One of the coolest and most important things
that our bodies do is maintain this thing called homeostasis,
the regulation of a stable internal environment,
no matter where we are or what we're doing.
After all, we put our bodies through a lot every single day:
We're always adding food and liquid and chemicals,
and we're constantly changing temperature
and our levels of activity,
but our bodies can roll with it.
It's like, no big deal for them.
All of our organ systems have some hand in maintaining homeostasis.
I mean, it's basically the thing that makes us not dead.
But the excretory system, aka the urinary system,
which includes the kidneys, the ureters, the bladder,
and the urethra, is the star quarterback of the homeostasis team
That's because your excretory system
is responsible for maintaining the right levels of water
and dissolved substances in your body.
This is called osmoregulation, and it's how our bodies
get rid of the stuff we don't need, like the byproducts
of metabolizing food, while also making sure we don't get dehydrated.
It's the body's greatest balancing act, and your body is doing
it right now, and all the time, as long as you're not dead.
As with other organ systems we've talked about,
not all excretory systems in the animal kingdom are created equal.
Different animals excrete waste different ways
based on their evolutionary history what environments they live in,
and what their hobbies and interests are.
These factors all influence how an animal regulates water,
and most metabolic waste needs to be dissolved
in water in order to be excreted.
The problem is, a main byproduct of metabolizing food is ammonia,
which comes from breaking down proteins, and it's pretty toxic.
So, depending on how much water is available to an animal
and how easy it is for the animal to lug a bunch of water
around inside it, animals convert this ammonia
into either urea or uric acid.
Mammals like us, as well as amphibians, and some marine animals
like sharks and sea turtles, convert ammonia into urea,
a compound made from combining ammonia and carbon dioxide,
in their livers.
The advantage of urea is its very low toxicity.
It can hang out in your circulatory systems for a while
with no ill effects.
But you have to have some extra water available
to dissolve it and get rid of it.
This isn't such a tall order, really,
I mean peeing isn't a huge inconvenience, I mean, is it?
It's not for me anyways.
Well, it would be, though,
if you a bird or an insect or a lizard livings in the desert.
Animals that have to be light enough to fly
or don't have a bunch of spare water hanging around,
convert ammonia into uric acid, which can be excreted
as a kind of paste, so not a lot of water is needed.
You've seen bird poop.
If you haven't taken a close look, next time, do that.
Just look.
The white stuff in the bird droppings is actually
the uric acid-y pee and the brown stuff is the poop.
So, now that we've established what is and what is not bird poop,
let's get down to the brass tacks of how humans
get all of this urea out of our blood and into our toilets.
The excretory system starts with the kidneys,
the organs that do all the heavy lifting,
from maintaining those levels of water and dissolved materials
in our bodies to controlling our blood pressure.
And even though they do an amazing job,
I'm not bad-mouthing your kidneys here,
the way that they do it is frankly
a little bit janky and inefficient.
They start out by filtering out a bunch of fluid
and the stuff dissolved in the fluid out of your blood,
and then they basically re-absorb 99% of it back
before sending that 1% on its way in the form of urine.
Seriously, 99% gets re-absorbed.
On an average day, your kidneys filter out about 180 liters
of fluid from your blood, but only 1.5 liters of that
ends up getting peed out.
So most of your excretory system isn't dedicated to excreting
it's dedicated to re-absorbing.
But the system works, obviously, I'm still alive.
So we can't argue with that.
Now it is time to get into the nitty gritty details
of how your kidneys do all this, and it's pretty cool.
But there's lots of weird words. So get ready.
Your kidneys do all this work using a network
of tiny filtering structures called nephrons.
Each one of your mango-sized kidneys has about a million of them
If you were, don't do this, but if you were to unravel
all of your nephrons and put them end to end,
they would stretch over 80 kilometers.
This is where all the crazy action happens,
so to understand how they work, we're just going to follow
the flow, from your heart to the toilet.
Blood from the heart enters the kidneys through renal arteries,
and just so you know, whenever you hear the word "renal"
it means you're dealing with kidney stuff.
As the blood enters, it's forced into a system of tiny capillaries
until it enters a tangle of porous capillaries called the glomerulus.
This is the starting point for a single nephron.
The pressure in the glomerulus is high enough
that it squeezes some of the fluid out of the blood,
about 20% of it, and into a cup-like
sac called the Bowman's capsule.
The stuff that's squeezed out is no longer blood,
it is now called filtrate.
It's made up of water, urea, some smaller ions and molecules
like sodium, glucose and amino acids.
The bigger stuff in your blood, like the red blood cells
and the larger proteins, they don't get filtered.
Now the filtrate is ready to be processed.
From the Bowman's capsule, it flows into a twisted tube
called the proximal convoluted tubule,
which means "the tube near the beginning and that is all wind-y."
WHY ARE WE SO BAD AT NAMING THINGS?!
Anyways, this is the first of two convoluted tubules in the nephron.
And these, along with other tubules we're talking about,
are where the osmoregulation takes place.
With all kinds of tricked out, specialized pumps
and other kinds of active and passive transport,
they re-absorb water and dissolved materials
to create whatever balance your body needs at the time.
In the proximal tubule, it's mainly organic solutes
in the filtrate that are reabsorbed like glucose, and amino acids,
and other important stuff that you want to hang on to.
But it also helps to re-capture some sodium, potassium
and water we're going to want later.
From here, the filtrate enters the Loop of Henle,
which is a long, hairpin-shaped tubule that passes through
the two main layers of the kidney.
The outermost layer is the renal cortex,
that's where the glomerulus, bowman's capsule,
and both convoluted tubules are, and the layer beneath
that is the renal medulla, which is the center of the kidney.
"Cortex," by the way, is Latin for tree bark,
so whenever you see it in biology, you know that it's
the outside of something.
"Medulla," on the other hand, meaning narrow or pith,
so you know that it's the inside.
Just to help you remember this stuff.
But, before we take a tour of this amazing loop
I have to do a couple of things.
First, go pee.
Because this is...you know.
And second, a Biolo-graphy!
So I'll be right back!
The Loop of Henle was discovered by 19th century German physician
and anatomist Friedrich Gustav Jakob Henle.
I'm pretty sure he was one of those guys that you can't gross out
since he spent most of his career dissecting kidneys,
eyeballs, and brains.
And also seemed to be a huge fan of mucus and pus.
He was by far the most important anatomist of his time.
His three-volume Handbook of Systematic Human Anatomy
was recognized as the definitive anatomy textbook
of its day and was famous for its exquisite attention
to detail and its intricate, even beautiful, illustrations.
Not only did Henle discover the Loop of Henle,
arguably the linchpin of kidney function in mammals,
he was an early adopter of the wildly unpopular
germ theory of disease.
His student Robert Koch is considered one of the founders
of microbiology, and the two worked together
to formulate the Henle-Koch Postulates,
which today remain the four conditions that must be met
to establish a causal relationship between a microbe and a disease.
Henle taught the world so much about the human body that there are,
right now, in you, no fewer than 9 features that bear his name.
From the Henle's fissures between the muscle fibers
of your heart to the Crypts of Henle,
which are microscopic pockets in the whites of your eyes.
Also the name of my Cradle of Filth cover band.
Alright, so, review time.
We've squeezed some filtrate out of the blood, and re-absorbed
some of the important organic molecules we want to keep.
But most of the re-absorption action happens here,
in the Loop of Henle, which does three really important things.
One, it extracts most of the water that we need from
the filtrate as it travels down to the medulla.
Two, it pumps out the salts that we want to keep
on the way back up to the cortex.
And three, in the process of doing all that,
it makes the medulla hypertonic, or super salty relative
to the filtrate.
Creating a concentration gradient that will allow the medulla
to draw out even more water one last time from the filtrate,
before the final journey to the toilet begins.
It's complicated and, again, kinda janky,
but it's what allows us mammals to create urine
that's as concentrated as necessary, using only
the amount of water that our bodies can spare at the time.
So first, filtrate starts going down the loop, and the thing
to know here is that the membrane is highly permeable to water,
not so much to salt or anything else, mainly water.
Now, compared to the filtrate, the tissue of the medulla
is already pretty salty.
And as the filtrate processes,
the surrounding tissue becomes increasingly hypertonic
the farther down you go, the saltier it gets.
So, applying everything we've learned about osmosis,
you know that as the filtrate moves along,
it loses more and more water through the membrane.
By the time the filtrate gets to the bottom of the Loop,
it's highly concentrated.
Now the filtrate enters the ascending end of the Loop,
and here it's basically the same but in reverse.
The membrane is NOT permeable to water, and instead it's lined with
channels that transport ions like sodium, potassium and chlorine.
And because the filtrate is so concentrated now, it's actually
hypertonic compared to the fluid outside in the medulla.
So as it ascends, huge amounts of salts start flowing out of the
filtrate, which makes the renal medulla really,
really, really salty.
This salty medulla also creates a concentration gradient
between the medulla and the filtrate which we're going to need
in the final step of pee-making.
But first! Once the filtrate is back up in the cortex
and out of the loop, it enters the second of our convoluted tubules,
called the distal convoluted tubule, or "farther-away curly tube."
While the first tubule worked mostly on reabsorbing
the organic compounds in the filtrate,
here the focus is on regulating levels of potassium,
sodium, and calcium.
This work is mainly done by pumps and hormones
that regulate the reabsorption process.
By the time it's done, we've finally taken everything
we want to keep out of the filtrate, so now it's mainly
just excess water, urea and other metabolic waste.
This stuff all gets dumped into collecting ducts
that channel it back down to the center of the kidney, the medulla.
And remember, the medulla is super-salty, right?
Now more hormones kick in that tell the collecting ducts
how porous to make their membranes.
If the membranes are made very porous,
more water is absorbed into the medulla, which makes the urine
yes, we can start calling it urine now
even more concentrated.
And here's a fun fact: If you've ever had one drink too many,
you might've noticed that you start to pee a lot,
and your pee is clear.
That's because alcohol interferes with these hormones
especially one called anti-diuretic hormone which tells
the collecting ducts to be very porous so that you
reabsorb most of the water.
With those hormones all confused and out of commission,
you just starting peeing out all kinds of water,
which also means you're getting dehydrated,
which means you're officially on a one-way trip to Hangover City.
So, now you know why that happens.
Now at this point, the urine leaves both kidneys
and flows down to the urinary bladder by tubes called ureters.
Once in the bladder, the urine just sits around,
waiting for us to decide when it's time to find a bathroom.
And when that time comes, a little sphincter muscle relaxes
and releases the urine from the bladder into a tube
called the urethra, which empties out wherever you point it.
So that's how your excretory system works!
And that's basically how it works for most mammals,
although some modifications are made based on, again,
where they live and what they do.
For instance, kangaroo rats, which are tiny and adorable
and live in the desert, have the most concentrated urine
of any animal anywhere, because it can't spare the water.
So it has a very, very long Loop of Henle that reabsorbs
most of the water from the filtrate.
On the other end of the spectrum, we have the beavers,
who have very short Loops of Henle, because they're like,
"Water reabsorption, schmater reabschmorption.
Do you see what I do all day?"
And so now you know the true origins of pee.
Thank you for coming to learn with us here at Crash Course Biology.
We hope that you learned something.
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Goodbye.