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.