The Excretory System: From Your Heart to the Toilet – CrashCourse Biology #29

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. You can go to and subscribe for more
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stuff that you didn't get.

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