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27 Nov 2025
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WJEC A-Level Biology: Notes on Homeostasis and Kidneys (Unit 3.7)
Caitlin Ellis
@caitlinellis0312
1 / 4
Kidney Structure and Homeostasis Basics
Homeostasis is your body's way of maintaining a constant internal environment, no matter what's happening outside. Think of it like a really sophisticated thermostat that controls not just temperature, but water levels, salt concentration, and waste removal too.
Your kidneys have two main regions doing different jobs. The cortex (outer layer) handles the initial filtering and reabsorption of useful stuff like glucose and water. The medulla (inner layer) is where your urine gets concentrated and osmoregulation (water balance control) happens.
Each kidney contains millions of tiny filtering units called nephrons. These are perfectly designed for their job with loads of surface area thanks to microvilli, plenty of mitochondria for energy, and tight connections to prevent any leakage.
Key Point: Negative feedback keeps everything balanced - when something changes, your body automatically reverses that change to get back to normal.
Ultrafiltration and Selective Reabsorption
The glomerulus is basically a high-pressure filter system that's incredibly clever. Blood enters through a wide tube (afferent arteriole) but leaves through a narrower one (efferent arteriole), creating massive pressure that forces filtration.
This three-layer filter lets small molecules like water and glucose through whilst keeping the big stuff (red blood cells, proteins) in your blood where they belong. Podocytes wrap around the capillaries like octopus tentacles, making the whole system super efficient.
In the proximal convoluted tubule (PCT), your body reclaims everything useful. Water moves back into blood by osmosis, whilst glucose hitches a ride with sodium through special protein channels. This cotransport mechanism is brilliant because it uses less energy than pumping glucose directly.
The loop of Henle creates a concentration gradient that's essential for water conservation. The descending limb loses water, whilst the ascending limb pumps out salt, making the whole system work like a biological desalination plant.
Remember: No glucose should appear in healthy urine - if it does, that's a sign of diabetes.
ADH and Water Balance Control
Antidiuretic hormone (ADH) is your body's water-saving superhero. When your blood gets too concentrated, your hypothalamus detects this and triggers ADH release from the pituitary gland.
ADH travels to your kidneys and opens more aquaporins (water channels) in the collecting duct walls. This means more water gets reabsorbed back into your blood instead of being lost as urine - pretty neat negative feedback in action.
When your kidneys fail, things go wrong fast. Kidney failure leads to waste product build-up and dangerous changes in water and salt balance. High-protein diets can worsen problems by creating more uric acid and kidney stones.
Excess salt consumption puts extra strain on your kidneys by increasing blood pressure and forcing them to work harder during ultrafiltration.
Health Tip: Staying hydrated and limiting processed foods helps keep your kidneys happy and functioning properly.
Treatment Options and Evolutionary Adaptations
Kidney transplants work best when donors are close relatives because similar antigens reduce rejection risk. Recipients need immunosuppressants to stop their immune system attacking the new kidney.
Haemodialysis uses countercurrent flow - your blood and cleaning fluid flow in opposite directions across a membrane. This maximises waste removal whilst keeping useful molecules in your blood. Peritoneal dialysis uses your abdomen's natural membrane as the filter instead.
Different animals have evolved brilliant solutions for waste removal. Aquatic animals produce highly toxic ammonia but dilute it quickly in water. Birds and insects make uric acid crystals that need hardly any water. Mammals like us produce urea as a compromise between toxicity and water conservation.
Desert animals have extra-long loops of Henle for maximum water reabsorption, whilst aquatic animals have shorter ones since water conservation isn't their priority.
Evolution Insight: Your kidney structure reflects your evolutionary history - longer loops mean better water conservation for surviving in dry environments.
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I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
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THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
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This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
WJEC A-Level Biology: Notes on Homeostasis and Kidneys (Unit 3.7)
Caitlin Ellis
@caitlinellis0312
Your kidneys are incredible biological machines that work 24/7 to keep your body's internal environment perfectly balanced. This process, called homeostasis, is essential for your survival and involves complex filtration systems that would put any water treatment plant to shame.
Kidney Structure and Homeostasis Basics
Homeostasis is your body's way of maintaining a constant internal environment, no matter what's happening outside. Think of it like a really sophisticated thermostat that controls not just temperature, but water levels, salt concentration, and waste removal too.
Your kidneys have two main regions doing different jobs. The cortex (outer layer) handles the initial filtering and reabsorption of useful stuff like glucose and water. The medulla (inner layer) is where your urine gets concentrated and osmoregulation (water balance control) happens.
Each kidney contains millions of tiny filtering units called nephrons. These are perfectly designed for their job with loads of surface area thanks to microvilli, plenty of mitochondria for energy, and tight connections to prevent any leakage.
Key Point: Negative feedback keeps everything balanced - when something changes, your body automatically reverses that change to get back to normal.
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Ultrafiltration and Selective Reabsorption
The glomerulus is basically a high-pressure filter system that's incredibly clever. Blood enters through a wide tube (afferent arteriole) but leaves through a narrower one (efferent arteriole), creating massive pressure that forces filtration.
This three-layer filter lets small molecules like water and glucose through whilst keeping the big stuff (red blood cells, proteins) in your blood where they belong. Podocytes wrap around the capillaries like octopus tentacles, making the whole system super efficient.
In the proximal convoluted tubule (PCT), your body reclaims everything useful. Water moves back into blood by osmosis, whilst glucose hitches a ride with sodium through special protein channels. This cotransport mechanism is brilliant because it uses less energy than pumping glucose directly.
The loop of Henle creates a concentration gradient that's essential for water conservation. The descending limb loses water, whilst the ascending limb pumps out salt, making the whole system work like a biological desalination plant.
Remember: No glucose should appear in healthy urine - if it does, that's a sign of diabetes.
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ADH and Water Balance Control
Antidiuretic hormone (ADH) is your body's water-saving superhero. When your blood gets too concentrated, your hypothalamus detects this and triggers ADH release from the pituitary gland.
ADH travels to your kidneys and opens more aquaporins (water channels) in the collecting duct walls. This means more water gets reabsorbed back into your blood instead of being lost as urine - pretty neat negative feedback in action.
When your kidneys fail, things go wrong fast. Kidney failure leads to waste product build-up and dangerous changes in water and salt balance. High-protein diets can worsen problems by creating more uric acid and kidney stones.
Excess salt consumption puts extra strain on your kidneys by increasing blood pressure and forcing them to work harder during ultrafiltration.
Health Tip: Staying hydrated and limiting processed foods helps keep your kidneys happy and functioning properly.
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Treatment Options and Evolutionary Adaptations
Kidney transplants work best when donors are close relatives because similar antigens reduce rejection risk. Recipients need immunosuppressants to stop their immune system attacking the new kidney.
Haemodialysis uses countercurrent flow - your blood and cleaning fluid flow in opposite directions across a membrane. This maximises waste removal whilst keeping useful molecules in your blood. Peritoneal dialysis uses your abdomen's natural membrane as the filter instead.
Different animals have evolved brilliant solutions for waste removal. Aquatic animals produce highly toxic ammonia but dilute it quickly in water. Birds and insects make uric acid crystals that need hardly any water. Mammals like us produce urea as a compromise between toxicity and water conservation.
Desert animals have extra-long loops of Henle for maximum water reabsorption, whilst aquatic animals have shorter ones since water conservation isn't their priority.
Evolution Insight: Your kidney structure reflects your evolutionary history - longer loops mean better water conservation for surviving in dry environments.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user