Lesson video

This lesson is called Controlling water balance: the kidneys and nephron structure and is from the unit Coordination and control: maintaining a constant internal environment.

Hi there, my name's Mrs. McCready and I'm here to guide you through today's lesson.

So thank you very much for joining me today.

In our lesson today, we're going to describe the role of the kidneys in filtering the blood to control the water balance of the human body, and we're going to describe the structure of the nephron.

Now, we're going to come across a good number of keywords in our lesson today, and they're shown up here on the screen for you now.

You may wish to pause the video to make a note of them, but I will introduce them to you as we come across them.

So in our lesson today, we're going to have a look at the function of the kidneys and how they maintain water balance.

So are you ready to go? I certainly am.

Let's get started.

So we know that the blood transports nutrients and waste products around the body, and waste products are what we're going to focus on in our lesson today.

Now these include carbon dioxide and urea.

Urea is created in the process of breaking down excess amino acids.

We're also going to consider excess water and mineral ions as part of waste in our lesson today.

And by excess mineral ions, sodium would be one of those.

Now, waste products must be removed from the body because they are harmful, and if left to remain for a long time, they can cause us real damage.

Now, carbon dioxide is excreted via the lungs as we exhale, so that's very straightforward to get rid of.

But the other products require a bit more processing.

So other products in the blood plasma have to be removed by the kidneys, and these include excess water and ions and all of the urea in the bloodstream.

So let's quickly check our understanding.

Which of these waste products is excreted via the kidneys? Is it excess ions, urea, or carbon dioxide? I'll give you five seconds to think about it.

Okay, so you should have chosen that the waste products excreted via the kidneys include excess ions and urea.

Well done.

Next, I'd like you to consider that urea is formed from the breakdown of which products? Is it glucose, carbon dioxide, or amino acids? Again, I'll give you five seconds to think about.

Okay, so you should have chosen that urea is formed by the breakdown of amino acids.

Well done.

Now, the kidneys are organs which belong to the excretory system.

The excretory system includes the kidneys, the ureter, which connects the kidneys to the bladder, and then the urethra, which connects the bladder to the outside world.

So this is the excretory system.

Now, kidneys filter blood as it passes through them and by doing so, it removes glucose, water, mineral ions, and urea from the blood plasma.

So blood is being passed through the kidneys and all of these things are being taken out of the blood plasma by the kidneys.

Now, blood cells such as white and red blood cells and large proteins cannot leave the bloodstream via this filtration process because they are too big.

So the kidneys are filtering small molecules out of the blood.

So the kidney has several discrete parts to it.

So there's the main body of it, which is the kidney itself.

There's the renal artery, which supplies blood to the kidney.

And the renal vein, which takes blood away from the kidney.

And then collecting ducts, which join up and join up and join up to eventually form the ureter, which takes urine down to the bladder.

So blood is entering the kidney via the renal artery.

The kidney is then filtering that blood, and glucose, ions, water, and urea leave the blood into the kidney tubules.

Then waste products including urea, excess water, and excess mineral ions are moved to the bladder via the ureter to form urine.

And then the filtered blood leaves the kidney via the renal vein.

So there are lots of functions within the kidney, lots of distinct parts within the kidney with this very step-by-step process that is going on with blood entering, being filtered.

And by filtering, we are talking about the removal of components from the blood plasma, the waste products being filtered off and taken down to the bladder for storage.

And then the clean blood, the filtered blood being passed back into the bloodstream and circulated around the body again.

Now I've said that the kidneys filter out all of the glucose from the blood.

That is true, but it then reabsorbs all of the glucose back into the blood by processes of diffusion and active transport.

So all of the glucose moves from the renal artery into the kidney tubule and then is absorbed back again into the renal vein, back into the blood plasma.

So that's true for glucose.

As far as other components of plasma are concerned, the amount of water and mineral ions that are reabsorbed varies.

And this is called selective reabsorption because the kidney can select to reabsorb water or not.

It can select to reabsorb mineral ions or not.

But it cannot select to reabsorb glucose because all glucose is reabsorbed from the kidney tubules back into the blood.

It is only water and mineral ions which have selective reabsorption applied to them by the kidneys.

So let's zoom in a little bit closer.

Now, each kidney contains about one million tubules, and these tubules, nephrons, are what are doing the filtering and that selective reabsorption.

So a kidney tubule is made up of a number of parts to it.

The glomerulus and the Bowman's capsule are at the start.

They then lead to the proximal convoluted tubule, convoluted meaning really bent and twisty.

Then the loop of Henle, which is a really long loop.

And there's also the collecting ducts, which are where the waste products drain into.

So let's consider this process in more detail.

So the blood is entering the kidney via the renal artery, and that means that the kidney can filter the blood.

So the blood enters at the glomerulus and the Bowman's capsule, and these filter glucose, ions, water, and urea out of the blood plasma and into the proximal convoluted tubule.

The proximal convoluted tubule is where all of the glucose and some of the ions are selectively reabsorbed.

And the filtrate continues through to the loop of Henle.

And the loop of Henle is where water is reabsorbed.

And then finally, the collecting duct is what collects the urea, the excess water, and the excess ions.

And that forms urine, which drains down to the bladder.

So the glomerulus and the Bowman's capsule are doing the filtering.

The proximal convoluted tubule is reabsorbing all of the glucose and selectively reabsorbing some of the ions.

The loop of Henle is selectively reabsorbing water.

And the collecting duct is gathering up the waste products and sending them down to the bladder.

After filtration, the urine drains through the collecting ducts into the ureter and down to the bladder for storage.

And the filtered blood leaves the kidneys via the renal vein to enter circulation once more.

So let's just check our understanding.

Who has correctly described the function of the kidneys and how would you correct the other mistakes? So Lucas has said, "The kidneys filter urine from the blood and store it in the bladder." Sam says, "Waste products are removed from the kidneys, processed, and reabsorbed." And Izzy says, "Blood enters the kidneys, it is filtered, and waste is stored in the bladder." But who is correct and what are the mistakes? I'll give you five seconds to think about it.

Okay, so you should have said that Izzy is correct.

Now Lucas has said that the kidneys filter urine, whereas he should have said that the kidneys filter urea.

And Sam has said that waste products are removed from the kidneys and processed and reabsorbed, and she should have said that some molecules are removed by the kidneys, scratch the processed part out of the sentence entirely, and said that they are reabsorbed.

So did you get all of those changes correct? Well done if you did.

So what I'd like you to do now is to firstly state the function of the kidneys and the bladder, and then I would like you to complete the flowchart to show how the kidneys filter products from the blood.

Once you've done both of those things, I'd like you to consider this table, which shows a number of substances and their concentration in grammes per decimeter cubed in the plasma, the filtrate, and the urine.

So take a look at that data in detail, and then I would like you to explain why protein is not found in the urine, and explain why glucose is found in the filtrate but is not found in the urine.

So pause the video and come back to me when you are ready.

Okay, let's see what you've written.

So I firstly asked you to state the function of the kidneys and the bladder.

So you should have said that the kidney filters all the urea and excess ions and water from the blood, and the bladder stores urine until we are ready to urinate.

Then I asked you to complete the flowchart to show how the kidneys filter products.

And you should have said that blood enters the kidneys via the renal artery.

The kidney tubules filter glucose, water, ions, and urea from the blood.

That urine is stored in the bladder until we are ready to urinate.

That some molecules and ions are selectively reabsorbed back into the blood.

And that cleaned blood leaves the kidneys via the renal vein.

Just check over your work really carefully to make sure you've got the functions of those different parts of the kidney and the process that happens with the kidneys correct.

Well done.

That was quite complicated.

And then I asked you to consider the data in this table and then explain why protein is not found in the urine.

So we can see that protein is present within the plasma, but it is not in the filtrate and it is not in the urine.

And this is because protein molecules are too big to be filtered by the kidney.

I also asked you to explain why glucose is found in the filtrate but not in the urine.

So we can see that glucose is in the plasma, is in the filtrate, but none of it is present in the urine.

And you should have said that that is because glucose is filtered out of the blood and then all of it is selectively reabsorbed back into the bloodstream.

So did you get those answers correct based on the data? Well done if you did.

Now that's all really tricky content, so well done indeed.

So let's have a look at how the kidneys maintain water balance now.

Now we've seen how water can be selectively reabsorbed to maintain the water balance in the body.

So in conditions where there is insufficient water, the water balance is low, more water can be reabsorbed.

Whereas in conditions of excess water where the water balance is high, higher than the optimum, more water can be excreted.

And this is really important because maintaining a constant water balance in the body is part of homeostasis, which is part of maintaining a constant internal environment.

And it is essential that homeostasis is maintained because only by maintaining constant internal environments within our cells can the cells function successfully and biological processes carry on in the way that they are expected and required in order to make our body work as we need it to.

So water can be selectively reabsorbed by the kidneys.

True or false? So you should have said that that is true, but why? Can you think of an explanation? So you should have said that water can be selectively reabsorbed by the kidneys to maintain homeostasis, which is a constant water balance in the body.

Well done if you got that correct.

Now, if we've lost lots of water through sweating and maybe we've not drunk very much, maybe it's been very warm or maybe we've been doing a lot of exercise, then the overall balance of water in our body will decrease.

So what this means is that the kidneys will reabsorb most of the water that it filters out.

So it takes the water out of the blood and then it reabsorbs it back into the blood.

This means that less water will be collected as urine.

So urine will be concentrated, it will be dark in colour, it'll have a dark yellow or orangey colour, and it will also have a strong smell.

And I'm sure you've all experienced that.

So that's what happens when we've lost lots of water and not replaced it sufficiently well enough.

Conversely, if we've drunk plenty of water, if maybe it's quite cool, we've not been really doing very much, we've not lost a lot of it through sweat, for instance, then the overall water balance in our body will increase.

So what this means is that the kidneys will only reabsorb some of the water it filters out.

So it's taking out lots of water from the blood plasma and then it will only reabsorb a little portion of that.

So what that means is that more water will make its way down the collecting ducts into the bladder as urine.

And because there is more water in urine, the urine will be dilute.

It will have a pale colour, a pale yellow colour, and a very weak smell.

And again, I'm sure you are all familiar with urine that has those features.

So this is what happens when we've drunk lots and not lost it through sweating.

So if we haven't lost it through sweating, we need to lose it in another way, and in that way we lose it via our urine.

So let's just quickly check our understanding.

If you have not drunk much water during the day, how will your urine look when you go to the toilet? Will it A, be light in colour and have little smell? Will it be B, dark in colour and smell strongly? Or will it be C, yellow in colour with no smell? I'll give you five seconds to think about it.

Okay, so if you haven't drunk much water during the day, you should notice that when you go to the toilet, your urine will be dark in colour and smell strongly.

Well done if you got that correct.

So what I'd like you to do now is to complete the table to summarise how the kidneys respond to different water balances in the body and the effects that this has on urine production.

Then when you've completed that table, I would like you to consider this scenario.

So the school students have been involved in a sports event on a hot July day.

Maybe you've been involved in one of those too.

So they've drunk some water, but they've worked hard, they've become very hot, and they've sweated a lot.

So what I would like you to do is to describe the changes in the water balance in the body during the day and the response of the kidneys to this change in internal environment.

So pause the video and come back to me when you are ready.

Okay, let's see what you've written.

So I asked you firstly to complete the table to show what happens if you've not drunk much or if you've drunk plenty.

So you should have said that the response of the kidneys, if you haven't drunk much, is to reabsorb more water.

Whereas if you've drunk plenty, it is to excrete more water.

That the volume of urine produced if you haven't drunk much will be small.

Whereas if you have drunk plenty, there will be a large volume of urine produced.

Now, if you haven't drunk much, the concentration of urine produced will be concentrated.

Whereas if you have drunk plenty, it will be dilute.

And the colour of urine, if you've not drunk much, will be dark yellow or orange.

Whereas if you have drunk plenty, it will be pale yellow.

And the smell of urine, if you've not drunk much, will be strong.

Whereas if you've drunk plenty, it will have a weak smell.

So just check your answers over, make sure you've got those responses around the right way.

Then I asked you to consider the scenario of the students involved in a sports event on a hot July day and what changes will be happening in the water balance and how will the kidneys respond to this.

So you should have written that as the day passes, the water balance reduces, so there is less water in the body.

And this is because less water has been drunk and water has been sweated out because of all of the activity that they've been doing.

So what this means in terms of a response by the kidneys is that the kidneys will reabsorb greater quantities of water during the day so that as little water as possible is excreted.

And ultimately, they'll have very concentrated, strong, dark urine as a result.

Just check over your answer, make sure you've got the correct points, you've got it round the right way.

And well done again on both of those tasks.

So we've come to the end of our lesson today, and in our lesson today we've seen that kidneys are organs in the excretory system and they filter waste products from the blood plasma to produce urine.

So water and urea diffuse out of the blood plasma into the kidney tubules and are excreted as urine, and some water is reabsorbed to maintain water balance in the body depending on need.

Now, urine can become concentrated, a dark yellow/orange colour and have a strong smell if we consume insufficient quantities of water.

However, the opposite is true when lots of water has been drunk.

So I hope you've enjoyed our lesson today.

Thank you very much for joining me and I hope to see you again soon.

Bye.