Hank: We all have our reasons for eating nachos at three 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 come 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. (Gasp)

But you can't just breathe in the stuff you need to build DNA, or actin, or a phospholipid bi-layer. 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 hundred calorie bite of nachos, it's probably with this much cheese, wouldn't 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 a hundred 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 work of converting the stuff in food into the stuff 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. (crunch)

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When it comes to what your digestive system ultimately does, just think of it of sort of a dis-assembly line. You could have an order of nachos, with the works. We're talking beef, and onions, sour cream and slices of jalapeno and your digestive system will deconstruct it, both mechanically and chemically one step at a time.

It's gotta do this 'cause your cells work best with materials 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 macro molecules, and everything that you eat, I hope, is at least partially made of them.

And there are four main kinds. You've got the lipids, the carbohydrates, the proteins, and the nucleic acids. Each possesses its own density of chemical energy, or caloric value. Like, for example, one gram of carbohydrate contains about four calories, while a gram of fat contains about nine 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 polymer's 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 macro molecules 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 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 doughnut.

So the layer of stratified squamous and columnar epithelial cells that line your tract is actually a barrier between your 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, in 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 these columnar cells secrete mucus, which lubricates everything and protects your cells from being digested by your own digestive enzymes. So the inner most 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 musclaris externa layer, which as you might guess, you find the muscles responsible for moving the food through your tube. 

Beyond these layers, the GI tract gets tons of support from the accessory digestive organ, like your teeth, and your tongue, and gall bladder, salivary glands, liver and pancreas. They're kind of like a pit crew and mostly help by secreting various enzymes that help take apart food as it comes down the tube.

Together, these two groups on the digestive dis-assembly 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 dis-assembly begins, in your face hole, which scientists call your mouth.

Now, we're going to get into the details of what happens here another time, but remember that food dis-assembly is both mechanical and chemical. So, your teeth pulverize the bite of nacho or whatever, while your salivary glands begins that food's hours long enzyme bath. 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 very quickly turned over to the involuntary action of peristalsis. In peristalsis, the smooth muscles of the walls of your digestive organs take turns contracting and relaxing to squeeze food throughout the lumen, or cavity of your alimentary tract.

Waves of peristalsis continue through the esophagus, the stomach, and the 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 are 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 small muscle segments push the food back and forth to keep it crumbling 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. In here, the accessory digestive organs, namely the liver, pancreas and gall bladder, secrete enzymes into the alimentary canal, where they ambush the mush, and break it down into its most 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, and digestion allows for the absorption of those nutrients as they pass from the small intestine, into the blood, by both passive and active 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 practising 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 your 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 the small intestines. And some chemical breakdown continues in the large intestines, thanks to our little bacterial farm down 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 whose doing what, where, how, and why. But for now, I've got some nachos to finish, so I've got to go.

And eating those nachos will provide me, as you learned today, 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, Leirvåg, Mikael Modin, and Jeremy Bradley for co-sponsoring this episode of Crash Course: Anatomy and Physiology. 

This episode was filmed in the Dr. 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 Café.

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