If you remember, I told you in the previous video that for urine to be formed, the first thing that needs to happen is the substances from the glomerulus or that coiled blood vessel, it has to be forced out and some of those substances will have to enter the Bowman's capsule. Now, that particular part of the process is known as ultrafiltration. And ultrafiltration is a very important process that happens between the glomerulus and the Bowman's capsule as I'm circling over here in green.
What I'm going to do here is I'm going to take out the glomerulus and also the Bowman's capsule as you can see over there. It's quite simple. Now, and the blood vessel represents, there are two blood vessels. One is going towards the glomerulus and one is going out of the glomerulus. Now, what exactly happens during alpha filtration?
Now, in alpha filtration, as I've mentioned, the blood pressure is very high within the glomerulus. And this is important because you want to force the substances into the Bowman's capsule. You want to push it out into the Bowman's capsule. When you see the word alpha filtration, I want you to break down that word alpha filtration into two. ultra and also filtration.
In this case, ultra just refers to the high pressure within the glomerulus. Filtration, which comes from the word filter, what exactly does that mean? It just means to separate some substances.
Because when you think about it, the glomerulus contains blood. It is transporting blood. Do you want everything from the blood to go into the Bowman's capsule?
No, you don't. You just want to remove some of the substances. So that is why it's called filtration in this case.
Now, as you can see in this drawing here, I'm just drawing and I'm just kind of making an oversimplified diagram. I know the glomerulus is supposed to be coiled. I get it, but I'm just drawing a simpler blood vessel over here. In reality, the glomerulus has to be coiled, by the way, and the pressure in the glomerulus has to be very high.
And in that case, it is able to push substances out such as water, salts, urea, glucose, amino acid and other wastes. But what about the red blood cells which are represented in the red colored dots within the blood vessel? Do the red blood cells also get forced outwards?
No, they don't. The red blood cells do not get filtered out. So that's the filtration element.
So only some things go out of the glomerulus, not everything. There are two very important questions that we have to talk about when we are covering alpha filtration. The first important question is how is the high pressure in the glomerulus generated and the second question is what filters are present between the glomerulus and the Bowman's capsule which prevents everything from being forced outwards. This is what is important for you to know for A levels. So For the first question, how exactly is the high pressure generated in the glomerulus?
Remember, the glomerulus is that tightly coiled blood vessel and blood goes into the glomerulus through the afferent arteriole and it goes up through the efferent arteriole. Some students will get confused between afferent and efferent. What I like to remember is A, B, C, D, E. So A is first and then E is later.
That's how I remember it. So, I mean, that's just the way I've remembered it for many, many, many, many, many years. So you have to try to find a way to remember it as well. Okay, so as we can see over here, the blood goes from the afferent arteriole towards the glomerulus and then it goes out through the efferent arteriole. So I'm just going to draw like a slightly more complicated diagram here, the efferent arteriole.
efferent arteriole together with the glomerulus quite tightly coiled. Now, I want you to notice something very interesting here. Notice the diameter of the efferent arteriole and the efferent arteriole. What do you notice over there?
The diameter of the efferent arteriole is much larger than the diameter of the efferent arteriole. The reason why this is important is when blood is going towards the glomerulus, it becomes much smaller, so it has much... much smaller space. So this causes pressure inside the glomerulus to build up because the blood is trying to move through that very tightly cramped up space. And the efferent arteriole is also not making it easier because the efferent arteriole's diameter, yes, it's getting a bit bigger, but it's still much smaller than the efferent arteriole.
So in this case, it causes a backlog of blood or the blood has to move through a tightly packed space. which creates that very high pressure within the glomerulus. This is good because when the pressure is very high inside the glomerulus, it forces the substance out of it. So long story short, in this case over here, if a question asks you, how is the high pressure in the glomerulus generated? The high pressure is generated by allowing the diameter of the afferent arteriole to be higher than the diameter of the efferent arteriole.
That's basically what you just have to mention for the exam. So now we know that the pressure inside the glomerulus is very high due to the difference in the diameter of the arterioles and it forces the substance out. But just because it forces the substances out doesn't mean we want everything to go out of the blood.
We only want certain things to go out. That's why it's called filtration. Therefore, we need to ask the next question. What filters are present during alpha filtration?
Because think about it, you have the afferent arteriole and the pink color represents the blood and in between the coiled part, the curved area, that's the glomerulus and a simplified structure of the glomerulus. And look at the wall of the glomerulus. In reality, the glomerulus is actually a special type of capillary.
It does have capillary characteristics. One cell thick, it has that flat endothelium. And the lumen is quite narrow as well.
And look at the wall. The walls of the glomerulus have those tiny little gaps. Why are those tiny little gaps important? Because look at the red dots. The red dots are the red blood cells.
Can they go out of the tiny little gaps? No, they cannot because the gaps are too small for the large red blood cells to pass through. The tiny gaps are to prevent the red blood cells, white blood cells, or platelets from leaking outwards. So that's the first filter we have.
But that's not good enough. We need to have another type of filter, which I've represented in that green color line, and that filter is called the basement membrane. So what exactly is the function of the basement membrane?
The basement membrane, by the way, is just a network of glycoproteins. You don't technically need to know that in detail. You just have to know it's called the basement membrane. Now, in reality, your blood contains not just blood cells, it contains the liquid called plasma. And within the liquid, you also have something called plasma proteins, which are quite large.
Examples of plasma proteins will be things like albumin and also fibrinogen. Now, As you can see here, because of the pressure, it might force the plasma protein outwards. So the plasma protein, as you can see, it can pass through the gaps, all right? But do we want those to be filtered outwards? No, because proteins are quite important and it is imperative for those proteins to remain inside the blood as well, all right?
Even though they can pass through the tiny gaps of the glomerulus, the basement membrane provides another barrier to prevent... the proteins from passing through. So they have no choice but to re-enter the blood. So the function of the basement membrane is to just prevent very large proteins from leaking out of the glomerulus. Examples, plasma proteins.
Sometimes in the Mark scheme, you will see, they will mention it there, that the function of the basement membrane is to prevent proteins which have... molecular mass of 67,000 and higher from being filtered out. You don't have to memorize that 67,000 molecular mass value. You just have to say prevents very large proteins from leaking out of the glomerulus.
That's good enough. All right. So you see, even though the pressure in the glomerulus is very high, not everything can go out because the tiny gaps prevent the blood cells from leaking out, like red blood cells.
white blood cells and platelets, and the basement membrane prevents large proteins from leaking outwards. So then after that, remember, whatever substances that are forced out will then enter the Bowman's capsule. But the Bowman's capsule themselves, which are these cup-like structures, they themselves have a one-cell thick wall. And the inner wall of the Bowman's capsule, which is forming the kind of mouth, that inner wall is known as the podocytes. And the podocytes are these weird structures that have tiny little gaps to allow filtrate to leak through.
So whatever substance that leaks out through the glomerulus into the Bowman's capsule, that is referred to as something called the glomerular filtrate. So therefore, how many filters do we have present over here? We have three filters present. Tiny gaps of the glomerulus wall.
to prevent red blood cells, white blood cells, and platelets from leaking out, basement membrane, which prevents very large proteins from leaking out, and podocytes, which allows the filtrates to pass through and enter the lumen of the Bowman's capsule. So as you can see here, I'm just drawing out a section of the glomerulus and also the Bowman's capsule. If you can, I hope you can appreciate that drawing.
I hope you understand that that yellow box is just, so I'm just zooming in on what we are supposed to see in that yellow box. Okay. So you have those tiny gaps of the glomerulus.
Okay. And the red blood cell represented by the red dot, the green color line. What is that? That is the basement membrane.
Now If the podocytes were not present, look here, the plasma, too much filtrate will be able to pass, well, too much filtrate will leak outwards. And sometimes we just don't want too much filtrate to leak out. So that is why the podocytes are also present, because with the help of the podocytes, it only allows the liquid to leak through that tiny little gaps. So it limits the amount of filtrate that passes through and enter the lumen of the Bowman's capsule.
The lumen of the Bowman's capsule is just the, you know, the space within the Bowman's capsule. That's what it is. All right. So these are the three filtrates that we actually have over here to make sure that only some substances are forced out from the glomerulus into the Bowman's capsule.
So I hope you understand this part.