The good news for selective reabsorption is we don't technically need to know the details of the selective reabsorption in the loop of Henle and also the DCT. It is just good to be aware that the loop of Henle and DCT will reabsorb some water and salts back into the blood during urine formation, but you don't actually have to go into the detail of that. For Cambridge A-levels, the main focus is just the PCT, which we saw in the previous video. And in this video, we are going to be looking at the selective reabsorption that happens in the collecting duct.
And I did tell you that in the collecting duct, before it forms the urine, reabsorption of water will happen in this portion of the nephron. And I also did mention as well that the collecting duct does something pretty magnificent where it reabsorbs water based on the amount of water in your body which means that if your blood does not have a lot of water your collecting duct will reabsorb more water and if your body has too much water the collecting duct will reabsorb less water so how does it do that so let's get into it When you're talking about the collecting duct, it is very important to also cover a concept known as osmoregulation. Now you need to know the definition of osmoregulation where it is the control of the water potential of the blood and tissue fluid. Some textbooks will just say that osmoregulation is the control of the amount of water inside our body.
Now you see, water is an extremely important thing that you need to survive. We all know that. But too much water can actually be very toxic to us, just like glucose, chocolates, caffeine, friends, right?
Anything that's too much can be quite dangerous for us. And we also know that when there is too little water in our body, it's also quite bad as well. Now, why is it quite dangerous if the amount of water in your body is not optimum or balanced?
Let's take an example over here. We have three situations. We have a body cell. and we also have the blood capillary.
Now, imagine in this blood capillary over here, the water, there's too much water inside the blood. And if there's too much water inside the blood, it causes the blood to have a very high water potential. And in that case, what might happen is the water might enter the cell by osmosis and it may cause the cells to burst. Okay. And we don't want that to happen again, but if it's very little water in the blood, it's...
It will cause the blood to have very low water potential and it will draw water out of the body cell by osmosis and it dehydrates our body cells. And if our body cell shrinks and it has very little cytoplasm, it may not be able to function normally and it might even die in the process. But if the amount of water in the blood is not too high but not too low, it will have an almost equal water potential with the body cell.
So in this case, the body cell is not damaged. The body cell is able to receive some water. It's able to give out some water depending on the situation. So this is the optimum condition or as I like to call it, the Goldilocks situation. The Goldilocks situation, if you are familiar with the fairy tale, is this girl who goes into the three bears house and she eats the porridge.
One porridge is too hot, one porridge is too cold and the third porridge is just nice. So Goldilocks is trying to achieve homeostasis as well. And she's quite a little bit of a thief.
But anyway, moving on. So regulating the blood water potential is an example of homeostasis. Osmoregulation is also an example of homeostasis because you are trying to make sure, you are trying to control the optimum internal environment of our body.
And one example of the internal environment is the amount of water in our blood. And in the first video on homeostasis, I did mention that homeostasis is regulated through a series of steps. it must start with the stimulus detected by receptor, signal is sent to the control center, another signal is sent to the effector and the effector produces a response or a corrective action and the entire thing is called the negative feedback mechanism.
So let's apply this now. As an example here, I'm just drawing out a blood vessel and the blood vessel's water potential changes. Let's say there's too much water inside the blood or too little water inside the blood.
That is referred to as a stimulus. Remember, a stimulus must always be detected by receptors. In this case, the change in the blood water potential is detected by something called osmoreceptors. When you see the word osmoreceptors, these are particular specialized type of cell that will be able to detect whether the amount of water or the water potential of the blood is too high or too low.
Where are these osmoreceptors located? They are located in a very small little organ in your brain. called the hypothalamus.
The hypothalamus, I mean, in my drawing here, it looks a little bit huge, but it's not. It's almost like a peanut-sized organ, by the way, all right? But just because it is small doesn't mean it's not important. So the osmoreceptors in the the hypothalamus will detect osmoreceptors.
Hypothalamus, sounds like I'm rapping. But anyway, so the osmoreceptors in the hypothalamus detects the changes in the blood water potential, and the hypothalamus is your control center. And what the hypothalamus does, okay, here's where it gets a little bit confusing.
The hypothalamus will then send a signal through a neuron to the posterior pituitary gland. The pituitary gland is just basically another organ. that is attached to your hypothalamus, they are also located inside the brain.
Alright, now when they send a signal to the posterior pituitary gland, not the anterior, the posterior, the posterior pituitary gland will have two choices. To release ADH or not to release ADH. Alright, we are going into the Shakespearean route over here. To release or not to release. And a common question, a common mistake that students make here is they always say that the posterior pituitary gland is produce.
produces ADH. That is wrong by the way. In the exam, you have to either write it as release ADH or not release ADH.
So now you are like, okay, what the hell is this ADH however? So let's talk about ADH. I know I'm introducing a lot of important things, but it will all fall into place.
Don't worry. Now, ADH is just antidiuretic hormone. Now, hormones are just chemical signals that travel inside our blood.
Okay. Diuretics are just chemicals that make us produce more urine. For example, caffeine. and such. So, antidiuretic is basically a chemical that makes us produce less urine, okay?
That's what it does. So, antidiuretic hormone is a chemical signal or a hormone that tells the body to reduce urine production, okay? So, like as an example, and caffeine, as I've mentioned here, diuretics are like caffeine, but antidiuretics are things that make makes us produce less urine. So the antidiuretic hormone is essentially just a hormone that tells our body to produce less urine or to reduce the urine production.
So how exactly does the ADH work? So as an example here, I'm just drawing out the blood vessels, okay? You have the, and you also have the hypothalamus. PPG is posterior pituitary gland. You cannot write that in the exam here, by the way.
You have to write it out as its full name. And I'm also just going to... raw part of the nephron, which is the collecting duct, because only the collecting duct is affected by the ADH.
No other parts of the nephron will be affected by the hormone ADH, by the way. So it is just the collecting duct. That is why only that part is involved in this process.
Now, let's take an example. The amount of water in the blood or the water potential in the blood is low, as you can see in the blue color dots. So there's very little water inside the blood.
That's a stimulus. So it is detected by the osmoreceptors. The osmoreceptors in the hypothalamus will send a signal to the posterior pituitary gland and the posterior pituitary gland will release ADH. Now, the posterior pituitary gland is in the brain and the collecting duct is in the kidney.
They are quite far away, but we know that hormones travel through the blood. So the ADH travels inside the blood and it will affect the collecting duct. And in this case over here, the collecting duct is in the anterior pituitary gland.
duct is the effector. So what does it do? Look at the amount of water inside the collecting duct.
It is quite high. So do we want to lose all that water? Do we want all that water to become urine?
No, we don't because there is very little water inside the blood. So we need to increase the amount of water inside the blood. So how do we do that? In that case, the collecting duct becomes more permeable to water, which means to say more water will be reabsorbed. See here.
So when more water is reabsorbed, as an example, a lot of water from the collecting duct goes out and then it goes back into the blood. Look at the amount of water inside the blood right now. It is much higher.
And look at the amount of water left in the collecting duct. It is very low, so less urine is produced. Remember, that's why What ADH does, it reduces the amount of urine. So the urine is more concentrated, which means to say it has more salts and more urea.
So the urine will have like a tea color or like a very brownish amber color. So what happens then if the amount of water in your blood is now very high? Remember I told you if your amount of water or the water potential is very high inside the blood, it may cause your cells to swell and burst.
We don't want that to happen. happen. So same thing, water potential of the blood is high. It is a stimulus detected by osmoreceptors in the hypothalamus.
It sends a signal to the posterior pituitary gland, which will not release ADH or less ADH will be released. So in this case, when there's less ADH released, the collecting duct is not affected and therefore the collecting duct will be less permeable to water. So in this case, less water is reabsorbed because why do we want to reabsorb a blood?
lot of water from the collecting duct. We don't want to reabsorb so much water because the blood already has too much water. So in this case, we can get rid of that excess water from the nephron in this case and therefore urine production will be much, guess what, it will be much higher volume of urine produced, correct, and it will be more dilute. So ADH makes the collecting duct more permeable to water so it reabsorbs more.
more water, but no ADH or less ADH makes the collecting duct less permeable to water. So less water is reabsorbed. So that's how you memorize it. More ADH, collecting duct more permeable to water, more water reabsorbed. But when there's less or no ADH, the collecting duct is less permeable to water, so less water reabsorbed.
That's how you try to remember it. And ADH is an antidiuretic home. hormone. So ADH tells the body to produce less urine and no ADH tells the body to produce more urine.
And the urine, because more water was reabsorbed, the urine will be very low volume and very high concentration. But in the case of the less ADH, less permeable, less water are reabsorbed. So there's more water that becomes urine.
So the urine will have high volume and a low concentration. That means it's dilute. So as you can see here, the ADH can affect the collecting duct by making it more permeable to water.
But how does it exactly do that? So the answer to this is, so I'm going to throw out the collecting duct cells over here, as you can see there. And these are the wall of the collecting duct. Yes, they also look quite weird, just like the PCT cells.
But they don't have microvilli, by the way. But they have a little bit of the basal membrane. And on the left side, it's facing the lumen of the collecting duct. duct and on the right side, the cells are facing the blood capillaries, okay? So it's the same thing.
I'm just magnifying that green color, small green box, okay? So this is what we are seeing. And the pink color that I'm coloring in is just the blood, okay? That's obvious because blood is in the blood capillaries, duh.
All right, now notice the wall of the collecting duct. On their cell surface membrane, they have these receptors. And you guessed it, those are the ADH receptors.
That is why only the collecting duct is sensitive. sensitive to ADH. No other parts of the nephron, the PCT, loop of Henle, the beginning part of the DCT, they do not have ADH receptors. That is why those parts of the nephron do not react to the ADH.
Only the collecting duct does. So if you notice, there's something inside the collecting duct cells that I'm drawing, and those are vesicles. But here's where it's very weird.
Normally, there are substances inside. the vesicle, correct? Like for example, digestive enzymes or antibodies or neurotransmitters. But here's where it's a little bit weird.
There are no substances inside the vesicle, but there are certain things on the vesicle membrane itself. And that thing is known as aquaporin. And aquaporins are just channel proteins for water. So that's weird. It's not inside the vesicle, it's on the vesicle.
vesicle membrane. There's a little bit of a distinction that you have to notice over there. So why is this important? It's because I'm going to draw out, okay, so let's simplify this, all right?
I don't want to draw those, you know, that weird looking cells. So you have the collecting duct wall made out of the collecting duct cells, you have the blood capillaries, and on the lumen side, you have those blue colored dots which are just water. You may be wondering that, hey, there are salts, there's urea, but we don't care about those things.
because the collecting duct is only supposed to reabsorb water all right so in this case here the parts where i'm highlighting is the cell surface membrane of the collecting duct is it very permeable to water no it's not not. The permeability to water is very low because in this case, water might be able to pass through the phospholipid bilayers as we have seen this in chapter 4, but the water cannot do so very easily. The reason is is because water is quite, water is slightly polar, okay? But the fact that water is quite small gives it the opportunity to pass through the phospholipid bilayer, but again, it does so with great difficulty. So in this case, reabsorption of water is extremely low, all right?
However, let's say that there's not enough water inside our blood, all right? That means the blood capillary doesn't have enough water. Again, remember, the stimulus is detected by osmotic receptors, it sends a signal to the posterior pituitary gland and the posterior pituitary gland will release that hormone called ADH.
Now, the ADH travels in the blood and it binds to the receptors on the cell surface membrane of the collecting duct cells. There you go. Now, in chapter four of cell membrane and transport, we talked about the cell signaling.
All right. And in that part of the chapter, I told you that it will activate some enzymes. and produce a second messenger, which begins a cascade reaction, as you can see here in these notes, and then it amplifies the signal.
For the purpose of A2, as you can see here, so this is an example. So it begins a cascade reaction in the cell, and you just have to remember the last part. It produces something called an active phosphorylase enzyme. That's the enzyme that you have to remember for the purpose of the exam.
Hence, I'm highlighting that one like that. Okay, now once it activates that active phosphorylase enzyme, phosphorylase enzyme, it will tell the vesicle, it will cause the vesicles to move to the, what is the vesicle doing? The vesicle, as you can see here, the vesicle is moving to the cell surface membrane.
Now, it's all happening in the same cell, by the way. I'm just showing you a time lapse, all right? It may look like different cells are doing it.
No, it's not. It's the same cell, but just read it like a comic book, okay? The first panel is the cell at the top, the second panel is the cell at the bottom.
So, it's just showing you the cell. sequence of events. So the cell, the vesicles with aquaporin are moving to the cell surface membrane and it fuses with the cell surface membrane.
Now some students will say, oh this is exocytosis. No, this is not. Exocytosis means it releases substances, but there's nothing inside the vesicle, so nothing is being released out of the vesicle. But what is happening is the aquaporin or that channel which was on the vesicle now becomes part of the cell.
surface membrane as I'm highlighting over there so in this case the vesicles fuse with the cell surface membrane and see here look at the cell surface membrane at the top then that I'm circling in red was it permeable to water no it wasn't but look at the one at the bottom that I'm highlighting the collecting duct is now more permeable to water. How is that so? Because it has more channel proteins for water or aquaporins.
Therefore, in this case, the collecting duct is more permeable and it reabsorbs more water. water. And that's what it was supposed to do. The ADH was supposed to reabsorb more water anyway.
Right? And look at the volume of the urine that is left. Whatever that was not reabsorbed in the lumen, that is the one that becomes the urine.
And so the volume of the urine will be very low in this case. So this is good. All right. And eventually, if your blood has too much water, the ADH will detach from the receptors.
As you can see there, the wall was quite permeable. And it causes the cell surface membrane to fold inwards. And when it folds inwards, what happens is...
the 10 to 15 minutes later, all right, the vesicle falls inwards, no more aquaporin on the cell surface membrane and the cell surface membrane is less permeable to water. So less reabsorption of water takes place. So in homeostasis, if we look at this graph here, if you drank too much water, the amount of water in your blood increases.
So it goes above optimum. This is a stimulus detected by, again, osmoreceptors, sensor signal to posterior pituitary gland. It does not release ADH in this case, or less ADH is released.
So less ADH, collecting duct is less permeable to water, less water is reabsorbed, so you produce more urine. And when you produce more urine, you lose more water. So as you lose more water from your body, what happens?
It goes back down to normal. But let's say you are sweating too much. And when you're sweating too much, the amount of water in your blood decreases. So in this case, what happens is the stimulus detected by osmoreceptors, again, in the hypothalamus, sends a signal to posterior pituitary gland, which releases ADH or releases more ADH. And remember, more ADH means collecting that is more permeable to water.
which means more reabsorption of water happens. So as you keep reabsorbing more water back into the blood, it goes back up. But what happens to urine production in this case? Bingo!
In this case, urine production will be very little because you've reabsorbed most of the water back into your blood. So I hope you understand this part of how the kidney does osmoregulation. And with that being said, we are actually done with the kidneys. Thank you.