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

HANK GREEN: 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 2 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 2 same things.

Both you and food are made of stuff, by which I mean matter made of certain atoms,

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

All living things need to take in stuff and energy

and convert it into slightly different stuff and energy.

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.

How does your body really acquire stuff?

That's where the nachos come in.

This cheesy, crunchy dish is made of

all different biological matter like carbohydrates and fat and protein.

It contains a certain, probably shocking, amount of calories,

which is how we measure energy stored in the chemical bonds in foods.

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 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,

it's pretty darn tiny.

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 6 main steps,

some of which you are intimately familiar with, others less so.

But every step of the way, your body is working to produce

all the different molecules in food into their tiniest and most basic forms.

The first step is probably everybody's favorite. [MUSIC]

When it comes to what your digestive system ultimately does,

just think of it as a 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 got to 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 2 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 macro molecules.

Everything that you eat,

I hope, is at least partially made of them.

There are 4 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.

For example, 1 gram of carbohydrate contains about 4 calories.

1 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 traffics in are those polymers'

individual components called monomers.

There are 4 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.

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 it's open at both ends.

You're a donut. 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.

They include your mouth, pharynx, esophagus,

stomach, and small and large intestines.

And your mouth and your esophagus and at the other end of things like your anus,

you have stratified squamous epithelial tissue just like

your epidermis to help resist the abrasive action of chewing,

like corn chips, maybe.

Your stomach on down, though—the GI tract—is lined with

simple columnar epithelial cells, which secrete

all sorts of stuff and which absorb and process various nutrients.

Most of these columnar cells secrete mucus, which lubricates everything and

protects yourselves from being digested by your own digestive enzymes.

The innermost epithelial layer of the tube is known as

the mucosal layer, and it contains some connective tissue as well,

which supplies 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.

Outside that, you have the muscularis external 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 are like a pit crew, and they mostly helped by secreting

various enzymes that help take apart food as it comes down the tube.

Together, these 2 groups on the digestive disassembly line work in 6 steps to

destroy your food and release and recycle its nutrient source.

First you got to introduce the food through 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 gonna get into the details of what happens here another time.

But remember that food disassembly is both mechanical and chemical.

Your teeth pulverized 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 much further down your tube.

This stage is called a propulsion.

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.

They're so strong that even if you are 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.

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 it crumbling up.

The goal of all this polarization 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.

Step 4: chemical digestion.

Really the actual process of digestion only occurs

when the main action becomes more chemical than mechanical.

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 basic currency of monomers like amino acids,

fatty acids, and simple sugars.

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 at once,

indigestible substances like fiber are escorted out of your body.

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 makes sure that they get the most

out of their lunch by giving the whole process

another round and practicing coprophagia,

a.k.a 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 6 steps require cooperation among multiple organs.

For example, both mechanical and chemical digestion

starts 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 got some nachos to finish, so I got to go.

Eating those nachos, as you learn 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.

Eventually, there will be pooping.

Thanks to all of our Patreon patrons who help make

Crash Course possible through their monthly contributions.

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 [INAUDIBLE],

Mikhail Moden, 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. [MUSIC]