Voiceover: So, we've already talked about
this part of the nephron, right?
Remember, it's called the glomerulus, the glomerulus,
and it's the structure that receives an arteriole
that's called the afferent, meaning going towards,
arteriole, that's the arteriole that branches off of
the renal artery, and it gives off this vessel right there.
This is called the efferent arteriole,
efferent meaning going away from,
and these are all talking about the glomerulus.
So, the afferent arteriole goes in, becomes really
really squiggly, and then it comes off as a
single vessel, the efferent arteriole.
In this process, a ton of fluid is filtered out of the blood
and goes into this yellow capsule right here.
This is Bowman's capsule, Bowman's capsule,
which is the first part of the nephron to actually
collect fluid that's going to be filtered to become urine,
and in this sense, the kidneys are pretty amazing.
Do you know in desert rats, these are animals
that may never see free water in their entire life,
because they live in the desert, their kidneys
are so good at concentrating urine and absorbing water
that sometimes their pee can actually be solid crystal.
That sounds pretty painful,
but it works for the desert rodents.
Let's figure out how our nephrons work for us.
After the glomerulus, the next part of the nephron is
this guy right here, which is pretty convoluted,
wouldn't you agree, but it's close to the glomerulus,
so, we name it according to those features.
It's proximal to the glomerulus, so we call this a proximal,
and because it's so windy, we call it convoluted.
It's the proximal convoluted tubule,
proximal convoluted tubule.
Kind of a mouthful, but these words
perfectly describe what it is.
The proximal convoluted tubule is very important
for reabsorbing ions, like sodium and chloride,
but also some of our other builders of macromolecules,
like amino acids and even glucose,
and remember when we're absorbing things like this,
and especially with sodium,
we're going to take water with it as well.
So, water is reabsorbed at the
proximal convoluted tubule too.
In fact, I've read somewhere that the
proximal convoluted tubule reabsorbs about
65 percent of all of these important nutrients.
That's the most of anywhere else in the nephron
that we're going to talk about.
So, it's pretty important.
Where does the nephron go next?
Well, it actually becomes this loop right here.
It descends deep into the kidney,
and then it ascends back up again.
This entire structure is affectionately called
the loop of Henle, the loop of Henle,
and it's got two limbs to it, as I've shown here,
and they're going in opposite directions.
One is called the descending limb,
descending because it's diving deep into the kidney,
so this is the descending limb, and the other part of it,
this guy right here, is called the ascending limb,
ascending because it rises up, ascending limb,
and the reason why this is significant is because
remember the kidney is kind of broken up
into two main overall parts, right?
There's a part that we call
the renal cortex, it's above here,
and then there's a part we call the renal medulla,
which is down here, and the thing to remember is
that the renal medulla is very salty,
very salty because we have a lot of
ion reabsorption happening here.
Now, I should further specify that
the descending limb and the ascending limb
of the loop of Henle reabsorb very separate things.
The descending limb reabsorbs water,
so we have mainly water coming here, and, in fact,
there are no ions that are reabsorbed at this point.
It is impermeable to ions.
On the other hand, the ascending limb
does the exact opposite.
Here, we reabsorb things like sodium, chloride, potassium,
and, in fact, this part is impermeable to water.
No water will be reabsorbed here in the ascending limb,
and because of this, we have
a very beautiful system that occurs as a result.
This is called countercurrent multiplication,
countercurrent multiplication, which is also, I'll admit,
a mouthful, but it completely makes sense, I promise.
Countercurrent multiplication.
We say countercurrent because the descending limb
and the ascending limb go in opposite directions.
That's why it's countercurrent.
Multiplication means that when we reabsorb ions
in the ascending limb here and make the medulla salty
by not reabsorbing water, that drives water to be
reabsorbed passively in the descending limb,
and we have a video that goes into detail
about transport processes in the nephron,
but here just remember that water is reabsorbed passively,
so no energy is expended to reabsorb water,
and this is because we have used energy
here in the ascending limb to reabsorb these ions.
So, active transport is used here,
and by actively pumping ions into the medulla,
and no water in the ascending limb to make it salty,
we can multiply the amount of water that is
reabsorbed passively, because it's driven into this
space around the tubule or the nephron.
The space around the tubule is just called interstitium.
I'll write that off right here.
So, this is the interstitium.
This is anything that is not the tubule or vessels,
and that's just space around here.
This is all just interstitium,
just hanging out right here, and so all this ions
that are reabsorbed into the medullary interstitium
down here, the interstitium of the medulla,
drive the passive reabsorption of water.
All right, so I think we have a pretty good understanding
of the loop of Henle and the countercurrent multiplication
process that happens here.
The next part of the nephron is this guy,
that kind of loops back and just kisses
the glomerulus right there, and I'm sure you have
astutely noted that just like the proximal convoluted tubule
this tubule is also certainly very convoluted.
So, it's a tubule that is convoluted, let's say,
but it's not as close to the glomerulus
as the proximal convoluted tubule was.
So, instead, we shall call this guy
the distal convoluted tubule, and this dude
is responsible for the reabsorption of other ions,
like sodium and chloride, and it helps to just pick up
more of these important nutrients that we'd like to have
in our bloodstream, that we don't want to pee away.
The other thing I should mention now,
that I promise we go into more depth in
in a separate video, is this very scientific kiss
that happens here, when the distal convoluted tubule
comes by the glomerulus again.
This produces something that's called,
and this is a mouthful,
the juxtaglomerular, juxtaglomerular apparatus,
juxtaglomerular apparatus, and all this is responsible for
is to control blood pressure.
This is part of the kidney that's used
to control blood pressure, and we'll talk about this
in detail in a separate video.
So, now that the distal convoluted tubule's
come up here and kissed the glomerulus
and kind of come out here, it's time to collect whatever
leftover fluid we have, and we do so in this guy right here.
This is called the collecting tubule or the collecting duct.
So, it collects things that we have left over
in the lumen, or inside of this nephron,
and one thing to note is that there are many
DCT's, or distal convoluted tubules, that feed into this
single collecting duct.
So, there is a DCT, there is a DCT, and there is
another one down here, right?
And we actually reabsorb a couple of things
in the collecting tubule as well.
The main thing that we reabsorb into our interstitium
is water, and another thing we reabsorb,
that I'll show deep in the medulla right here, is urea.
Urea is one of the main waste components
that we actually pee away, but sometimes
the kidneys like to hold onto urea to increase
the osmolarity in the medulla, to help drive
water reabsorption in the loop of Henle.
This goes into a process that's called urea recycling,
if you've heard of that term before,
but we're not going to go into detail for right now on that.
Instead, we'll just mention here that
urea is reabsorbed to maintain osmolarity,
maintain the osmolarity in our medulla that will help
drive water reabsorption in the loop of Henle.
And, finally, I want to close the loop on what happens
to this poor efferent arteriole right here,
because we came off the afferent arteriole,
and this I promised you would turn into a capillary
and then a venule, and here's where I'm going to
talk about that, because we've reabsorbed
all of these awesome things here in blue,
but we don't have a way to put them
into the bloodstream yet.
Well, the efferent arteriole gives us a way to do that,
and it does so by kind of coming off this way,
and just like all good arterioles, it branches off
into even smaller branches, so much so that we
branch off into smaller capillaries,
and these capillaries will dance across our nephron
and collect all this good stuff that we've talked about,
here in blue, that gets reabsorbed
into our interstitium, and I should mention that because
these capillaries kind of hang out all over the place,
where our tubules are, we say that they are peritubular,
peritubular meaning just around the tubule,
so, sure enough, their official name is
peritubular capillary, or we call them all
peritubular capillaries, and so after we've
collected our nutrients in these peritubular capillaries,
we come back here, where we then start doing
the exact opposite, because now we've lost our oxygen
but we've reabsorbed these nutrients
into our bloodstream, and then this will
kind of come back together and head off into,
I think you guessed it, the renal vein,
and the renal vein will then take this back to the
rest of the body, and that leaves the rest
of what we've collected here in our tubules,
in our lumen right here, that goes away,
and this is going to become our urine.
Our collecting tubule is where we first have urine
that's going to be sent off into our renal calyces
and then further on to be peed away.