Urinary System, Part 2: Crash Course Anatomy & Physiology #39

It has filled countless diapers,
caused discomfort for any number

of airline passengers,
and it totally ruined the Dude’s rug,

which really tied the room together, man.

Anatomists call it micturition,
and I don’t know why

because the rest of us call it urination,
which seems like a fine word.

All mammals and most animals
urinate to remove toxins

and to help maintain water-volume
homeostasis, or blood pressure.

And while some of us spray it around
to attract mates or mark territory,

or deter predators; as far as I know,
only humans actually study pee.

In fact, we’ve been doing it
for thousands of years.

Even before Hippocrates extolled
the diagnostic virtues of pee-sniffing,

early Sumerian and Babylonian physicians
were making urine-related observations.

Medieval doctors as well diagnosed
diseases based on on smelling,

inspecting, or even tasting urine samples.

And although they were often
totally off-base,

which makes me feel bad for those guys
who sipped urine for no reason;

they were kind of on to something.

Today, urological tests can help detect
a lot of ailments based on

the color, the smell, the clarity,
and chemical composition of a sample.

Freshly peed urine is usually about
95% water,

slightly acidic with a pH of around 6,
a little aromatic,

and usually somewhere
between clear and dark yellow in color

depending on your level of hydration.

Urine also contains over
3000 different chemical compounds,

and their varying levels of concentration
can tell us a lot about

what’s going on in the body.

For example,
if you give me a urine sample,

I will have no idea what to do with it.

But if you give it to a doctor
and they can see that

it’s cloudy with white blood cells,
that’s a good sign

you’ve got a urinary tract infection.

If it smells sweet
and contains a lot of glucose,

you might have diabetes.

If it looks pink,
then unless you’ve recently eaten beets,

you probably have
internal bleeding somewhere.

And if it is chocked full of proteins,
you could be pregnant,

or working out too hard,
or have high blood pressure,

or be headed to heart failure.

So as you can see, even if the most
thought you’ve given the subject is

to wonder if you should pee now
or wait until the end of the movie;

the whole process of producing, storing,
and eliminating pee

is no where near as simple as it may seem.

From osmotic pressure
to stretch receptors to hormones,

our circulatory, nervous,
and endocrine systems regulate

how much urine we produce,
what goes into it,

and when to get rid of it.

So join me
as we journey into the world of pee.

Wait, can we rephrase that?

[♪ Intro Music ♪]

All right, how about:
“let’s look at where your pee comes from.”

No, actually,
that doesn’t sound good either.

Let’s begin by looking at
what regulates the production of urine.

That works.

Last time, we discussed
how your kidneys filter your blood,

but the actual production of urine
can be affected

by a whole host of factors.

One thing that might have
crossed your mind last time

is that the production of urine
must by its very nature

be influenced by blood.

Specifically, its volume and its pressure.

Because, step one in pee-making
is the process of glomerular filtration

where blood is filtered
in the little blood-filled balls of yarn

that are the glomeruli.

So, just like water in a hose,
higher pressure in the blood

must push more plasma
out of the capillaries

and into the glomeruli.

But here’s a problem:
Your kidneys can only handle

so much filtrate at a time.

So they have to maintain
a constant rate of flow inside of them.

This is known as
the glomerular filtration rate,

or how much blood passes through
the glomeruli every minute.

And your kidneys have ways
of regulating this rate

despite changes in blood pressure.

If your blood pressure happens
to increase, for example,

the higher pressure causes the arterioles
leading to your glomeruli to stretch.

And then the smooth muscle
in the walls of the glomeruli

respond to this stretching stimulus
by constricting,

automatically reducing the amount
of blood flow into the glomeruli

and leaving the flow rate
relatively unchanged.

This kind of intrinsic control,
or autoregulation, is helpful

in controlling the filtration rate
through normal ranges of blood pressures.

But the kidneys mostly regulate
urine concentration

at the other end of the nephron tubules.

This kind of regulation
I’m sure you’re familiar with.

If you’ve ever had too much coffee
or gone on a bit of a bender,

you may have experienced the pleasure
of having to pee every five minutes.

That’s because your endocrine system
has a lot to say

about your bathroom breaks.

So you have some strong hormonal
mechanisms that affect

when and how often you go.

And as it happens,
both caffeine and alcohol inhibit

the release of one of these hormones
called antidiuretic hormone, or ADH;

which is secreted by
the posterior pituitary gland

to help the body retain water
and stay hydrated.

How ADH works is kind of complex,
but first let’s remember

that cell membranes are generally
not that permeable to water.

But in the parts of the nephron
that reabsorb water,

like the descending limb of the loop
of Henle,

water has to move easily through cells
from the filtrate to the blood.

This is possible because of
special protein channels

in their membranes called aquaporins
that are on both the apical,

or filtrate-facing side, and the basal
or capillary-facing side of the cells.

By contrast, the cells lining
the collecting duct

only have aquaporins on the basal side,
so not a lot of reabsorption

takes place there usually.

But ADH triggers those cells
to move aquaporins they have in storage

over to the apical side,
which allows more water

to leave the urine.

And since caffeine
and alcohol inhibit ADH,

that means no moving aquaporins,
which means very little

water reabsorption,
and ultimately tons of peeing,

and dehydration.

So, yeah, lots of factors
affect the production of urine.

But once it’s produced,
it doesn’t just leave the building.

It has to be moved and stored
until the time is right.

Once the urine leaves the kidneys,
it enters the ureters,

a pair of slender tubes that drop down
to the posterior urinary bladder.

Contrary to what you might think,
your ureters aren’t just passive tubes

and your pee doesn’t wind up
in your bladder because of gravity alone.

Rather like the small intestines,
each ureter features a layer

of smooth muscle that contracts
to move urine using peristalsis.

The frequency and strength
of these peristaltic waves varies

depending on how fast urine
is being produced;

and a series of valves prevent pee
from backing up

making sure that instead
it reaches the bladder.

The bladder is a hollow, collapsible sac
that temporarily stores urine.

Like the kidneys, it’s retroperitoneal,
located posterior to the pubic bone

and anterior to the rectum.

The bladder wall consists of 3 layers:
an inner mucosa

surrounded by a thick muscular layer
called the detrusor

wrapped in a fibrous,
protective outer membrane.

The inner mucosal layer
consists of transitional epithelium,

which allows the bladder to expand
so it can hold more urine.

A handy feature for social mammals
like us who prefer dry underwear

and peeing in private.

When it’s empty,
it collapses into a triangular shape,

folding up on itself
like a deflated balloon.

Then as urine accumulates,
the bladder thins and expands

into a pear-shape,
and all those folds disappear.

A full bladder can comfortably hold
around 500 mL of pee,

though it can usually expand
to hold a maximum of around one liter.

At that point, though,
you’re pushing your luck

because prolonged overdistention
could, in theory, lead to a burst bladder.

Although you’d probably
just pee your pants first.

But let’s assume for the sake
of polite conversation

that you have found an appropriate
location to relieve yourself.

Your urine enters the thin
but muscular urethra

by passing through the internal
urethral sphincter.

Now we don’t actually have voluntary
control over this particular sphincter,

but the autonomic nervous system
keeps it cinched up

whenever you’re not peeing
to prevent leakage.

Once the urine is through the sphincter,
it heads down

through the urogenital diaphragm
which includes the last stop:

the external urethral sphincter;
which is probably the one

that you’re familiar with
because it’s made of skeletal muscles

and is the one that you control voluntarily.

Only now are we finally ready to explore
the act of micturition itself,

the actual excretion of urine, urination.

As the pee from your morning coffee
builds up,

it causes the bladder to push out,
activating the stretch receptors

in its walls.

The resulting nerve impulses zip along
afferent fibers

to the sacral region of the spinal cord,
along interneurons, and toward the brain;

eventually exciting
the parasympathetic neurons

and inhibiting the sympathetic system.

This tells the detrusor to contract
while the internal urethral sphincter

simultaneously opens,
and the external sphincter relaxes

so that the pee can flow out.

This, you may or not recall,
is kind of an acquired skill.

When you’re a baby,
those stretch-receptor impulses

trigger a simple spinal reflex
that coordinates this whole process,

and you have no real control over
when you pee.

But within a couple of years of birth,
your brain’s circuits have developed

the ability to override
simple reflexive urination

and to choose a different neural pathway.

So how’s that possible?

Well, an area of your brainstem
called the pons

contains two different centers
that lock down your urination control,

or lack of it.

There’s the pontine storage area,
which inhibits urination;

and the pontine micturition center,
which gives it the green light.

As your bladder fills up,
impulses triggered by stretch receptors

head to the pons
and other higher brain centers

that give you that conscious feeling
that you have to pee.

If your bladder isn’t full
and you’re too busy to find a bathroom,

it mostly activates
the pontine storage area

that keeps you from peeing by
inhibiting your parasympathetic activity

and increasing sympathetic output.

Of course, the longer you hold it, the more your bladder fills up, and eventually the
need to pee becomes too strong to ignore, at which point the pontine micturition center
jumps into action, overriding the previous orders, and opening the sphincters so you can finally tinkle.

And that’s how your own personal waterworks … works.

Whether you’re a baby in diapers, or a grown-up science student … or a guy who was sent
to “leave a message” on Jeffrey Lebowski’s rug.

Today you learned how the urinary system regulates the production of urine, by maintaining a
study glomerular flow rate.

We also talked about the anatomy of storing and excreting
urine -- from the ureters to the urethra -- and we went over the nervous system’s role in
controlling the act of urination.

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 you want to help us keep making videos
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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 by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer is Michael
Aranda, and the Graphics team is Thought Cafe.