The Kidney and Nephron

Overview of how the nephrons in the kidney filter blood and reabsorb water and other molecules.

What I want to do in this video
is talk a little bit

about the kidney-- and this is
a big picture of a kidney--

and to talk about how it
operates at its-- I guess you

could call it its smallest
functional level and that's

the nephron.

So we're going to talk about
the kidney and the nephron.

And I think you might already
know the kidney.

We have two of them.

They're the organ that, I guess,
is most famous for

producing or allowing
us to excrete waste.

But part of that process, it
also helps us maintain our

water, the correct level, and
actually the amount of salts

or electrolytes we have and our
blood pressure, but I'll

just say maintain water.

And it also produces hormones
and things, and I'm not going

to go into a lot of detail
on that right now.

I really just want to focus on
these first two to kind of

just understand the overview
function of the kidney.

And most of us have
two of these.

They're kind of closer to our
back on either sides of our

spine behind our liver.

And this is a zoomed-in
version of it.

If you're watching this in full
screen, it's not going to

be as big as this picture is,
but we've sliced it so we can

see what's going on
inside the kidney.

Just to understand the different
parts here, just

because it will actually be
significant when we start

talking about the functional
units or the nephron within

the kidney, this area right here
from here to here, this

is called the renal cortex.

Whenever we talk about something
with the kidney, if

you see a renal anything, that's
actually referring to

the kidney.

So this right here is
a renal cortex, that

outer part right there.

And then this area right here,
this is the renal medulla.

And medulla comes from middle.

So you can almost view it as
the middle of the kidney.

Besides just understanding these
words, we're going to

see that they actually play a
very important role in this

actual filtration or this
excretion of waste and this

ability to not dump too much
water or excrete too much

water when we're trying to
filter out our blood.

So I've said before, and you
might have heard it already

from other lectures or from
other teachers, that the

functional unit of the kidney
is the nephron.

And the reason why it's called
a functional unit-- I'll put

it in quotes-- is because that's
the level at which

these two things
are happening.

The two major functions of the
kidney: the waste excretion

and the maintenance
of the water level

in our blood system.

So just to get an idea of how a
nephron fits in within this

picture of a kidney-- I got this
picture from Wikipedia.

The artist tried to draw a
couple of nephrons over here.

So a nephron will look something
like this, and it

dips down into the medulla, and
then it goes back into the

cortex, and then it dumps into
collecting ducts, and

essentially the fluid will end
up in the ureters right here

and end up in our urinary
bladder that we can later

excrete when we find
a suitable time.

But that's about-- I guess
you can imagine

the length of a nephron.

This is where it starts and
then it dips down again.

So multiple nephrons are going
to keep doing that, but

they're super thin.

These tubes or these tubules,
maybe I should

say, are super thin.

Your average kidney will contain
on the order of one

million nephrons.

You can't really say, my
nephrons are microscopic.

They kind of have a-- at least
their length when they dip

down, you can say, I can
see that distance.

You can still jam a lot of them
inside of one kidney.

With that said, let's actually
figure out how a nephron

filters the blood and actually
makes sure that not too much

water or not too much of the
good stuff in our blood ends

up the urine.

So let me draw here a nephron.

So I'm going to start
like this.

We'll start with
the blood flow.

So the blood's going to come
in an arterial-- that's an

arterial capillary,
you could say.

So it's going to come
in like that.

This is actually called
the afferent arterial.

You don't have to know
the names, but you

might see that sometime.

Blood is coming in.

Then it goes into this
big windy place.

It really winds around
like that.

This is called the glomerulus.

And then it leaves via the
efferent arterial.

Efferent just means away
from the center.

Afferent towards, efferent
away from the center.

And I'll talk about it more
in the future, but it's

interesting that we're
still dealing with an

artery at this point.

It's still oxygenated blood.

Normally, when we leave a
capillary system like the

glomerulus right there, we're
normally dealing with the

venous system, but here we're
still an arterial system.

And it's probably because
arterial systems have higher

blood pressure, and what we
need to do is we need to

squeeze fluid and stuff that's
dissolved in the fluid out of

the blood and in the glomerulus
right here.

So this glomerulus is very
porous and it's surrounded by

other cells.

This is kind of a
cross-section.

It's surrounded like that by
this structure, and these are

cells here so you can imagine
these are all cells over here.

And, of course, the actual
capillaries have cells that

line them so there
are cells here.

So when I draw these lines,
these lines are actually made

up of little cells.

What happens is the
blood comes in

at really high pressure.

This is very porous.

These cells out here, they're
called podocytes.

They're a little bit more
selective in what gets

filtered out, and essentially
about a fifth of the fluid

that's coming in ends up going
into this space right here

that's called the
Bowman's space.

Well, actually, this whole
thing is called

the Bowman's capsule.

It's a sphere with an opening in
here that the capillary can

kind of wind around in, and the
space right here, this is

the Bowman's space.

It's the space inside the
Bowman's capsule, and the

whole thing has cells.

All these structures are
obviously made-- or maybe not

so obviously-- they're
made up of cells.

And so we end up having
filtrate in it.

Filtrate is just the stuff
that gets squeezed out.

We can't call it urine just yet
because there's a lot of

steps that have to occur for
it to earn the name urine.

So it's only filtrate right now,
and essentially what get

squeezed out, I said it's about
a fifth of the fluid,

and things that are easily
dissolved in fluid, so small

ions, sodium, maybe some small
molecules like glucose, maybe

some amino acids.

There are tons of stuff
in here, but this is

just to give an idea.

The things that do not get
filtered are things like red

blood cells or larger molecules,
larger proteins.

They will not get filtered.

It's mainly the micromolecules
that'll get filtered, that'll

be part of this filtrate
that shows up here

in the Bowman space.

Now, the rest of what the
nephron does, the Bowman's

capsule is kind of the beginning
of the nephron, and

just to get an idea of our big
picture of our kidney, let's

say we're near an arterial.

This is a Bowman's capsule
right here.

It looks something like that,
and the whole nephron is going

to be convoluted like this.

It's going to dip down into
the medulla, and then come

back, and then it's going to
eventually dump into a

collecting duct, and I'll
talk more about that.

So what I've drawn just here,
this is a zoomed-in version of

that part right there.

Now what I want to do is zoom
out a little bit because I'm

going to run out of space.

So let me zoom out.

So we had our arterial go in.

It gets all bunched in the
glomerulus, and then most of

the blood leaves, but
one-fifth of it gets

essentially filtered in to
the Bowman's capsule.

That's the Bowman's capsule
right there.

I've just zoomed out
a little bit.

So we have our filtrate here.

Maybe I'll make it a
little bit yellow.

The filtrate that just comes out
at this point, sometimes

it's called the glomerular
filtrate because it's been

filtered by the glomerulus, but
it's also been filtered by

those podocyte cells
on the inside of

the Bowman's capsule.

But now it's ready to go
to the proximal tubule.

Let me draw something
like this.

And obviously, this is not
exactly what it looks like,

but it gives you the sense.

This right here, this is
the proximal tubule.

And it sounds like a very fancy
word, but proximal just

means near and tubule, you can
imagine, is just a small tube.

So it's a small tube that's
near the beginning.

That's why it's called
a proximal tubule.

And it has two parts.

The whole thing is
often called a

proximal convoluted tubule.

That's because it's
all convoluted.

The way I've drawn
it is all curvy.

And I just drew it curvy
in two dimensions.

It's actually curvy in
three dimensions.

But the reality is there's a
curvy part and then there's a

straight part near the end
of the proximal tubule.

So we'll call this whole thing
the proximal tubule.

This is the convoluted part.

That's the straight
part, but we don't

have to get too picky.

But the whole point of this part
of the nephron-- and just

to remember where we are, we're
now at this point of the

nephron right there-- the
whole point is to start

reabsorbing some of the stuff
that is in the filtrate that

we don't want to lose.

We don't want to lose glucose.

That's hard-earned stuff
that we ate that

was good for energy.

We don't want to lose
necessarily as much sodium.

We've seen in multiple videos
that that's a useful ion to

have around.

We don't want to lose
amino acids.

Those are useful for building up
proteins and other things.

So these are things we don't
want to lose so we start

absorbing them back.

I'll do a whole video on exactly
how that happens, but

it's done actively.

Since we're using ATP, and just
as a bit of a summary,

you're using ATP to actually
pump out the sodium and then

that actually helps bring
in the other things.

That's just kind of a tidbit
on what's happening.

So we're reabsorbing, so just
imagine what's happening.

You have cells lining the
proximal tubule right now.

And actually, they have little
things that jut out.

I'll do a whole video on that
because it's actually

interesting.

So you have cells out here.

On the other side of the cells,
you have an arterial

system, or a capillary system,
I should say, actually.

So let's say you have a
capillary system here that is

very close to the cells lining
the proximal tubule, and so

this stuff actually gets
actively pumped, especially

the sodium, but all of it, using
energy, gets pumped back

into the blood selectively,
and maybe a

little bit of our water.

So we're pumping back some
sodium, some glucose, and

we'll start pumping a little
bit of the water back in

because we don't want to
lose all of that water.

If all of the water that was
originally in the filtrate we

were just leaving in our urine,
we'd be excreting

gallons and gallons of water
every day, which we do not

want to do.

So that's the whole point.

We're starting the absorption
process.

And then we'll enter the loop
of Henle, and actually, this

is, in my mind, the most

interesting part of the nephron.

So we're entering the loop of
Henle, and it dips down, and

then comes back up.

And so most of the length
of the nephron

is the loop of Henle.

And if I go back to this diagram
right here, if I'm

talking about the loop of Henle,
I'm talking about this

whole thing right there.

And you can see something
interesting here.

It crosses the border between
the cortex, this light brown

part, and the renal medulla,
this kind of reddish or orange

part right there, and it does
that for a very good reason.

I'm going to draw it here.

So let's say this is the
dividing line right here.

This right here was
the cortex.

This right here is
the medulla.

So the whole point-- well,
there's two points

of the loop of Henle.

One point is to make the renal
medulla salty, and it does

this by actively pumping
out salts.

So it actively pumps out salts,
and it does that in the

ascending part of the
loop of Henle.

So it actively pumps out salts:
sodium, potassium,

chloride, or chlorine,
I should say.

Chlorine ions.

It actively pumps out these
salts right here to make the

entire medulla salty, or if we
think about it in terms of

kind of osmosis, make
it hypertonic.

You have more solute out here
than you have in the filtrate

that's going through
the tubules.

And it uses ATP to do this.

All of this stuff requires ATP
to actively pump against a

concentration gradient.

So this is salty and it's
salty for a reason.

It's not just to take back these
salts from the filtrate,

although that's part of the
reason, but by making this

salty, the ascending part is
only permeable to these salts

and these ions.

It's not permeable to water.

The descending part of the
loop of Henle is only

permeable to water.

So what's going to happen?

If this is all salty because the
ascending part is actively

pumping out salt, what's going
to happen to water as it goes

down the descending loop?

Well, it's hypertonic
out here.

Water will naturally want to go
and kind of try to make the

concentrations balance out.

I've done a whole
video on that.

It doesn't happen by magic.

And so the water will-- because
this is hypertonic,

it's more salty, and this is
only permeable to water, the

water will leave the membrane on
the descending part of the

loop of Henle right now.

And this is a major part
of water reabsorption.

I've thought a lot about why
don't we use ATP somehow to

actively pump water?

And the answer there
is, there's no

easy way to do that.

Biological systems are good at
using ATP to pump out ions,

but it can't actively
pump out water.

Water's kind of a hard thing
for proteins to operate on.

So the solution is to make it
salty out here by pumping out

ions and then water, if you
make this porous only to

water, water will naturally
flow out.

So this is a major mechanism of
gaining back a lot of the

water that gets filtered
out up here.

And the reason why this is so
long is to give time for this

water to secrete out, and that's
why it dips nice and

pretty far down into
this salty portion.

So then we'll leave the loop of
Henle and then we're almost

done with the nephron.

Then we're in another convoluted
tubule, and you

might even guess the name of
this convoluted tubule.

If this was the proximal one,
this is the distal one.

And actually, just to make my
drawing correct, it actually

passes very close to the
Bowman's capsule, so let me do

it in a different color.

The distal convoluted tubule
actually goes pretty close to

the Bowman's capsule.

And once again, I've made
it all convoluted in two

dimensions, but it's actually
convoluted in three.

And it's not that long, but I
just had to get over here and

I wanted to get over that
point right there.

It's called distal.

Distal is further away.

It's convoluted and
it's a tubule.

So this right here is the distal
convoluted tubule, and

here we have more reabsorption:
calcium, more

sodium reabsorption.

We're just reabsorbing more
things that we didn't want to

lose in the first place.

There's a lot of things we
could talk about what get

reabsorbed, but this is
just the overview.

And we're also reabsorbing a
little bit of more water.

But then at the end
right here, our

filtrate has been processed.

A lot of the water's
been taken out.

It's a lot more concentrated.

We've reabsorbed a
lot of the salts,

electrolytes that we want.

We've reabsorbed the glucose and
a lot of the amino acids.

Everything that we want,
we've taken back.

We've reabsorbed.

And so this is mainly waste
products and water that we

don't need anymore and then
this gets dumped into

collecting ducts.

And you can kind of view this
as the trash chute of the

kidney, where multiple
nephrons are going

to dump into this.

So that might be the distal
tubule of another nephron

right here and this is a
collecting duct, which is just

a tube that's collecting
all the

byproducts of the nephrons.

And the interesting thing is
that the collecting duct

further goes into the
medulla again.

It goes into the medulla again
to the salty part again.

So if we're talking about the
collecting duct, maybe the

collecting duct's coming back
into the medulla, collecting

all of the filtrate from
the different nephrons.

And because it goes back through
that super salty spot

in the medulla, we actually
have four hormones called

anti-diarrhetic hormone that
can dictate how porous this

collecting tube is, and if it
makes it very porous, it

allows more water to leave as we
go to the medulla, because

this is very salty,
so the water will

leave if this is porous.

And when we do that, what that
does is it makes the

filtrate-- and we can maybe
start calling it urine now--

even more concentrated so we
lose even less water, and it

keeps collecting, collecting,
collecting until we end up

here, and it leaves the kidney
and goes via our ureters to

the urinary bladder.

So hopefully, you found
that helpful.

I think the neatest thing here
is just how we actively

reabsorb the water and how we--
well, actually, in my

mind, that is the neatest part
in the loop of Henle.