0:00:00.000,0:00:00.340


0:00:00.340,0:00:02.190
What I want to do in this video[br]is talk a little bit

0:00:02.190,0:00:05.500
about the kidney-- and this is[br]a big picture of a kidney--

0:00:05.500,0:00:08.200
and to talk about how it[br]operates at its-- I guess you

0:00:08.200,0:00:10.500
could call it its smallest[br]functional level and that's

0:00:10.500,0:00:11.420
the nephron.

0:00:11.420,0:00:16.700
So we're going to talk about[br]the kidney and the nephron.

0:00:16.700,0:00:18.780
And I think you might already[br]know the kidney.

0:00:18.780,0:00:19.680
We have two of them.

0:00:19.680,0:00:23.270
They're the organ that, I guess,[br]is most famous for

0:00:23.270,0:00:27.860
producing or allowing[br]us to excrete waste.

0:00:27.860,0:00:32.229
But part of that process, it[br]also helps us maintain our

0:00:32.229,0:00:36.410
water, the correct level, and[br]actually the amount of salts

0:00:36.410,0:00:39.010
or electrolytes we have and our[br]blood pressure, but I'll

0:00:39.010,0:00:40.870
just say maintain water.

0:00:40.870,0:00:42.960
And it also produces hormones[br]and things, and I'm not going

0:00:42.960,0:00:45.190
to go into a lot of detail[br]on that right now.

0:00:45.190,0:00:48.580
I really just want to focus on[br]these first two to kind of

0:00:48.580,0:00:52.210
just understand the overview[br]function of the kidney.

0:00:52.210,0:00:54.130
And most of us have[br]two of these.

0:00:54.130,0:00:57.480
They're kind of closer to our[br]back on either sides of our

0:00:57.480,0:00:59.210
spine behind our liver.

0:00:59.210,0:01:02.470
And this is a zoomed-in[br]version of it.

0:01:02.470,0:01:04.239
If you're watching this in full[br]screen, it's not going to

0:01:04.239,0:01:07.150
be as big as this picture is,[br]but we've sliced it so we can

0:01:07.150,0:01:10.980
see what's going on[br]inside the kidney.

0:01:10.980,0:01:15.130
Just to understand the different[br]parts here, just

0:01:15.130,0:01:17.660
because it will actually be[br]significant when we start

0:01:17.660,0:01:20.520
talking about the functional[br]units or the nephron within

0:01:20.520,0:01:24.930
the kidney, this area right here[br]from here to here, this

0:01:24.930,0:01:28.910
is called the renal cortex.

0:01:28.910,0:01:31.550
Whenever we talk about something[br]with the kidney, if

0:01:31.550,0:01:34.110
you see a renal anything, that's[br]actually referring to

0:01:34.110,0:01:34.580
the kidney.

0:01:34.580,0:01:36.800
So this right here is[br]a renal cortex, that

0:01:36.800,0:01:38.230
outer part right there.

0:01:38.230,0:01:44.380
And then this area right here,[br]this is the renal medulla.

0:01:44.380,0:01:46.480
And medulla comes from middle.

0:01:46.480,0:01:49.050
So you can almost view it as[br]the middle of the kidney.

0:01:49.050,0:01:52.400


0:01:52.400,0:01:55.680
Besides just understanding these[br]words, we're going to

0:01:55.680,0:01:58.350
see that they actually play a[br]very important role in this

0:01:58.350,0:02:01.720
actual filtration or this[br]excretion of waste and this

0:02:01.720,0:02:05.730
ability to not dump too much[br]water or excrete too much

0:02:05.730,0:02:09.020
water when we're trying to[br]filter out our blood.

0:02:09.020,0:02:11.720
So I've said before, and you[br]might have heard it already

0:02:11.720,0:02:14.330
from other lectures or from[br]other teachers, that the

0:02:14.330,0:02:16.490
functional unit of the kidney[br]is the nephron.

0:02:16.490,0:02:22.510


0:02:22.510,0:02:25.040
And the reason why it's called[br]a functional unit-- I'll put

0:02:25.040,0:02:28.290
it in quotes-- is because that's[br]the level at which

0:02:28.290,0:02:30.250
these two things[br]are happening.

0:02:30.250,0:02:32.980
The two major functions of the[br]kidney: the waste excretion

0:02:32.980,0:02:36.580
and the maintenance[br]of the water level

0:02:36.580,0:02:37.730
in our blood system.

0:02:37.730,0:02:41.730
So just to get an idea of how a[br]nephron fits in within this

0:02:41.730,0:02:45.840
picture of a kidney-- I got this[br]picture from Wikipedia.

0:02:45.840,0:02:48.420
The artist tried to draw a[br]couple of nephrons over here.

0:02:48.420,0:02:50.630
So a nephron will look something[br]like this, and it

0:02:50.630,0:02:53.680
dips down into the medulla, and[br]then it goes back into the

0:02:53.680,0:02:56.640
cortex, and then it dumps into[br]collecting ducts, and

0:02:56.640,0:02:59.620
essentially the fluid will end[br]up in the ureters right here

0:02:59.620,0:03:02.960
and end up in our urinary[br]bladder that we can later

0:03:02.960,0:03:05.880
excrete when we find[br]a suitable time.

0:03:05.880,0:03:07.770
But that's about-- I guess[br]you can imagine

0:03:07.770,0:03:10.780
the length of a nephron.

0:03:10.780,0:03:13.030
This is where it starts and[br]then it dips down again.

0:03:13.030,0:03:14.840
So multiple nephrons are going[br]to keep doing that, but

0:03:14.840,0:03:15.840
they're super thin.

0:03:15.840,0:03:18.140
These tubes or these tubules,[br]maybe I should

0:03:18.140,0:03:20.800
say, are super thin.

0:03:20.800,0:03:26.190
Your average kidney will contain[br]on the order of one

0:03:26.190,0:03:27.440
million nephrons.

0:03:27.440,0:03:31.320


0:03:31.320,0:03:34.470
You can't really say, my[br]nephrons are microscopic.

0:03:34.470,0:03:39.390
They kind of have a-- at least[br]their length when they dip

0:03:39.390,0:03:41.880
down, you can say, I can[br]see that distance.

0:03:41.880,0:03:45.510
You can still jam a lot of them[br]inside of one kidney.

0:03:45.510,0:03:49.580
With that said, let's actually[br]figure out how a nephron

0:03:49.580,0:03:52.880
filters the blood and actually[br]makes sure that not too much

0:03:52.880,0:03:56.530
water or not too much of the[br]good stuff in our blood ends

0:03:56.530,0:03:58.330
up the urine.

0:03:58.330,0:04:04.070
So let me draw here a nephron.

0:04:04.070,0:04:07.200
So I'm going to start[br]like this.

0:04:07.200,0:04:08.850
We'll start with[br]the blood flow.

0:04:08.850,0:04:13.480
So the blood's going to come[br]in an arterial-- that's an

0:04:13.480,0:04:15.780
arterial capillary,[br]you could say.

0:04:15.780,0:04:17.589
So it's going to come[br]in like that.

0:04:17.589,0:04:21.820
This is actually called[br]the afferent arterial.

0:04:21.820,0:04:23.230
You don't have to know[br]the names, but you

0:04:23.230,0:04:24.480
might see that sometime.

0:04:24.480,0:04:26.670


0:04:26.670,0:04:28.170
Blood is coming in.

0:04:28.170,0:04:32.060
Then it goes into this[br]big windy place.

0:04:32.060,0:04:34.810
It really winds around[br]like that.

0:04:34.810,0:04:36.060
This is called the glomerulus.

0:04:36.060,0:04:43.490


0:04:43.490,0:04:45.870
And then it leaves via the[br]efferent arterial.

0:04:45.870,0:04:57.100


0:04:57.100,0:04:58.610
Efferent just means away[br]from the center.

0:04:58.610,0:05:02.750
Afferent towards, efferent[br]away from the center.

0:05:02.750,0:05:04.340
And I'll talk about it more[br]in the future, but it's

0:05:04.340,0:05:05.830
interesting that we're[br]still dealing with an

0:05:05.830,0:05:07.040
artery at this point.

0:05:07.040,0:05:08.770
It's still oxygenated blood.

0:05:08.770,0:05:12.260
Normally, when we leave a[br]capillary system like the

0:05:12.260,0:05:14.950
glomerulus right there, we're[br]normally dealing with the

0:05:14.950,0:05:18.650
venous system, but here we're[br]still an arterial system.

0:05:18.650,0:05:21.280
And it's probably because[br]arterial systems have higher

0:05:21.280,0:05:22.760
blood pressure, and what we[br]need to do is we need to

0:05:22.760,0:05:28.730
squeeze fluid and stuff that's[br]dissolved in the fluid out of

0:05:28.730,0:05:30.970
the blood and in the glomerulus[br]right here.

0:05:30.970,0:05:35.420
So this glomerulus is very[br]porous and it's surrounded by

0:05:35.420,0:05:37.460
other cells.

0:05:37.460,0:05:38.710
This is kind of a[br]cross-section.

0:05:38.710,0:05:44.470


0:05:44.470,0:05:49.200
It's surrounded like that by[br]this structure, and these are

0:05:49.200,0:05:53.350
cells here so you can imagine[br]these are all cells over here.

0:05:53.350,0:05:56.980
And, of course, the actual[br]capillaries have cells that

0:05:56.980,0:05:58.960
line them so there[br]are cells here.

0:05:58.960,0:06:01.140
So when I draw these lines,[br]these lines are actually made

0:06:01.140,0:06:03.420
up of little cells.

0:06:03.420,0:06:04.540
What happens is the[br]blood comes in

0:06:04.540,0:06:05.900
at really high pressure.

0:06:05.900,0:06:06.960
This is very porous.

0:06:06.960,0:06:10.410
These cells out here, they're[br]called podocytes.

0:06:10.410,0:06:11.880
They're a little bit more[br]selective in what gets

0:06:11.880,0:06:14.630
filtered out, and essentially[br]about a fifth of the fluid

0:06:14.630,0:06:20.640
that's coming in ends up going[br]into this space right here

0:06:20.640,0:06:22.550
that's called the[br]Bowman's space.

0:06:22.550,0:06:23.690
Well, actually, this whole[br]thing is called

0:06:23.690,0:06:24.940
the Bowman's capsule.

0:06:24.940,0:06:28.270


0:06:28.270,0:06:30.650
It's a sphere with an opening in[br]here that the capillary can

0:06:30.650,0:06:34.050
kind of wind around in, and the[br]space right here, this is

0:06:34.050,0:06:36.890
the Bowman's space.

0:06:36.890,0:06:41.270
It's the space inside the[br]Bowman's capsule, and the

0:06:41.270,0:06:42.150
whole thing has cells.

0:06:42.150,0:06:44.200
All these structures are[br]obviously made-- or maybe not

0:06:44.200,0:06:46.830
so obviously-- they're[br]made up of cells.

0:06:46.830,0:06:48.730
And so we end up having[br]filtrate in it.

0:06:48.730,0:06:53.190
Filtrate is just the stuff[br]that gets squeezed out.

0:06:53.190,0:06:56.380
We can't call it urine just yet[br]because there's a lot of

0:06:56.380,0:07:01.720
steps that have to occur for[br]it to earn the name urine.

0:07:01.720,0:07:04.400
So it's only filtrate right now,[br]and essentially what get

0:07:04.400,0:07:07.220
squeezed out, I said it's about[br]a fifth of the fluid,

0:07:07.220,0:07:10.450
and things that are easily[br]dissolved in fluid, so small

0:07:10.450,0:07:16.430
ions, sodium, maybe some small[br]molecules like glucose, maybe

0:07:16.430,0:07:19.570
some amino acids.

0:07:19.570,0:07:21.086
There are tons of stuff[br]in here, but this is

0:07:21.086,0:07:22.080
just to give an idea.

0:07:22.080,0:07:25.370
The things that do not get[br]filtered are things like red

0:07:25.370,0:07:31.340
blood cells or larger molecules,[br]larger proteins.

0:07:31.340,0:07:32.510
They will not get filtered.

0:07:32.510,0:07:36.700
It's mainly the micromolecules[br]that'll get filtered, that'll

0:07:36.700,0:07:40.910
be part of this filtrate[br]that shows up here

0:07:40.910,0:07:42.390
in the Bowman space.

0:07:42.390,0:07:45.110
Now, the rest of what the[br]nephron does, the Bowman's

0:07:45.110,0:07:46.960
capsule is kind of the beginning[br]of the nephron, and

0:07:46.960,0:07:51.280
just to get an idea of our big[br]picture of our kidney, let's

0:07:51.280,0:07:54.760
say we're near an arterial.

0:07:54.760,0:07:56.980
This is a Bowman's capsule[br]right here.

0:07:56.980,0:07:59.075
It looks something like that,[br]and the whole nephron is going

0:07:59.075,0:08:00.580
to be convoluted like this.

0:08:00.580,0:08:03.150
It's going to dip down into[br]the medulla, and then come

0:08:03.150,0:08:06.220
back, and then it's going to[br]eventually dump into a

0:08:06.220,0:08:07.980
collecting duct, and I'll[br]talk more about that.

0:08:07.980,0:08:12.250
So what I've drawn just here,[br]this is a zoomed-in version of

0:08:12.250,0:08:14.660
that part right there.

0:08:14.660,0:08:16.250
Now what I want to do is zoom[br]out a little bit because I'm

0:08:16.250,0:08:17.550
going to run out of space.

0:08:17.550,0:08:19.290
So let me zoom out.

0:08:19.290,0:08:23.220
So we had our arterial go in.

0:08:23.220,0:08:26.520
It gets all bunched in the[br]glomerulus, and then most of

0:08:26.520,0:08:30.160
the blood leaves, but[br]one-fifth of it gets

0:08:30.160,0:08:33.390
essentially filtered in to[br]the Bowman's capsule.

0:08:33.390,0:08:34.780
That's the Bowman's capsule[br]right there.

0:08:34.780,0:08:36.450
I've just zoomed out[br]a little bit.

0:08:36.450,0:08:39.530
So we have our filtrate here.

0:08:39.530,0:08:41.080
Maybe I'll make it a[br]little bit yellow.

0:08:41.080,0:08:44.440


0:08:44.440,0:08:46.800
The filtrate that just comes out[br]at this point, sometimes

0:08:46.800,0:08:49.270
it's called the glomerular[br]filtrate because it's been

0:08:49.270,0:08:52.170
filtered by the glomerulus, but[br]it's also been filtered by

0:08:52.170,0:08:55.060
those podocyte cells[br]on the inside of

0:08:55.060,0:08:56.470
the Bowman's capsule.

0:08:56.470,0:08:59.120
But now it's ready to go[br]to the proximal tubule.

0:08:59.120,0:09:03.080


0:09:03.080,0:09:06.530
Let me draw something[br]like this.

0:09:06.530,0:09:08.300
And obviously, this is not[br]exactly what it looks like,

0:09:08.300,0:09:09.580
but it gives you the sense.

0:09:09.580,0:09:17.470
This right here, this is[br]the proximal tubule.

0:09:17.470,0:09:20.560
And it sounds like a very fancy[br]word, but proximal just

0:09:20.560,0:09:23.590
means near and tubule, you can[br]imagine, is just a small tube.

0:09:23.590,0:09:25.740
So it's a small tube that's[br]near the beginning.

0:09:25.740,0:09:27.880
That's why it's called[br]a proximal tubule.

0:09:27.880,0:09:29.700
And it has two parts.

0:09:29.700,0:09:31.245
The whole thing is[br]often called a

0:09:31.245,0:09:32.870
proximal convoluted tubule.

0:09:32.870,0:09:35.870


0:09:35.870,0:09:37.390
That's because it's[br]all convoluted.

0:09:37.390,0:09:38.830
The way I've drawn[br]it is all curvy.

0:09:38.830,0:09:40.750
And I just drew it curvy[br]in two dimensions.

0:09:40.750,0:09:43.170
It's actually curvy in[br]three dimensions.

0:09:43.170,0:09:45.470
But the reality is there's a[br]curvy part and then there's a

0:09:45.470,0:09:48.220
straight part near the end[br]of the proximal tubule.

0:09:48.220,0:09:50.570
So we'll call this whole thing[br]the proximal tubule.

0:09:50.570,0:09:52.060
This is the convoluted part.

0:09:52.060,0:09:53.670
That's the straight[br]part, but we don't

0:09:53.670,0:09:55.340
have to get too picky.

0:09:55.340,0:09:59.040
But the whole point of this part[br]of the nephron-- and just

0:09:59.040,0:10:02.810
to remember where we are, we're[br]now at this point of the

0:10:02.810,0:10:05.890
nephron right there-- the[br]whole point is to start

0:10:05.890,0:10:09.700
reabsorbing some of the stuff[br]that is in the filtrate that

0:10:09.700,0:10:10.670
we don't want to lose.

0:10:10.670,0:10:11.880
We don't want to lose glucose.

0:10:11.880,0:10:13.900
That's hard-earned stuff[br]that we ate that

0:10:13.900,0:10:15.090
was good for energy.

0:10:15.090,0:10:18.790
We don't want to lose[br]necessarily as much sodium.

0:10:18.790,0:10:24.040
We've seen in multiple videos[br]that that's a useful ion to

0:10:24.040,0:10:24.480
have around.

0:10:24.480,0:10:26.490
We don't want to lose[br]amino acids.

0:10:26.490,0:10:30.490
Those are useful for building up[br]proteins and other things.

0:10:30.490,0:10:32.320
So these are things we don't[br]want to lose so we start

0:10:32.320,0:10:33.670
absorbing them back.

0:10:33.670,0:10:35.840
I'll do a whole video on exactly[br]how that happens, but

0:10:35.840,0:10:37.480
it's done actively.

0:10:37.480,0:10:40.575
Since we're using ATP, and just[br]as a bit of a summary,

0:10:40.575,0:10:43.900
you're using ATP to actually[br]pump out the sodium and then

0:10:43.900,0:10:45.660
that actually helps bring[br]in the other things.

0:10:45.660,0:10:48.160
That's just kind of a tidbit[br]on what's happening.

0:10:48.160,0:10:52.190
So we're reabsorbing, so just[br]imagine what's happening.

0:10:52.190,0:10:56.370
You have cells lining the[br]proximal tubule right now.

0:10:56.370,0:10:58.310
And actually, they have little[br]things that jut out.

0:10:58.310,0:10:59.790
I'll do a whole video on that[br]because it's actually

0:10:59.790,0:11:00.630
interesting.

0:11:00.630,0:11:01.910
So you have cells out here.

0:11:01.910,0:11:04.660
On the other side of the cells,[br]you have an arterial

0:11:04.660,0:11:08.110
system, or a capillary system,[br]I should say, actually.

0:11:08.110,0:11:12.080
So let's say you have a[br]capillary system here that is

0:11:12.080,0:11:16.580
very close to the cells lining[br]the proximal tubule, and so

0:11:16.580,0:11:19.140
this stuff actually gets[br]actively pumped, especially

0:11:19.140,0:11:23.000
the sodium, but all of it, using[br]energy, gets pumped back

0:11:23.000,0:11:24.890
into the blood selectively,[br]and maybe a

0:11:24.890,0:11:25.870
little bit of our water.

0:11:25.870,0:11:32.480
So we're pumping back some[br]sodium, some glucose, and

0:11:32.480,0:11:34.710
we'll start pumping a little[br]bit of the water back in

0:11:34.710,0:11:36.540
because we don't want to[br]lose all of that water.

0:11:36.540,0:11:38.620
If all of the water that was[br]originally in the filtrate we

0:11:38.620,0:11:41.560
were just leaving in our urine,[br]we'd be excreting

0:11:41.560,0:11:44.240
gallons and gallons of water[br]every day, which we do not

0:11:44.240,0:11:45.310
want to do.

0:11:45.310,0:11:46.500
So that's the whole point.

0:11:46.500,0:11:49.130
We're starting the absorption[br]process.

0:11:49.130,0:11:51.400
And then we'll enter the loop[br]of Henle, and actually, this

0:11:51.400,0:11:52.610
is, in my mind, the most

0:11:52.610,0:11:55.370
interesting part of the nephron.

0:11:55.370,0:11:59.930
So we're entering the loop of[br]Henle, and it dips down, and

0:11:59.930,0:12:03.680
then comes back up.

0:12:03.680,0:12:05.740
And so most of the length[br]of the nephron

0:12:05.740,0:12:07.900
is the loop of Henle.

0:12:07.900,0:12:10.916
And if I go back to this diagram[br]right here, if I'm

0:12:10.916,0:12:13.270
talking about the loop of Henle,[br]I'm talking about this

0:12:13.270,0:12:15.440
whole thing right there.

0:12:15.440,0:12:17.450
And you can see something[br]interesting here.

0:12:17.450,0:12:21.050
It crosses the border between[br]the cortex, this light brown

0:12:21.050,0:12:24.820
part, and the renal medulla,[br]this kind of reddish or orange

0:12:24.820,0:12:27.170
part right there, and it does[br]that for a very good reason.

0:12:27.170,0:12:28.000
I'm going to draw it here.

0:12:28.000,0:12:32.710
So let's say this is the[br]dividing line right here.

0:12:32.710,0:12:35.460
This right here was[br]the cortex.

0:12:35.460,0:12:39.550
This right here is[br]the medulla.

0:12:39.550,0:12:42.060
So the whole point-- well,[br]there's two points

0:12:42.060,0:12:43.590
of the loop of Henle.

0:12:43.590,0:12:48.570


0:12:48.570,0:12:57.830
One point is to make the renal[br]medulla salty, and it does

0:12:57.830,0:13:00.502
this by actively pumping[br]out salts.

0:13:00.502,0:13:03.090
So it actively pumps out salts,[br]and it does that in the

0:13:03.090,0:13:06.140
ascending part of the[br]loop of Henle.

0:13:06.140,0:13:12.390
So it actively pumps out salts:[br]sodium, potassium,

0:13:12.390,0:13:14.440
chloride, or chlorine,[br]I should say.

0:13:14.440,0:13:17.500
Chlorine ions.

0:13:17.500,0:13:21.990
It actively pumps out these[br]salts right here to make the

0:13:21.990,0:13:27.670
entire medulla salty, or if we[br]think about it in terms of

0:13:27.670,0:13:29.990
kind of osmosis, make[br]it hypertonic.

0:13:29.990,0:13:33.320
You have more solute out here[br]than you have in the filtrate

0:13:33.320,0:13:36.010
that's going through[br]the tubules.

0:13:36.010,0:13:37.240
And it uses ATP to do this.

0:13:37.240,0:13:39.910
All of this stuff requires ATP[br]to actively pump against a

0:13:39.910,0:13:41.570
concentration gradient.

0:13:41.570,0:13:46.110
So this is salty and it's[br]salty for a reason.

0:13:46.110,0:13:50.820
It's not just to take back these[br]salts from the filtrate,

0:13:50.820,0:13:53.230
although that's part of the[br]reason, but by making this

0:13:53.230,0:13:57.620
salty, the ascending part is[br]only permeable to these salts

0:13:57.620,0:13:59.270
and these ions.

0:13:59.270,0:14:00.830
It's not permeable to water.

0:14:00.830,0:14:06.110


0:14:06.110,0:14:09.050
The descending part of the[br]loop of Henle is only

0:14:09.050,0:14:10.300
permeable to water.

0:14:10.300,0:14:13.390


0:14:13.390,0:14:14.380
So what's going to happen?

0:14:14.380,0:14:17.430
If this is all salty because the[br]ascending part is actively

0:14:17.430,0:14:21.260
pumping out salt, what's going[br]to happen to water as it goes

0:14:21.260,0:14:23.110
down the descending loop?

0:14:23.110,0:14:24.870
Well, it's hypertonic[br]out here.

0:14:24.870,0:14:28.830
Water will naturally want to go[br]and kind of try to make the

0:14:28.830,0:14:30.540
concentrations balance out.

0:14:30.540,0:14:31.510
I've done a whole[br]video on that.

0:14:31.510,0:14:33.680
It doesn't happen by magic.

0:14:33.680,0:14:36.710
And so the water will-- because[br]this is hypertonic,

0:14:36.710,0:14:39.620
it's more salty, and this is[br]only permeable to water, the

0:14:39.620,0:14:42.930
water will leave the membrane on[br]the descending part of the

0:14:42.930,0:14:44.560
loop of Henle right now.

0:14:44.560,0:14:48.710
And this is a major part[br]of water reabsorption.

0:14:48.710,0:14:52.870
I've thought a lot about why[br]don't we use ATP somehow to

0:14:52.870,0:14:54.360
actively pump water?

0:14:54.360,0:14:55.850
And the answer there[br]is, there's no

0:14:55.850,0:14:56.650
easy way to do that.

0:14:56.650,0:15:01.130
Biological systems are good at[br]using ATP to pump out ions,

0:15:01.130,0:15:03.360
but it can't actively[br]pump out water.

0:15:03.360,0:15:06.360
Water's kind of a hard thing[br]for proteins to operate on.

0:15:06.360,0:15:09.580
So the solution is to make it[br]salty out here by pumping out

0:15:09.580,0:15:12.540
ions and then water, if you[br]make this porous only to

0:15:12.540,0:15:14.750
water, water will naturally[br]flow out.

0:15:14.750,0:15:18.170
So this is a major mechanism of[br]gaining back a lot of the

0:15:18.170,0:15:20.820
water that gets filtered[br]out up here.

0:15:20.820,0:15:23.390
And the reason why this is so[br]long is to give time for this

0:15:23.390,0:15:27.560
water to secrete out, and that's[br]why it dips nice and

0:15:27.560,0:15:31.490
pretty far down into[br]this salty portion.

0:15:31.490,0:15:34.710
So then we'll leave the loop of[br]Henle and then we're almost

0:15:34.710,0:15:36.110
done with the nephron.

0:15:36.110,0:15:38.790
Then we're in another convoluted[br]tubule, and you

0:15:38.790,0:15:41.710
might even guess the name of[br]this convoluted tubule.

0:15:41.710,0:15:44.860
If this was the proximal one,[br]this is the distal one.

0:15:44.860,0:15:47.070
And actually, just to make my[br]drawing correct, it actually

0:15:47.070,0:15:51.560
passes very close to the[br]Bowman's capsule, so let me do

0:15:51.560,0:15:52.810
it in a different color.

0:15:52.810,0:15:57.230


0:15:57.230,0:16:01.920
The distal convoluted tubule[br]actually goes pretty close to

0:16:01.920,0:16:03.970
the Bowman's capsule.

0:16:03.970,0:16:06.000
And once again, I've made[br]it all convoluted in two

0:16:06.000,0:16:07.740
dimensions, but it's actually[br]convoluted in three.

0:16:07.740,0:16:09.840
And it's not that long, but I[br]just had to get over here and

0:16:09.840,0:16:11.830
I wanted to get over that[br]point right there.

0:16:11.830,0:16:12.830
It's called distal.

0:16:12.830,0:16:14.510
Distal is further away.

0:16:14.510,0:16:16.750
It's convoluted and[br]it's a tubule.

0:16:16.750,0:16:24.450
So this right here is the distal[br]convoluted tubule, and

0:16:24.450,0:16:27.690
here we have more reabsorption:[br]calcium, more

0:16:27.690,0:16:28.820
sodium reabsorption.

0:16:28.820,0:16:31.250
We're just reabsorbing more[br]things that we didn't want to

0:16:31.250,0:16:32.600
lose in the first place.

0:16:32.600,0:16:34.180
There's a lot of things we[br]could talk about what get

0:16:34.180,0:16:36.330
reabsorbed, but this is[br]just the overview.

0:16:36.330,0:16:39.870
And we're also reabsorbing a[br]little bit of more water.

0:16:39.870,0:16:41.420
But then at the end[br]right here, our

0:16:41.420,0:16:42.800
filtrate has been processed.

0:16:42.800,0:16:44.290
A lot of the water's[br]been taken out.

0:16:44.290,0:16:45.760
It's a lot more concentrated.

0:16:45.760,0:16:47.700
We've reabsorbed a[br]lot of the salts,

0:16:47.700,0:16:49.290
electrolytes that we want.

0:16:49.290,0:16:52.100
We've reabsorbed the glucose and[br]a lot of the amino acids.

0:16:52.100,0:16:53.770
Everything that we want,[br]we've taken back.

0:16:53.770,0:16:55.630
We've reabsorbed.

0:16:55.630,0:16:59.710
And so this is mainly waste[br]products and water that we

0:16:59.710,0:17:01.670
don't need anymore and then[br]this gets dumped into

0:17:01.670,0:17:02.920
collecting ducts.

0:17:02.920,0:17:05.140


0:17:05.140,0:17:07.230
And you can kind of view this[br]as the trash chute of the

0:17:07.230,0:17:12.384
kidney, where multiple[br]nephrons are going

0:17:12.384,0:17:13.740
to dump into this.

0:17:13.740,0:17:16.960
So that might be the distal[br]tubule of another nephron

0:17:16.960,0:17:22.010
right here and this is a[br]collecting duct, which is just

0:17:22.010,0:17:23.569
a tube that's collecting[br]all the

0:17:23.569,0:17:27.040
byproducts of the nephrons.

0:17:27.040,0:17:28.620
And the interesting thing is[br]that the collecting duct

0:17:28.620,0:17:31.040
further goes into the[br]medulla again.

0:17:31.040,0:17:34.210
It goes into the medulla again[br]to the salty part again.

0:17:34.210,0:17:36.090
So if we're talking about the[br]collecting duct, maybe the

0:17:36.090,0:17:40.400
collecting duct's coming back[br]into the medulla, collecting

0:17:40.400,0:17:43.070
all of the filtrate from[br]the different nephrons.

0:17:43.070,0:17:46.860
And because it goes back through[br]that super salty spot

0:17:46.860,0:17:50.240
in the medulla, we actually[br]have four hormones called

0:17:50.240,0:17:54.540
anti-diarrhetic hormone that[br]can dictate how porous this

0:17:54.540,0:18:00.910
collecting tube is, and if it[br]makes it very porous, it

0:18:00.910,0:18:04.430
allows more water to leave as we[br]go to the medulla, because

0:18:04.430,0:18:06.110
this is very salty,[br]so the water will

0:18:06.110,0:18:07.690
leave if this is porous.

0:18:07.690,0:18:10.740
And when we do that, what that[br]does is it makes the

0:18:10.740,0:18:13.100
filtrate-- and we can maybe[br]start calling it urine now--

0:18:13.100,0:18:17.170
even more concentrated so we[br]lose even less water, and it

0:18:17.170,0:18:19.490
keeps collecting, collecting,[br]collecting until we end up

0:18:19.490,0:18:23.650
here, and it leaves the kidney[br]and goes via our ureters to

0:18:23.650,0:18:25.030
the urinary bladder.

0:18:25.030,0:18:27.200
So hopefully, you found[br]that helpful.

0:18:27.200,0:18:29.650
I think the neatest thing here[br]is just how we actively

0:18:29.650,0:18:33.150
reabsorb the water and how we--[br]well, actually, in my

0:18:33.150,0:18:36.770
mind, that is the neatest part[br]in the loop of Henle.

0:18:36.770,0:18:37.133