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