Urinary System, part 1: Crash Course A&P #38

Even though you probably don't choose to spend a lot of time thinking about it, your pee is kind of a big deal. Today we're talking about the anatomy of your urinary system, and how your kidneys filter metabolic waste and balance salt and water concentrations in the blood. We'll cover how nephrons use glomerular filtration, tubular reabsorption, and tubular secretion to reabsorb water and nutrients back into the blood, and make urine with the leftovers.

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Table of Contents
Kidneys Filter Metabolic Waste & Balance Salt & Water Concentrations in the Blood 1:25
Nephrons 4:13
Glomerular Filtration 4:37
Tublar Reabsorption 5:14
Tubular Secretion 8:17
Urine 8:40

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We’ve been spending a lot of time lately talking
about eating, and digesting, and metabolizing food.

And those are some of my favorite things in
the world! It’s been a really great time.

But, as with all good parties, or brunch buffets,
in the end, we’re left with a mess.

And I’m not talking about spilled beer and
Dorito crumbs, I’m talking about toxic levels

of garbage that need to be cleaned up before
they kill you.

In your body, a lot of the cleanup that comes
after metabolism is handled by the liver, which

plays a tremendous role in directing dead cells and
leftover chemicals to the digestive and urinary systems.

But your liver can’t actually escort waste
out of your person.

Your lungs can lend a hand, exhaling carbon
dioxide, and of course your colon will eventually

poop out unusable stuff and old cell-parts.
But much of your chemical waste still needs

to be sorted and disposed of, so one system
steps in to bat clean-up.

And that, is your urinary system.

This system -- and specifically your kidneys
-- does all sorts of important homeostatic

stuff, like regulating your water volume,
ion salt concentrations, and pH levels, and

influencing your red blood cell production
and blood pressure.

But its main purpose -- what we’re going
to be focusing on for the next two lessons

-- is how it filters toxic leftovers from
your blood -- like the nitrogenous waste made

by metabolizing protein -- and ferries it
out of the body.

And — spoiler alert! — this all involves the
how, and the why, and the what of your pee.

Now you probably know that kidneys are filters,
and you may imagine them as sieves that strain

out the bad stuff, leaving it sitting like
a hairball at the bottom of the bathtub.

But that is, in fact, kind of the opposite
of what you should be thinking.

Most of what’s in your blood is totally
removed by the kidneys. Then your body pulls

back what it wants to hold onto, before the
rest is sent on a one-way trip to the bladder.

It’s kinda like this: you don’t clean out your fridge
by just taking out the rotten fruit and fuzzy leftovers.

Instead, you’ve got to take everything out,
and put it on the counter, and then sort through

what goes back in the fridge and what goes
in the trash.

That’s how your urinary system cleans you
up. And it is really good at its job.

So this morning I decided to go the healthy
route and instead of eating my normal breakfast

of nothing, I had a big 32-ounce protein smoothie.

My digestive system did its thing, and all
the protein was hydrolyzed into amino acids,

which were absorbed by my blood, and sent
all over my body to build and repair cells.

It’s a beautiful thing, but not without
consequence.

Because metabolizing nutrients -- especially
protein -- makes a mess.

You may remember that amino acids are unique,
in that they have nitrogen in their amine groups.

And because we can’t store amino acids,
extra ones get processed into storable carbs or fats.

But the amine group isn’t used in those
storage molecules, so it’s converted to

NH3, or ammonia, which happens to be toxic. So the

liver converts the ammonia into a less-toxic compound,
urea, which our kidneys filter out into our pee.

Once out of the body, urea can degrade back
into ammonia, which is why dirty, pee-soaked

toilets and cat litter boxes smell like ammonia.

Now this business of taking out the nitrogenous
trash is one of the urinary system’s biggest jobs.

Its other major duty is to regulate the balance
of salt and water in your blood, and both

of these tasks are processed in the whole
system of tubes that is your urinary system.

So let’s take a look at some basic pee-making
anatomy.

Your kidneys are a pair of dark red, fist-sized,
bean-shaped organs that sit on each side of

your spine against the posterior body wall.

Kidneys are retroperitoneal, which means they
lie between the dorsal wall and the peritoneum

-- the membrane that surrounds the abdominal
cavity -- rather than inside the cavity itself,

like your intestines and stomach do.

Each kidney has three distinct layers, beginning
with the outermost cortex.

Beneath that is the medulla,
a set of cone-shaped masses

of tissue that secrete urine into tiny sac-like tubules.

And finally, the innermost layer is the renal
pelvis, a funnel-shaped tube surrounded by

smooth muscle that uses peristalsis to move urine
out of the kidney, into the ureter, and into the bladder.

Because the kidneys’ main job is to filter
blood continuously, they end up seeing a lot of it.

In fact, at any given moment they hold over
20 percent of your total blood volume.

Oxygenated blood enters the kidneys through
the large renal arteries, which deliver nearly

a quarter of all blood pumped through the
heart every minute. That means your kidneys

filter about 120 to 140 liters of blood EVERY
DAY.

As they enter the kidneys, renal arteries branch many,
many times, ending in tons of little capillary groups.

So a kidney isn’t just one big filter; instead,
each one is made up of about a million twisty

microscopic filtering units called nephrons.

Structurally and functionally, nephrons are
where the real business of blood-processing

-- which, like, “pee-making” -- begins, in three steps:
filtration, reabsorption, and secretion.

Each nephron consists of a round renal corpuscle
that resides up in the cortex, followed by

a long and winding renal tubule that loops
around between the cortex and the medulla.

The outer part of the corpuscle is a cup-shaped
feature called the glomerular capsule, because

inside it there’s a whole tangle of capillaries
called the glomerulus -- that’s from the

Latin word for “ball of yarn,” which is
pretty much what it looks like.

And the endothelium of these capillaries is
very porous. So they allow lots of fluid,

waste products, ions, glucose, and amino acids
to pass from the blood into the capsule -- but

they block out bigger molecules like blood
cells and proteins, so they stay in the blood

and exit through the peritubular capillaries,
also known as the vasa recta.

Now, all the stuff that get squeezed out of
the blood into the glomerulus is called filtrate,

which is then sent along to the elaborately
twisting three-centimeter-long renal tubule.

Even though it looks like it’s just a tube,
it has three major parts, some of which are

permeable to certain substances, but not others.

First along is the proximal convoluted tubule,
or PCT, which is about as convoluted-looking

at its name suggests; then the tube drops
into a dramatic hairpin turn called the nephron

loop, or the loop of Henle -- I term I kinda
like better, personally -- and finally it

ends in the distal convoluted tubule or DCT,
which empties into a collecting duct.

All this twisting might make the tubule look,
like, super inefficient, but it actually serves

an important purpose, as you might expect.

Just like with your small intestines, the
long, curly shape of the nephron provides

more time and space for it to re-absorb whatever
useable stuff it can.

And this meandering path also allows the parts
of the tubule that are toward the end, to

have an affect on processes that take place
closer to the beginning, as they pass each other.

Because a lot of the stuff that winds up in
the tube are valuable commodities -- like

ions and glucose and water -- and we don’t
want to just pee all of them out if we can help it.

So, let’s trace the whole process, starting at the
top, with the proximal convoluted tubule or PCT.

The walls here are made of cuboidal epithelial
cells, with big ol’ mitochondria that make

ATP, to power pumps that pull lots of sodium
ions from the filtrate, using active transport.

These cells also are covered in microvilli
that increase their surface area and help

re-absorb much of the good stuff from the
filtrate and back into the blood.

The remaining filtrate passes from the PCT
into the loop of Henle, which starts in the

cortex, then dips into the medulla before
coming back into the cortex.

And the form of this loop is key to its function,
because its primary task is to drive the re-absorption

of water, by creating a salt concentration
gradient in the tissue of the medulla.

It does this mainly by actively pumping out
salts in the ascending limb. This creates

some very salty interstitial fluid in the
medulla, so when new filtrate comes down the

descending loop in front of it, water passively
flows out, and into the super salty interstitial space.

Since most of this water is picked up by the
blood pretty quickly, the saltiness of the

interstitial space doesn’t get diluted.
So it can keep drawing water out of the next

batch of filtrate in the descending limb.

Needless to say, this is super important,
because if we peed out all the water that

went into our kidneys, we would die of dehydration
really quick.

But even after all that, we are still only
two thirds of the way through the process.

As we move out of the loop of Henle, into
the distal convoluted tubule, and on to the

collecting duct, the remaining filtrate is
now officially urine. But there’s one more

component that we have to squeeze the most
out of before we excrete the stuff. Urea.

Even though we think of urea as a waste product
-- just one more part of that protein shake

that has to be dumped -- the kidneys actually
need it.

They use it to ramp up the concentration gradient
earlier in the process, making the medulla

even saltier for the filtrate that’s back
there going through the ascending limb.

So in the final steps, after the filtrate
leaves the DCT, it enters the collecting duct,

which runs back into the medulla. And while
the salt passively draws even more water out

of the collecting duct, some urea passively
leaves the urine as well.

Making the medulla even more salty -- and,
in turn, more effective at drawing out water

from the ascending limb a few steps back.

So there’s essentially a traveling pool
of urea that escapes the urine, finds its

way back into the loop of Henle, and then
runs the whole course again back to the collecting

duct -- an ammonia-scented cycle called urea
recycling.

Now all that’s left is a kind of last call
to selectively sneak out any extra waste -- like

hydrogen, potassium, and certain organic acids
and bases -- using active transport.

This is called tubular secretion, and it transports
only select kinds of waste that have already

made their way into the blood that’s in the
peritubular capillaries, ready to leave the kidneys.

This step is kind of like emptying your pockets
of any last wads of tissue or crumpled receipts

as you’re walking a bag of trash to the
curb.

And that’s how your kidneys clean up the
mess left over from the giant party that is

you metabolizing food. So if you thought that
your kidneys were just a kinda fine mesh that

filtered out bad stuff? Now you know that’s
not true.

If you thought your urinary system was basically
a matter of: Water goes in, pee goes out?

That’s DEFINITELY not true.

And if you thought we were done talking about
your urine, that is also not true, either,

because next time, we’re going to learn
how your body regulates what’s absorbed

and what’s excreted, and we’ll find out
can happen when that regulation goes awry.

But for now, you learned the anatomy of your
urinary system, and how your kidneys filter

metabolic waste and balance salt and water
concentrations in the blood. Specifically

you learned how nephrons use glomerular filtration,
tubular reabsorption, and tubular secretion

to reabsorb water and nutrients back into
the blood, and make urine with the leftovers.

Thank you to our Headmaster of Learning, Linnea
Boyev, and thank you to all of our Patreon

patrons whose monthly contributions help make
Crash Course possible, not only for themselves,

but for everyone. If you like Crash Course
and want to help us keep making videos like

this one, you can go to patreon.com/crashcourse.

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 and edited by Nicole Sweeney;
our sound designer is Michael Aranda, and

the Graphics team is Thought Cafe.