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

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.

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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.