Ok point #1.  So, there’s this bacteria called C. diff.

If it takes over too much of someone’s colon,   it can cause fever, diarrhea, and cramps. Not a great time.  But there are treatments,  including… fecal transplants.

Yep. You heard that right.  Doctors take feces from a patient with a healthy  population of gut microbes, mix it with water, and   spray it onto the walls of a sick person’s colon. All in the name of introducing some   much-needed bacteria.

And sure, I know what   you’re thinking: “Dr. Sammy! Why???” But there’s a larger point here.

Point #2: When we think   about animals getting resources and getting  rid of waste, we often just think about food.  We consume things that provide nutrients on one  end and ditch what isn’t needed on the other.  But these systems are about more than that:  they’re about keeping a body in balance.  Which can include food, but also  things like gases and bacteria. The more we know about how this balancing  act works, the better we can maintain it,   and develop treatments to help  us when things get off-kilter.  Even if that means spraying a feces  solution into someone’s colon. Hi!

I’m Dr. Sammy,   your friendly neighborhood entomologist. I hope you’re not eating lunch right now,   because this is Crash Course Biology,  and today we’re talking about poo.  But you know what don’t stank?

This theme music. [THEME MUSIC]

All organisms need a way to   get resources and dispose of waste. Vertebrates — animals with backbones — do   this using the digestive system and the urinary  system — so, eating and going to the bathroom.  But there’s also a third, hugely important player:   the respiratory system, which acquires  oxygen and gets rid of carbon dioxide. Oxygen is one key ingredient in the  reactions that get our cells energy,   alongside the glucose we get  from eating and digesting food.  While animals like us can last without food and  water for days, without O2… we can’t go for long.  In fact, humans can rarely survive more  than ten minutes without breathing oxygen.

Virtually all animals need some kind  of respiratory system to breathe,   though how they look and operate varies. Like, instead of using lungs to breathe air,   many fish absorb oxygen from the  water around them through their gills. After oxygen is breathed in, it has to make  its way to cells where respiration takes place.  In animals with lungs, the  oxygen molecules automatically   move from where there’s a bunch of oxygen,  to where there isn’t much—the bloodstream.

This is a process called diffusion, and  it’s similar to how the smell of fresh   cookies starts in the kitchen and wafts  into the studio while you’re trying to   record a video… and… I… [Offline:  Sammy as cookie monster graphic] At the same time, carbon dioxide  is going the other way: There’s   more of it in the bloodstream, so it’s  diffusing from the blood to the lungs.  And when you breathe out… there it goes. This whole process is called gas exchange. Now, despite what it feels like  when you take a good, deep breath,   lungs aren’t a pair of balloons in your chest.  They’re more like sponges,  with lots of little openings.

You can think of gas exchange like a busy  grocery store checkout on a Saturday morning.  If there’s only one register open,  the line will get so long that   it’ll start wrapping around the store. But if there are ten registers open,   the lines will move much faster,  and more people can get through.  The same thing goes for lungs—more  entry points means faster gas exchange. Overall, it can be easy to crown  oxygen the MVP of this process.  But both parts — getting oxygen in and getting  carbon dioxide out — are equally important.  We need some CO2 in our blood,  but too much is toxic to cells.  And too much oxygen in the  blood can be poisonous, too.  So, with both gases — as with most  things — it’s all about balance.

While your lungs are working on oxygen   and CO2, another system is trying to  balance other stuff—sugars, proteins,   fats, vitamins and more. It’s the digestive system!  And there’s no better way to get to know it than  to follow the avocado toast I had for breakfast. This delicious morning meal is about to go  on a five-step journey through my digestive   tract, also known as my alimentary canal.

And I can already check off step one: ingestion.  I ate the avocado toast. Amazing.  Love it! Can’t say enough about this part.

Next, almost immediately, digestion begins. Digestion is breaking food into smaller bits,   either mechanically — like  by chewing — or chemically,   like with the enzymes in my saliva and stomach.  And while digestion happens in the mouth, it  also happens in the stomach and small intestine.  The microbes living in your small  intestine can help here too. Most vertebrates digest food the same way  we do, though there are a few exceptions.  Like, typically birds don’t have teeth,  so they swallow their food whole.  And then to break it down, the  bird swallows rocks and holds them   in part of its stomach called the gizzard.

The gizzard contracts and grinds those rocks   around with the bird’s food to break it apart. So… birds are basically living garbage disposals. And, as we’ve come to expect by now,   our friend the platypus does  things to the beat of its own drum.  Platypuses don’t have teeth or  stomachs because of course they don’t.  Food is ground up by plates in their  jaws before passing more or less directly   from their throat to their intestines.

Always have to one-up us, don't you pal? Now, whether you’re a bird,   a platy-anomaly, or a human, step three in  the digestive system is the same: absorption.  See, it’s not enough for my  avocado toast to have been   chewed and dissolved into vaguely-nutritious goop.  Those nutrients also have to  get absorbed into my body. In absorption, the broken-down  bits from digestion move from   the digestive tract to the bloodstream.

Sometimes this happens through diffusion,   like in the lungs. But sometimes,   your cells need molecules to move toward an  area where there’s already a lot of them.  This is called active transport  because it’s, well, active.  It costs energy rather than  happening automatically. It turns out most absorption  doesn’t happen in the stomach,   as you might imagine, but in the intestines.

Like with your lungs, your intestines have a   lot of open cash registers. They’re long and wrinkly,   with lots of folds and valleys that molecules  pass through to enter the bloodstream. So after my toast breaks down a bit, its  nutrients are absorbed by my intestines.  The carbohydrates and B vitamins in the whole wheat bread, and the  fat in the avocado mostly get absorbed in the   small intestine, along with a little water.

Then the large intestine absorbs a few more   nutrients and lots of water while  it gathers anything I can’t digest. Once those nutrients enter the bloodstream,   that blood heads over to the liver,  where the nutrients are processed.  Some of them are modified  and stored to be used later.  And if there are any toxic compounds, a lot  of those can be inactivated in the liver, too. Most vertebrates use a  system roughly like this one.  We might not all have teeth or gizzards, but we  all have small intestines that absorb nutrients.  Meanwhile, our invertebrate cousins  can be incredibly different.

Take a starfish, for example. It doesn’t have a lot of room   in its body for a huge, clam-filled stomach. So, after it breaks apart a clam and is ready   to chow down, it just vomits up its stomach.

The stomach shoots out of its mouth and into   the clamshell, where it releases some  juicy enzymes to break down the meal.  Once that clam is a pile of goo, those  nutrients get absorbed straight into   the stomach, and the whole organ gets  sucked back into the starfish’s body. In a way, it’s not all that  different from what humans do.  Our bodies digest food, too, and  those nutrients get absorbed.  Starfish just have a… unique way of doing it. Before we get to the final  stop on our digestive journey,   there’s been something happening to my toast  in the background this whole time: propulsion.  This is where food moves  through the digestive tract.

Propulsion includes swallowing, but  also peristalsis: the process where   muscles move food along the digestive tract. Ever wonder about the sound you hear when your   stomach is getting all gurgly? That’s peristalsis!

It’s so important that one of the  first big things NASA tested when   it sent people to space was whether or not  peristalsis works when you’re weightless.  And thankfully, it does! Peristalsis is so powerful   that the muscles in our digestive  systems work against gravity. Alright!  So, from ingestion to digestion to absorption,  we’ve officially followed my breakfast as it   was propelled through my digestive system.

Now it’s time for one last, glorious step,   the moment you’ve all been waiting for, the  pride of the digestive system: defecation!  Also known as getting rid of  undigested and unabsorbed food.  Or… pooping. The plops if you will. In humans, this begins in the large  intestine, which includes the colon.  The intestine forms a big blob of  water, undigested food, bacteria from   your digestive tract, and various other waste  products that aren’t handled by your urinary   system, which we’ll get to in a second.

And that blob is none other than feces,   which then hangs out in your rectum  until it’s time to make a grand exit. Like with anything, not all organisms  defecate the same way we do.  Take jellyfish for example. Many have only one main opening   in their bodies, so effectively,  they eat and poop with the same hole.  And one type of jellyfish has an anus  that only appears when it needs to go.  An Anus ex Machina, for my fellow drama nerds.  So yeah, in the animal kingdom, everybody  poops and there’s no wrong way to do it.

Last but not least, we have the   urinary system, which is the main  way animals get rid of waste.  The urinary system’s plumbing is made of  several organs, including the bladder, which   stores urine—the fluid that carries the waste  products— until it’s a convenient time to pee.  But the champions of this  system are really the kidneys. Kidneys filter blood. They use both diffusion   and active transport to remove waste products  from it, as well as things your body has too   much of, like sodium or water.

And the byproduct is urine! This means that when you pee, it’s  not because water passed through   your stomach and intestines and out the  other end, like a baby-wets-a-lot doll.  It’s because that water was absorbed  through your intestines into your blood,   and then was filtered out by your kidneys. And kidneys are amazing—they’re constantly  communicating with different parts of the body to   know how much of what stuff needs to be removed.

For example, kidneys release an enzyme called   renin that helps regulate blood pressure  and the amount of calcium in our bones. For living things, acquiring resources   and getting rid of waste is an endless balancing  act, whether it’s happening in the intestines,   the lungs, or the kidneys. And after billions of years,   animals in particular are pretty  amazing at this, all things considered.

Sure, you can disrupt your gut bacteria  with medications or a diet change.  But with every breath you take  and every piece of food you eat,   your body is doing its best to keep you balanced. Every breath you take, every  food you plate, probiotic shakes,   and tummy aches we're balancing your stool. Next time, we’ll learn more about how  animal bodies manage to stay organized,   and what stops us from being  a random mess of organs.  See you then.

Peace! This series was produced in  collaboration with HHMI BioInteractive.  If you’re an educator, visit  BioInteractive.org/CrashCourse for   classroom resources and professional development  related to the topics covered in this course. Thanks for watching this episode of Crash  Course Biology which was filmed at our studio   in Indianapolis, Indiana, and was made  with the help of all these nice people.  If you want to help keep Crash  Course free for everyone,   forever, you can join our community on Patreon.