Digestive System, Part 1: Crash Course Anatomy & Physiology #33

Summary

This video explains the digestive system through a fun analogy involving nachos. It highlights how food provides energy and materials for our bodies, breaking down complex molecules into simpler components like sugars, fatty acids, and amino acids. The process involves mechanical and chemical digestion, starting in the mouth and continuing through the stomach and intestines. Enzymes play a key role in breaking down macromolecules into monomers, which are absorbed by cells for energy and tissue-building. The video also emphasizes the importance of proper digestion and ends with a humorous nod to the elimination process.

Takeaways

• 🌮 The main reason we eat food, like nachos, is to obtain energy and raw materials needed for survival and body maintenance.
• ⚛️ Both food and the human body are made of matter (atoms) and contain stored energy in the bonds between those atoms.
• 🦠 The digestive system converts food into usable forms for the body through six main steps: ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.
• 🧬 Enzymes play a crucial role in digestion by breaking down macromolecules (like lipids, carbohydrates, proteins, and nucleic acids) into smaller monomers (fatty acids, sugars, amino acids, and nucleotides) that the body can use.
• 🚶 Your body’s cells use the absorbed nutrients to generate energy, build tissues, and maintain bodily functions, adapting to current needs.
• 🍽️ The gastrointestinal (GI) tract is a hollow tube that runs from the mouth to the anus, allowing for the movement, digestion, and absorption of food.
• 🧱 The digestive tract consists of various tissue layers, including mucosal, submucosal, and muscularis externa, each playing a role in food processing and nutrient absorption.
• 🧪 Mechanical digestion physically breaks down food, while chemical digestion uses enzymes to further decompose food into basic molecules.
• 🚽 After nutrient absorption, indigestible materials are excreted from the body through defecation, completing the digestive process.
• 👩‍🔬 The digestive process requires cooperation between multiple organs and accessory organs, such as the liver, pancreas, and gallbladder, which assist with enzyme production and digestion.

Q & A

Why do humans eat food like nachos?

Humans eat food to obtain energy for staying alive and to acquire the raw materials required to build tissues. Food provides both matter and energy, which are essential for life.

What are the two main things that both humans and food have in common?

Both humans and food are made of matter, which is composed of atoms, and both contain energy stored in the bonds between these atoms.

How does the body convert food like nachos into usable energy and raw materials?

The digestive system breaks down food into smaller molecules that cells can absorb and use. The process involves mechanical and chemical digestion, with enzymes breaking down carbohydrates, proteins, and fats into monomers like sugars, amino acids, and fatty acids.

What are the four main types of biological molecules found in food?

The four main types of biological molecules in food are lipids, carbohydrates, proteins, and nucleic acids.

Why does the digestive system need to break down food into its smallest components?

Cells can only absorb and use the smallest building blocks, or monomers, from food. Polymers like carbohydrates and proteins must be broken down into sugars and amino acids so the body can use them for energy and tissue building.

What role do enzymes play in digestion?

Enzymes are proteins that act as catalysts to speed up chemical reactions. In digestion, enzymes break down large biological molecules into smaller ones, such as breaking down carbohydrates into sugars and proteins into amino acids.

What happens to food once it enters the mouth?

Food is broken down mechanically by chewing, and chemically by enzymes in saliva. This begins the process of breaking down the food into smaller, more usable forms.

What is peristalsis and why is it important?

Peristalsis is the involuntary contraction and relaxation of the smooth muscles in the digestive organs, which moves food through the alimentary canal. It ensures that food is pushed along the digestive tract, even if someone is upside down.

What are the six main steps of human digestion?

The six steps are ingestion (eating), propulsion (moving food through the digestive tract), mechanical digestion (breaking food down physically), chemical digestion (breaking food down with enzymes), absorption (cells absorbing nutrients), and defecation (removal of indigestible substances).

How does the body protect itself from digesting its own tissues during digestion?

The inner lining of the digestive tract is protected by mucus, which is secreted by columnar epithelial cells. This mucus lubricates the tract and protects the tissue from being digested by the body’s own enzymes.

Outlines

00:00

🥨 Why We Eat Food (And Nachos) in the First Place

This paragraph explores the reasons why we consume food, specifically nachos. It explains that we eat food to gain energy and raw materials for our body's tissues. Both humans and food are made of matter and contain energy in the bonds between atoms. The body processes food to access these materials and energy through digestion, with nachos serving as a practical example. The discussion includes the steps our digestive system takes to break down food into usable substances, emphasizing how digestion converts biological matter into energy and essential nutrients.

05:03

🍕 The Role of the Digestive Tract and Tissue Layers

Here, the focus shifts to the anatomy of the digestive tract, particularly the types of epithelial tissues that line it, from the mouth to the anus. The paragraph explains how different tissues (stratified squamous and columnar epithelial cells) serve specific purposes in protecting and aiding digestion. Additionally, the structure of the GI tract is described in layers: the mucosal, submucosal, and muscularis externa layers, each playing a role in moving food and supporting digestion. The role of accessory organs like the liver, pancreas, and gallbladder in secreting enzymes is also introduced.

10:04

🎥 Support for Crash Course and Team Acknowledgments

This paragraph is a closing note thanking supporters and contributors of the Crash Course series. It acknowledges Patreon patrons who help make the videos possible and extends gratitude to specific individuals who sponsored the episode. The production team, including writers, editors, and consultants, is also credited for their work on the episode.

Keywords

💡Energy

Energy refers to the stored chemical potential in food that the body uses for daily activities and maintaining life. In the video, it is explained that both food and the human body contain energy stored in chemical bonds, which is essential for performing physical tasks, such as walking or running. The nachos serve as an example of how food provides the energy required to sustain the body.

💡Macromolecules

Macromolecules are large, complex molecules found in food, including carbohydrates, proteins, lipids, and nucleic acids. The digestive system breaks these down into smaller units to make them usable by the body. In the script, nachos are composed of these biological macromolecules, which are reduced to monomers like amino acids and fatty acids that the body can absorb.

💡Digestion

Digestion is the process by which food is broken down both mechanically and chemically into smaller, absorbable components. This is a central theme of the video, with digestion described as a six-step process involving the deconstruction of food (like nachos) into its simplest molecules to allow the body to use it for energy and tissue building.

💡Monomers

Monomers are the small, basic building blocks of macromolecules. These include fatty acids, sugars, amino acids, and nucleotides. In the video, digestion is explained as a process of breaking down macromolecules in food into monomers, which cells can use for energy or tissue construction. For example, fats are broken down into fatty acids, which the body can use.

💡Enzymes

Enzymes are proteins that speed up chemical reactions, playing a crucial role in breaking down food into absorbable molecules. The video explains how enzymes are secreted by digestive organs like the liver and pancreas, facilitating the chemical digestion of food components such as carbohydrates and proteins. They help convert food into usable monomers.

💡Alimentary Canal

The alimentary canal, also known as the gastrointestinal (GI) tract, is a continuous tube that runs from the mouth to the anus, through which food passes during digestion. The video compares the human body to a hollow donut, with the alimentary canal being the tunnel through which food is broken down and nutrients are absorbed. Key organs like the stomach and intestines are part of this canal.

💡Peristalsis

Peristalsis refers to the wave-like muscle contractions in the digestive tract that move food from one part of the alimentary canal to another. In the video, peristalsis is highlighted as the mechanism that pushes food through the digestive system, even if someone is eating upside down, demonstrating its power and importance in the digestion process.

💡Chemical Digestion

Chemical digestion is the breakdown of food by enzymes into smaller, absorbable components. The video explains that after mechanical digestion, chemical digestion takes over to further break down food like nachos into its most basic building blocks, such as amino acids and sugars, which the body can then absorb.

💡Absorption

Absorption is the process through which nutrients from digested food pass from the digestive system into the bloodstream. In the video, absorption is described as the ultimate goal of digestion, allowing the body to use the nutrients and energy from food to build tissues or perform physical activities like walking a mile after eating nachos.

💡Accessory Digestive Organs

Accessory digestive organs are organs that aid digestion but are not part of the alimentary canal. These include the liver, pancreas, gallbladder, and salivary glands. In the video, these organs are described as the 'pit crew' that secrete enzymes and other substances to help break down food and make digestion more efficient.

Highlights

We eat food to obtain energy and raw materials needed to stay alive and build tissues.

Both you and food contain energy stored in the bonds between atoms, which is necessary for life.

Food needs to be converted into forms that our cells can work with at a microscopic level.

Digestion occurs in six main steps, breaking down food mechanically and chemically.

Enzymes, which speed up chemical reactions, play a major role in breaking down food during digestion.

Food contains macromolecules—carbohydrates, proteins, fats, and nucleic acids—each with caloric value.

Digestion breaks down macromolecules into their smaller components (monomers) like sugars, fatty acids, and amino acids.

Different parts of the digestive system work to reduce food into its simplest forms, starting from the mouth.

The process involves both mechanical pulverization and chemical enzymatic breakdown to increase surface area for digestion.

Peristalsis, a series of muscle contractions, moves food through the digestive tract, even when upside down.

The liver, pancreas, and gallbladder secrete enzymes into the digestive system to aid in breaking down food.

Nutrients are absorbed through the small intestine, using active and passive transport mechanisms.

Indigestible substances are removed from the body through defecation, completing the digestive process.

The entire digestive system works cooperatively across multiple organs to break down food and absorb nutrients.

Digestion is not only mechanical but highly dependent on chemical interactions facilitated by enzymes from various organs.

Transcripts

We all have our reasons for eating nachos at 3 in the afternoon.

I happen to have my own. And don’t ask -- it’s personal.

But more generally, we all eat any kind of food to accomplish two simple things: to obtain the energy

we need to stay alive and to get the raw materials required for building all of our tissues and stuff.

That’s because, when it comes down to it, both you and the food you eat contain those two same things:

Both you and food are made of “stuff” -- by which I mean, matter, made of certain

kinds of atoms -- and both you and food have energy stored in the bonds between those atoms.

So all living things need to take in stuff and energy, and convert it into slightly different stuff and energy.

And you can get some of the things you need pretty easily. Like, in order to get oxygen

for respiration, to unleash the chemical energy in your food, you just have to inhale.

But you can’t just breathe in the stuff you need to build DNA, or actin, or a phospholipid bilayer.

So, how does your body really acquire “stuff”?

That’s where the nachos come in.

This cheesy, crunchy dish is made of all different kinds of biological matter -- like carbohydrates

and fat and protein -- and it contains a certain, probably shocking, amount of calories, which

is how we measure energy stored in the chemical bonds in food.

So if I take, like, a 100-calorie bite of nachos -- which probably with this much cheese

wouldn’t even be a very big bite -- I can convert the chemical energy stored in those

carbohydrates and proteins and fats to feed my muscle and heart cells and maybe, like,

walk a mile -- an activity that happens to use about 100 calories.

But I can’t just swallow the nachos and watch the lump of them travel straight to my heart or leg muscles.

In order to actually use this food, I have to convert the biological matter into something

my body can work with on the cellular level, which as you know, is pretty darn tiny.

And, the work of converting the stuff in food, into the stuff that’s in my body, is done by my digestive system.

Human digestion occurs in six main steps -- some of which you are intimately familiar with. Others less so.

But every step of the way, your body is working to reduce all the different kinds of molecules

in food into their tiniest and most basic forms.

The first step? Is, uh, probably everybody’s favorite.

When it comes to what your digestive system ultimately does, just think of it as a sort of disassembly line.

You could have an order of nachos with The Works -- I’m talking beef and onions and

sour cream and slices of jalapeño -- and your digestive system will deconstruct it,

both mechanically and chemically, one step at a time.

It’s gotta do this because your cells work best with materials that are in their most basic form.

Your digestive system reduces food to that level in two main ways: by physically smashing

it to smithereens, and by bathing them, as much as it can, in enzymes.

Enzymes are proteins that living things use as catalysts, to speed up chemical reactions.

When used in digestion, enzymes break down the large molecules in your food into the

building blocks that your cells can actually absorb.

Those large molecules are called biological molecules -- also known as macromolecules

-- and everything that you eat, I hope, is at least partially made of them.

And there are four main kinds: you got the lipids, the carbohydrates, the proteins, and the nucleic acids.

Each possesses its own density of chemical potential energy, or caloric value, like for

example, 1 gram of carbohydrate contains about 4 calories, while a gram of fat contains about 9 calories.

But many of these biological molecules are polymers -- or sequences of smaller molecules

-- and your cells aren’t really equipped to take them up whole.

What your body trafficks in are those polymers’ individual components -- called monomers -- and

there are four main kinds of those, too: fatty acids, sugars, amino acids, and nucleotides.

The simple idea behind the whole digestive system is to break down the polymers of macromolecules

in your food, into the smaller monomers that your cells can use to build their own polymers,

while also getting the energy they need.

And, what your body needs to build at any given moment is always changing.

Maybe you need new fat stores so you can have energy to run a marathon, or new actin and

myosin to build bigger muscles, or more DNA so you can replace the skin cells you scraped

off your knee when you fell, or more enzymes so you can digest more food to get more building materials.

To meet your body’s constant, and constantly shifting demands, your digestive system requires a lot of organs

that perform a lot of specific tasks to break down and absorb the right nutrient at the right time.

Now, I’m quite sure that you’re familiar with the key players here -- they’re the

hollow organs that form the continuous tube that is your alimentary canal, aka the gastrointestinal

tract, which runs from your mouth to your anus.

It’s worth pointing out that these organs are hollow, because you are basically hollow, too.

Your digestive tract is really just one unbroken, insulated tunnel of outside that just happens

to run through your body, and is open at both ends. You’re a donut.

So the layer of stratified squamous and columnar epithelial cells that line your tract is actually

a barrier between the outside world and your inside world -- but it’s a barrier that

allows for the selective movement of materials between them.

It’s these hollow organs that do the actual moving, digesting, and absorbing of food,

and they include your mouth, pharynx, esophagus, stomach, and small and large intestines.

In your mouth, in your esophagus, and at the other end of things, at your anus, you have

stratified squamous epithelial tissue, just like your epidermis, to help resist the abrasive

action of like, chewing, like corn chips, maybe.

From your stomach on down, though, the inner GI tract is lined with simple columnar epithelial

cells, which secrete all sorts of stuff, and which absorb and process various nutrients.

Most of those columnar cells secrete mucus, which lubricates everything, and protects

your cells from being digested by your own digestive enzymes.

So, the innermost epithelial layer of the tube is known as the mucosal layer, and it

contains some connective tissue as well, which supplies it with blood.

Surrounding the mucosal layer is the submucosal layer, made of loose areolar connective tissue,

which helps provide the elasticity that the tube needs when you eat a whole pizza in one

sitting, and it contains more blood vessels.

And outside that, you have the muscularis externa layer, which as you might guess, is

where you find the muscles responsible for moving food through your tube.

Beyond these layers, the GI tract gets tons of support from the accessory digestive organs,

like your teeth, and your tongue, your gallbladder, salivary glands, liver, and pancreas.

They’re kind of like a pit crew, and they mostly help by secreting various enzymes that

help take apart food as it comes down the tube.

Together, these two groups on the digestive disassembly line work in six steps to destroy

your food and release and recycle its nutrients.

First, of course, you’ve got to introduce the food to your digestive system. What you

know as eating, or ingestion, is basically just creating a bulk flow of nutrients from

the outside world into your tissues.

This is where the work of disassembly begins: In your face-hole, which scientists call your mouth.

Now, we’re going to get to the details of what happens here another time, but remember

that food disassembly is both mechanical and chemical: So your teeth pulverize the bite

of nacho or whatever, while your salivary glands begin that food’s hours-long enzyme bath.

But the food, at this point, is not nearly “micro” enough to be of any use to your

cells, so you have to move that mush further down your tube.

This stage is called propulsion, and its initial mechanism is swallowing -- which, as you know,

is a voluntary action -- but then it’s quickly turned over to the involuntary process of peristalsis.

In peristalsis, the smooth muscles of the walls of your digestive organs take turns

contracting and relaxing to squeeze food through the lumen, or cavity, of your alimentary tract.

Waves of peristalsis continue through the esophagus, stomach, and intestines, and they’re

so strong that even if you were hanging upside down while eating your lunch and drinking

your tea, the food would still soldier on, fighting gravity, and eventually make it to its final destination.

Don’t do that, though. There’s other reasons why you shouldn’t be upside down.

Anyway, all of this shipping and handling mechanically breaks down the food even more,

and even after it goes through the stomach and its gastric acid, the mechanical work

still continues once it reaches your small intestine, as more smooth muscle segments

push the food back and forth to keep crumbling it up.

The goal of all this pulverization is to increase the surface area of that bite of food by breaking it down

into increasingly tiny pieces, to prepare it to encounter more enzymes in step four: chemical digestion.

Really, the actual process of digestion only occurs when the main action becomes

more chemical than mechanical.

And here, the accessory digestive organs -- namely, the liver, pancreas and gallbladder -- secrete

enzymes into the alimentary canal, where they ambush the mush and break it down into its

most basic chemical building blocks.

Like I said before, our cells prefer to do business in the really basic currency of monomers,

like amino acids, fatty acids, and simple sugars. And digestion allows for the absorption of

those nutrients as they pass from the small intestine into the blood, by both active and passive transport.

Once those nutrients are absorbed by your cells, you can finally use the energy inside

of them or use them to build new tissues.

The absorption of the nutrients is the goal of the entire process.

But, of course, it is not the end of it.

Once your body has sucked out all the nutrients it wants, indigestible substances like fiber

are escorted out of your body.

Yeah, I’m talking about pooping, or defecation.

And that is the end of the digestive line -- unless you are a capybara, or one of the

other animals who make sure that they get the most out of their lunch, by giving the

whole process another round and practicing coprophagia, aka eating their own poop.

Now, you should notice here that some of the processes of digestion occur in just one place,

and are the job of a single organ -- like hopefully you’re only ingesting through

your mouth and eliminating from the large intestine.

But most of these six steps require cooperation among multiple organs.

For example, both mechanical and chemical digestion start in the mouth, and continue

through the stomach and small intestines. And some chemical breakdown continues in the

large intestine, thanks to our little bacterial farm there.

Over the next couple of weeks we’re going to take you and your nachos on a stroll through

your digestive system and see who’s doing what, where, how, and why.

But for now, I’ve got some nachos to finish, so I gotta go.

And eating those nachos, as you learned today, will provide me with energy and raw materials,

by first ingesting something nutritious, propelling it through my alimentary canal where it will

be mechanically broken down, and chemically digested by enzymes until my cells can absorb

their monomers and use them to make whatever they need. And eventually, there will be pooping.

Thanks to all of our Patreon patrons who help make Crash Course possible through their monthly

contributions. And if you like Crash Course and want to help us keep making videos like

this one, you can go to patreon.com/crashcourse. Also, a big thank you to Peter Rapp, Sigmund

Leirvåg, Mikael Modin, and Jeremy Bradley for co-sponsoring this episode of Crash Course

Anatomy and Physiology.

This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written

by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson.

It was directed by Nicholas Jenkins, edited by Nicole Sweeney; our sound designer is Michael

Aranda, and the Graphics team is Thought Cafe.