among animals, adapting to their feeding behaviors and diets. Examples include the house fly's regurgitation before eating and the different digestive strategies of dogs and cows. The paragraph concludes by focusing on human digestion, emphasizing our omnivorous nature and the generalization of our digestive system to handle a variety of foods.

05:01

🔎 The Journey of Food Through the Digestive System

This paragraph delves into the process of digestion, starting from the mouth where salivary amylase begins breaking down starches. It describes the咀嚼 process as crucial for increasing the surface area of food, allowing enzymes and acids to act more effectively. The speaker then follows the path of a bolus through the esophagus, highlighting the role of peristalsis in moving food. The stomach's role is detailed, with its production of gastric juice containing hydrochloric acid and pepsin, which further break down food and kill bacteria. The paragraph also explains the function of the small intestine, where most nutrient absorption occurs, and its extensive surface area due to folds, villi, and microvilli. The role of bile in emulsifying fats is also discussed, emphasizing the small intestine's importance in nutrient absorption.

10:01

🌀 The Final Stages of Digestion and Excretion

The final paragraph covers the latter stages of digestion, where chyme moves from the small intestine to the large intestine. It explains the large intestine's role in absorbing water and bile salts, and its relatively shorter length compared to the small intestine. The appendix is mentioned as a potential safe house for beneficial bacteria. The paragraph concludes with the process of excretion, where the remaining waste passes through the rectum and is expelled through the anal sphincters. The speaker invites viewers to join the next episode on the excretory system, emphasizing the educational yet light-hearted tone of the series.

Keywords

💡Digestive System

The digestive system is a series of hollow organs that work together to break down food, absorb nutrients, and process energy for the body to function. It plays a central role in the video, as the speaker describes how digestion begins in the mouth and continues through different organs, each adapted for specific functions like absorption and nutrient breakdown.

💡Surface Area

Surface area refers to the total area of a surface that is exposed to chemical reactions in the body. In digestion, maximizing surface area is crucial for effective nutrient absorption, such as when food is chewed into smaller bits and when the small intestine uses villi and microvilli to increase absorption space. This concept is important for understanding how the body efficiently absorbs nutrients.

💡Peristalsis

Peristalsis is the wave-like muscle contractions that move food through the digestive system, particularly the esophagus. This process ensures that food, or bolus, is transported from the mouth to the stomach and through the intestines without relying on gravity. The video emphasizes peristalsis as a key mechanism in digestion.

💡Gastric Juice

Gastric juice is a mixture of hydrochloric acid, enzymes like pepsin, and mucus, found in the stomach. This substance breaks down food, kills bacteria, and starts the digestion of proteins. The speaker describes gastric juice as essential for turning food into chyme and ensuring that digestion in the stomach is efficient and thorough.

💡Enzymes

Enzymes are biological catalysts that speed up chemical reactions in the body, such as breaking down food into smaller molecules. In digestion, enzymes like salivary amylase and pepsin play critical roles in processing carbohydrates and proteins. The video highlights their importance in both the mouth and stomach for ensuring proper digestion.

💡Small Intestine

The small intestine is a long, narrow organ responsible for most of the nutrient absorption in the digestive system. It has a highly folded surface with villi and microvilli to increase surface area, allowing for efficient nutrient uptake. The video explains that despite its name, the small intestine is crucial due to its massive surface area, making it the center of nutrient absorption.

💡Villi and Microvilli

Villi are small, finger-like projections on the lining of the small intestine that increase its surface area to enhance nutrient absorption. Microvilli are even smaller structures on the villi, further increasing absorption capacity. These structures play a major role in human digestion, as explained in the video, ensuring the body can efficiently extract nutrients from food.

💡Bolus

A bolus is a ball of chewed food that is swallowed and travels down the esophagus into the stomach. The term is introduced in the video to describe the form food takes after it has been chewed and prepared for digestion in the stomach, where it will be broken down by gastric juices.

💡Chyme

Chyme is the semi-liquid mass of partially digested food that leaves the stomach and enters the small intestine. The video discusses chyme as the result of the stomach's powerful mixing and digestion, which is further processed in the intestines for nutrient absorption.

💡Bile

Bile is a digestive fluid produced by the liver and stored in the gallbladder, which helps break down fats into smaller molecules that can be absorbed in the small intestine. The video describes bile's emulsifying role, comparing it to dish soap breaking down grease, allowing fats to be digested and absorbed more efficiently.

Highlights

Introduction to the digestive system, its importance, and how it's the first system to develop in animals during embryonic development.

Different animals have specialized digestive systems based on their diets, such as house flies vomiting digestive juices to break down food.

Vertebrates generally process food by entering at one end and exiting at the other with the help of the autonomic nervous system.

Comparison of digestion between carnivores like dogs, which have short digestive tracts for quickly processing meat, and herbivores like cows, which take up to 80 hours to digest food.

The role of microorganisms in cows' stomachs that help break down tough cellulose from grass, enabling digestion.

Humans have a generalist digestive system as omnivores, breaking down both plant and animal matter.

Maximizing surface area is key in digestion. Chewing breaks food down into smaller particles, increasing the surface area for enzymes to act on.

Human digestive system: the small intestine has a surface area of 250 square meters due to folds, villi, and microvilli, maximizing absorption.

The stomach's role in digestion involves churning food with gastric juice, hydrochloric acid, and pepsin to break down food into a substance called chyme.

The protective function of mucus in the stomach to prevent self-digestion, with issues like peptic ulcers arising when mucus is insufficient.

How bile, produced by the liver and stored in the gallbladder, emulsifies fats in the small intestine, allowing them to be absorbed.

The appendix's modern function as a safe house for gut bacteria, helping recolonize the digestive system after illness.

Final stages of digestion occur in the large intestine, where water is absorbed from the chyme, turning it into stool.

The entire digestive process from intake to excretion can take up to 3 days.

Conclusion of the episode with a mention of the upcoming discussion on the excretory system.

Transcripts

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 the nutrients and energy we need to function.

Though...wait a second, if I remember correctly

digestion is actually pretty freaking disgusting...

so maybe I shouldn't be eating right now.

Oh whatever... Waiter!

The digestive system is so fundamental that it's basically

step number one in the guide: 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 house fly 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'd 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 humans actually have a 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 food down

so that it can be absorbed and used by our bodies.

But 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 there's like, an apple boulder sitting there in my mouth.

I've got enzymes in my saliva that immediately start

breaking it down, like, the outsides of the boulder.

If I swallowed 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 systems work 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

there are a bunch of little folds 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 work to increase their surface area.

Last episode I was all impressed that lungs had

a total surface area of 75 square meters...

well the small intestine has a surface area of 250 square meters!

Blegh... 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 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,

because the outside here is mostly bread.

the glands in my mouth start secreting 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 going to 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 has 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.

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 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 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 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.

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 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 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 liquidy slop that you might

be familiar with from the last time you had a stomach virus.

You knew 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, 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

and the breakdown of fats.

And again, the reason it's so good at absorbing

is because of all 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 in it, so that it can absorb nutrients.

And get this: each villus, which is only like half a millimeter long

is covered in teeeny tiny little microvilli,

providing even more surface area!

In fact, apparently, the small intestine has a texture

kind of like velvet, which is...

[DISGUST]

Oh 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 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've never had Chunky Monkey before

Mmmmm!

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 outside of the small intestine and then it calls it good

Also, I should mention, at the end of the cecum

there is a little tube where the 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 safe house

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 recolonize you after your illness.

So, I think you're familiar with the final step

in the digestive system.

That's the pooping.

Your food can spend as long as 3 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'll join us next time for more disgustingness,

as we discuss the details of the Excretory System!

Until then, bon appetit!

Thank you for watching this episode of Crash Course Biology.

If you want to go back and review anything.

Table of contents just click on it.

Thanks, of course, to everybody who helped put this episode together.

If you have questions for us, please leave them

in the comments below, or on Facebook or Twitter.

And our team of experts will attempt to answer.

Goodbye.