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M
Marisa Smith
14/12/2025
Biology
Biology Excretion Revision Notes
966
•
14 Dec 2025
•
A-Level OCR A Biology: Excretion Notes
M
Marisa Smith
@marisasmith_qauu
Excretion and the Liver's Role
Excretion is simply your body's way of removing metabolic waste - think of it as taking out the rubbish at a cellular level. The main culprits are carbon dioxide from cellular respiration and nitrogenous waste that ends up in your urine.
Your liver is basically a multi-tasking powerhouse that handles four key jobs: converting glucose into glycogen for storage, breaking down amino acids into urea, detoxifying your blood (alcohol, drugs, hormones), and producing bile. When you have excess amino acids, they can't just hang around because they're nitrogenous substances that would damage your body.
Deamination is the process where the liver removes the amine group from amino acids to form ammonia. What's left becomes a keto acid that can be respired or converted to glycogen. However, ammonia is too toxic for mammals to excrete directly, so the ornithine cycle converts it into urea using carbon dioxide. This urea then dissolves in your blood plasma and travels to your kidneys for excretion.
💡 Key Insight: Think of your liver as a chemical processing plant - it's constantly converting dangerous waste into safer forms your body can actually get rid of.
Liver Structure and Kidney Basics
Hepatocytes radiate from a central vein to form liver lobules, which make up your entire liver. Blood flows through sinusoids (specialised capillaries) past these hepatocytes, whilst Kupffer cells act like security guards, removing bacteria and broken red blood cells.
Your kidneys have two main regions: the outer cortex (which secretes aldosterone to control blood pressure) and the inner medulla (which secretes adrenaline to increase heart rate). Blood enters via the renal artery, gets filtered by nephrons through ultrafiltration and selective reabsorption, then exits via the renal vein.
The loop of Henle is where water reabsorption gets clever. Sodium and chlorine ions are actively pumped out to create a concentration gradient, the ascending limb stays impermeable to water, and the descending limb allows water to move out via osmosis. This creates a counter-current system that maximises water recovery.
Kidney Function and Control
Ultrafiltration happens in Bowman's capsule, where the glomerulus (a bundle of capillaries) filters small molecules like glucose, urea, water and salts from your blood under pressure. The system has three key adaptations: endothelium with gaps for substances to pass through, a basement membrane that acts like a fine mesh filter, and podocyte cells with finger-like projections.
Selective reabsorption occurs mainly in the proximal convoluted tubule (PCT), where useful substances like glucose get actively transported back into your blood. The PCT cells are covered in microvilli to maximise surface area - it's like having a massive sponge to soak up everything valuable.
Osmoregulation is controlled by ADH (antidiuretic hormone), which increases water reabsorption when your blood water levels drop. This is a classic negative feedback system - low water triggers ADH release, which increases water reabsorption, which raises blood water levels again.
💡 Exam Tip: Remember that kidney failure is diagnosed by measuring glomerular filtration rate (GFR) - the faster blood is filtered, the healthier your kidneys are.
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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
A-Level OCR A Biology: Excretion Notes
M
Marisa Smith
@marisasmith_qauu
Your body is constantly producing waste that needs to be removed - from the carbon dioxide you breathe out to the urea in your urine. Understanding how your liver and kidneys work together to keep you healthy is crucial for... Show more
Excretion and the Liver's Role
Excretion is simply your body's way of removing metabolic waste - think of it as taking out the rubbish at a cellular level. The main culprits are carbon dioxide from cellular respiration and nitrogenous waste that ends up in your urine.
Your liver is basically a multi-tasking powerhouse that handles four key jobs: converting glucose into glycogen for storage, breaking down amino acids into urea, detoxifying your blood (alcohol, drugs, hormones), and producing bile. When you have excess amino acids, they can't just hang around because they're nitrogenous substances that would damage your body.
Deamination is the process where the liver removes the amine group from amino acids to form ammonia. What's left becomes a keto acid that can be respired or converted to glycogen. However, ammonia is too toxic for mammals to excrete directly, so the ornithine cycle converts it into urea using carbon dioxide. This urea then dissolves in your blood plasma and travels to your kidneys for excretion.
💡 Key Insight: Think of your liver as a chemical processing plant - it's constantly converting dangerous waste into safer forms your body can actually get rid of.
Liver Structure and Kidney Basics
Hepatocytes radiate from a central vein to form liver lobules, which make up your entire liver. Blood flows through sinusoids (specialised capillaries) past these hepatocytes, whilst Kupffer cells act like security guards, removing bacteria and broken red blood cells.
Your kidneys have two main regions: the outer cortex (which secretes aldosterone to control blood pressure) and the inner medulla (which secretes adrenaline to increase heart rate). Blood enters via the renal artery, gets filtered by nephrons through ultrafiltration and selective reabsorption, then exits via the renal vein.
The loop of Henle is where water reabsorption gets clever. Sodium and chlorine ions are actively pumped out to create a concentration gradient, the ascending limb stays impermeable to water, and the descending limb allows water to move out via osmosis. This creates a counter-current system that maximises water recovery.
Kidney Function and Control
Ultrafiltration happens in Bowman's capsule, where the glomerulus (a bundle of capillaries) filters small molecules like glucose, urea, water and salts from your blood under pressure. The system has three key adaptations: endothelium with gaps for substances to pass through, a basement membrane that acts like a fine mesh filter, and podocyte cells with finger-like projections.
Selective reabsorption occurs mainly in the proximal convoluted tubule (PCT), where useful substances like glucose get actively transported back into your blood. The PCT cells are covered in microvilli to maximise surface area - it's like having a massive sponge to soak up everything valuable.
Osmoregulation is controlled by ADH (antidiuretic hormone), which increases water reabsorption when your blood water levels drop. This is a classic negative feedback system - low water triggers ADH release, which increases water reabsorption, which raises blood water levels again.
💡 Exam Tip: Remember that kidney failure is diagnosed by measuring glomerular filtration rate (GFR) - the faster blood is filtered, the healthier your kidneys are.
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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
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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
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David K
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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 😍😁😲🤑💗✨🎀😮
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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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