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Britain & the wider world: 1745 -1901
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Biology
8 Dec 2025
232
5 pages
Rebekah @rebekah_liz
Your kidneys are working 24/7 to keep you alive by filtering out toxic waste and controlling your body's... Show more
Think of your kidneys as your body's ultimate filtration system - they're constantly cleaning your blood and managing your fluid balance. The kidneys perform two crucial jobs excretion (removing toxic waste like urea and creatinine) and osmoregulation (controlling water levels in your body fluids).
Blood flows into each kidney through the renal artery, gets filtered, then leaves via the renal vein. The waste products and excess water form urine, which travels down the ureter to your bladder for storage.
Each kidney has an outer cortex where filtration happens and an inner medulla that's divided into pyramid-shaped sections. The real star of the show is the nephron - the kidney's functional unit that acts as a microscopic filter.
Quick Tip Remember that blood enters the nephron through an afferent arteriole, forms a ball of capillaries called the glomerulus inside Bowman's capsule, then exits via an efferent arteriole.
The Bowman's capsule is where the magic of filtration begins, and it's got three key layers working together. You've got the capillary endothelium with tiny pores, the crucial basement membrane that does the actual filtering, and specialised podocytes with foot-like processes creating filtration slits.
High hydrostatic pressure forces fluid out of the blood to create filtrate. The key equation you need to remember is net filtration force = ψ_plasma - ψ_filtrate. This pressure difference drives the filtration process.
Once filtrate forms, the proximal convoluted tubule (PCT) gets to work on selective reabsorption. About 70-80% of water gets reabsorbed back into the blood, along with useful stuff like glucose, amino acids, and some salts.
Exam Focus The PCT is perfectly adapted for reabsorption with microvilli, mitochondria for ATP, and protein carriers for active transport.
The distal convoluted tubule (DCT) might seem less exciting than other parts, but it's your body's chemistry lab. It actively pumps sodium ions and hydrogen carbonate ions to control your blood's acid-base balance and ionic composition.
This is also where your kidneys get rid of nasty toxic substances like creatinine by secreting them from blood into the filtrate. Think of it as your kidney's final quality control check.
The DCT's cuboidal epithelium comes equipped with microvilli and plenty of mitochondria, just like the PCT. This gives it the power to fine-tune your blood chemistry and maintain that perfect internal environment your cells need.
Remember The DCT raises blood pH when necessary and controls the ionic composition of surrounding blood.
Osmoregulation is your body's way of maintaining perfect water balance, and it's controlled by a hormone called antidiuretic hormone (ADH). The hypothalamus produces ADH, which gets released from the posterior pituitary when your body detects changes in blood concentration.
When you're dehydrated, sweating loads, or eating salty food, osmoreceptors detect the change and trigger ADH release. This hormone increases the permeability of the collecting duct walls, allowing more water to be reabsorbed back into your blood.
The result? You produce a smaller volume of concentrated (hypertonic) urine, conserving precious water. This is a classic negative feedback system - when water levels return to normal, ADH secretion drops.
Real-Life Connection This explains why you produce less urine when you're dehydrated and why it's much more concentrated and darker in colour.
The Loop of Henle is basically your kidney's secret weapon for producing concentrated urine. This U-shaped structure dips deep into the medulla and uses a brilliant mechanism called the countercurrent multiplier.
The descending limb is permeable to water, so water leaves by osmosis as the filtrate becomes more concentrated. Meanwhile, the ascending limb has thick walls packed with mitochondria and actively pumps out sodium and chloride ions, but water can't follow.
This creates a salt gradient in the surrounding tissue - the deeper you go into the medulla, the saltier it gets. When the collecting duct passes through this salty region, water gets drawn out by osmosis, concentrating your urine even more.
Key Concept The countercurrent multiplier allows mammals to produce hypertonic urine, which is essential for surviving in environments where water is scarce.
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.
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Anatomy Unit 1: Directional Terms (superior, inferior, ventral, dorsal, etc.) and Organ Systems
AP BiologyAll notes from AP Classroom slides and videos
AP BiologyAP Bio topic 2.9
AP BiologyReviews homeostasis for AP Bio
AP BiologyHere are some AP Biology Notes for the AP exam
AP BiologyPlant Organs
AP BiologyApp Store
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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
Rebekah
@rebekah_liz
Your kidneys are working 24/7 to keep you alive by filtering out toxic waste and controlling your body's water balance. Understanding how these remarkable organs work will help you grasp one of the most important homeostatic processes in your body.
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Think of your kidneys as your body's ultimate filtration system - they're constantly cleaning your blood and managing your fluid balance. The kidneys perform two crucial jobs: excretion (removing toxic waste like urea and creatinine) and osmoregulation (controlling water levels in your body fluids).
Blood flows into each kidney through the renal artery, gets filtered, then leaves via the renal vein. The waste products and excess water form urine, which travels down the ureter to your bladder for storage.
Each kidney has an outer cortex where filtration happens and an inner medulla that's divided into pyramid-shaped sections. The real star of the show is the nephron - the kidney's functional unit that acts as a microscopic filter.
Quick Tip: Remember that blood enters the nephron through an afferent arteriole, forms a ball of capillaries called the glomerulus inside Bowman's capsule, then exits via an efferent arteriole.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The Bowman's capsule is where the magic of filtration begins, and it's got three key layers working together. You've got the capillary endothelium with tiny pores, the crucial basement membrane that does the actual filtering, and specialised podocytes with foot-like processes creating filtration slits.
High hydrostatic pressure forces fluid out of the blood to create filtrate. The key equation you need to remember is: net filtration force = ψ_plasma - ψ_filtrate. This pressure difference drives the filtration process.
Once filtrate forms, the proximal convoluted tubule (PCT) gets to work on selective reabsorption. About 70-80% of water gets reabsorbed back into the blood, along with useful stuff like glucose, amino acids, and some salts.
Exam Focus: The PCT is perfectly adapted for reabsorption with microvilli, mitochondria for ATP, and protein carriers for active transport.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The distal convoluted tubule (DCT) might seem less exciting than other parts, but it's your body's chemistry lab. It actively pumps sodium ions and hydrogen carbonate ions to control your blood's acid-base balance and ionic composition.
This is also where your kidneys get rid of nasty toxic substances like creatinine by secreting them from blood into the filtrate. Think of it as your kidney's final quality control check.
The DCT's cuboidal epithelium comes equipped with microvilli and plenty of mitochondria, just like the PCT. This gives it the power to fine-tune your blood chemistry and maintain that perfect internal environment your cells need.
Remember: The DCT raises blood pH when necessary and controls the ionic composition of surrounding blood.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Osmoregulation is your body's way of maintaining perfect water balance, and it's controlled by a hormone called antidiuretic hormone (ADH). The hypothalamus produces ADH, which gets released from the posterior pituitary when your body detects changes in blood concentration.
When you're dehydrated, sweating loads, or eating salty food, osmoreceptors detect the change and trigger ADH release. This hormone increases the permeability of the collecting duct walls, allowing more water to be reabsorbed back into your blood.
The result? You produce a smaller volume of concentrated (hypertonic) urine, conserving precious water. This is a classic negative feedback system - when water levels return to normal, ADH secretion drops.
Real-Life Connection: This explains why you produce less urine when you're dehydrated and why it's much more concentrated and darker in colour.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The Loop of Henle is basically your kidney's secret weapon for producing concentrated urine. This U-shaped structure dips deep into the medulla and uses a brilliant mechanism called the countercurrent multiplier.
The descending limb is permeable to water, so water leaves by osmosis as the filtrate becomes more concentrated. Meanwhile, the ascending limb has thick walls packed with mitochondria and actively pumps out sodium and chloride ions, but water can't follow.
This creates a salt gradient in the surrounding tissue - the deeper you go into the medulla, the saltier it gets. When the collecting duct passes through this salty region, water gets drawn out by osmosis, concentrating your urine even more.
Key Concept: The countercurrent multiplier allows mammals to produce hypertonic urine, which is essential for surviving in environments where water is scarce.
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.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
8
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Anatomy Unit 1: Directional Terms (superior, inferior, ventral, dorsal, etc.) and Organ Systems
AP BiologyAll notes from AP Classroom slides and videos
AP BiologyAP Bio topic 2.9
AP BiologyReviews homeostasis for AP Bio
AP BiologyHere are some AP Biology Notes for the AP exam
AP BiologyPlant Organs
AP BiologyApp Store
Google Play
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
Sociology