Overview
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
The structure of the nephron
What is the kidneys’ role?
The kidney is a vital organ and has a key role in:
- Filtering the blood, removing waste products from it
- Maintaining and regulating water potential and electrolyte balance
- Excreting waste products as urine
Structure of kidneys
Mammals have 2 kidneys, each made of 3 main regions:
- Cortex
- Outer region
- This includes: Glomerulus, Bowman’s capsule, the proximal convoluted tubule & distal convoluted tubule
- Medulla (m for middle region)
- Middle region
- Loop of Henle
- Renal Pelvis
- Collecting urine from collecting ducts, passes to ureter
A nephron is the functional unit of the kidney (image below)
What does functional unit mean for nephrons?
What does functional unit mean for nephrons?
It’s an independent structure responsible for performing the kidney’s main functions: filtering blood, removing waste products, and forming urine.
Osmoregulation stages
The kidneys have to maintain the water potential of the blood plasma and the tissue fluid. This is done through the following stages:
- Ultrafiltration: Formation of glomerular filtrate
- Selective Reabsorption of glucose & water at Proximal Convoluted Tubule
- Maintenance of a sodium ion gradient in medulla by Loop of Henle
- Reabsorption of water by Distal Convoluted Tubule and collecting ducts
Exam Tip
The AQA specification requires the full names, proximal convoluted tubule and distal convoluted tubule, in exam answers. Abbreviations such as PCT and DCT are not accepted. If you’re ever unsure whether an abbreviation is allowed, check your specification.
Nephron with key structures and stages of osmoregulation
Ultrafiltration
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
The structure of the nephron and its role in: the formation of glomerular filtrate
Ultrafiltration
High hydrostatic pressure forces water and small molecules out of the glomerulus into Bowman’s capsule, forming glomerular filtrate. Blood cells and proteins cannot pass through and remain in the bloodstream.
How is a high hydrostatic pressure maintained?
- The diameter of the afferent arteriole (the arteriole that takes blood into the glomerulus) is greater than that of the efferent arteriole (which takes filtered blood towards the capillaries). This results in the blood in the glomerulus being under high hydrostatic pressure.
Glomerular Capillary Walls
The glomerular capillary wall has layers the blood has to pass through in the ultrafiltration process:
- Gaps in the endothelium — these have pores which prevent red blood cells passing through
- Basement membrane — the main filter which prevents proteins from being filtered out of the blood
- Epithelium — gaps between the podocytes
- These podocytes are specialised cells attached to the basement membrane, they allow filtrate to pass between them
These 3 layers prevent larger molecules (such as red blood cells and proteins) from passing through, so these remain in the blood. The substances (smaller molecules) that do pass through enter the Bowman’s capsule and are known as the glomerular filtrate.
Where do the proteins and blood cells go?
Where do the proteins and blood cells go?
They remain in the blood and travel via the efferent arteriole to the capillaries.
High levels of protein in the urine, known as proteinuria or albuminuria, can occur when the glomeruli are damaged, allowing proteins to pass through the basement membrane from the blood.
Selective Reabsorption
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
- The structure of the nephron and its role in: reabsorption of glucose and water by the proximal convoluted tubule
As this glomerular filtrate passes into the proximal convoluted tubule, selective reabsorption occurs.
This is where the body reabsorbs ‘useful’ substances (such as glucose and water) back into the bloodstream.
Why is it called 'selective' 'reabsorption'?
Why is it called 'selective' 'reabsorption'?
- Selective because it reabsorbs useful substances, while most waste, such as urea, stays in the filtrate
- Reabsorption because these substances were in the blood before ultrafiltration, and are now taken back into it
Adaptations of the Proximal Convoluted Tubule
To ensure the useful substances (glucose) can be reabsorbed, the proximal convoluted tubule has some specific features
- Microvilli on epithelium of the proximal convoluted tubule to provide a large surface area for reabsorption of useful materials from glomerular filtrate into capillaries
- Lots of mitochondria to produce ATP for active transport
- Many carrier proteins/channel proteins for facilitated diffusion
- Many carrier proteins for active transport
- Cells packed together (no fluid can pass between cells)
Tip
Active transport only uses carrier proteins, not channel proteins. Facilitated diffusion, on the other hand, can use both carrier and channel proteins.
Selective Reabsorption Process
- Active Transport: Sodium ions (Na+) are actively transported out of the proximal convoluted tubule epithelial cells and into blood capillaries. This reduces the Na+ concentration in epithelial cells lining the proximal convoluted tubule
- Co-transport: Na+ moves from the proximal convoluted tubule lumen into the epithelial cells, down its concentration gradient. Na+ is co-transported by carrier proteins with substances like glucose and amino acids into the epithelial cells.
- Facilitated diffusion and osmosis: Glucose and amino acids move into the capillaries from the epithelial cells via facilitated diffusion. Due to the movement of solutes, water also moves via osmosis into the proximal convoluted tubule epithelial cells and capillaries.
Loop of Henle
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
The structure of the nephron and its role in: maintaining a gradient of sodium ions in the medulla by the loop of Henle
The Loop of Henle is a long curved tubule (see image) which extends into the medulla. It’s responsible for maintaining the gradient of sodium ions in the medulla so water can be reabsorbed into the blood. We will look at how the kidney does this below.
Important
- Ascending limb (the thicker one) is impermeable to water
- Descending limb is permeable to water
Sodium ions are actively transported out of the ascending limb.
This creates a lower water potential in the medulla, due to the concentration of these sodium ions.
What feature of the ascending limb do you think will be important?
What feature of the ascending limb do you think will be important?
Mitochondria to produce ATP for active transport.
Water moves out of descending limb
As there is a low water potential in the medulla, water moves out of the descending limb into the medulla. It moves into interstitial space (the space between kidney tubules and blood vessels), and can then be reabsorbed into the blood through the capillary network.
Why does water not move out of ascending limb?
Why does water not move out of ascending limb?
The ascending limb is impermeable to water so water can’t move out of this limb.
Filtrate becomes more concentrated
As the filtrate travels down the descending limb it becomes progressively more concentrated (most concentrated at the bottom of the hairpin turn).
Sodium ions diffuse out of bottom of ascending limb
Sodium ions also diffuse out of the bottom of the ascending limb. This causes the water potential of the medulla to decrease even further.
Reabsorption of water
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
- The structure of the nephron and its role in: reabsorption of water by the distal convoluted tubule and collecting ducts.
With a low water potential in the medulla, water can move out of the distal convoluted tubule and collecting duct by osmosis into the medulla (interstitial space). This water can then be reabsorbed into the blood through the capillary network.
The volume of water which is reabsorbed into the blood via the capillary network is controlled by changing the permeability of the distal convoluted tubule and collecting duct. This is done by antidiuretic hormone (ADH).
What does the countercurrent multiplier mean?
What does the countercurrent multiplier mean?
- Countercurrent: Filtrate flows down the descending limb and up the ascending limb in opposite directions
- Multiplier: Ascending limb actively pumps out Na+ (& Cl-) ions
This means that the water potential in the medulla is lower than that of the filtrate, so the water will continue to move by osmosis and be reabsorbed back into the blood.
ADH
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
Osmoregulation as control of the water potential of the blood. The roles of the hypothalamus, posterior pituitary and antidiuretic hormone (ADH) in osmoregulation.
Osmoreceptors detect low water potential of blood
- Osmoreceptors in the hypothalamus detect low water content of the blood (decrease in water potential). This is an example of negative feedback
- The hypothalamus produces ADH
- ADH is passed into the posterior pituitary gland, from where it is released into the bloodstream
ADH binds to receptors on distal convoluted tubule & collecting duct
- ADH passes to the kidneys and binds to receptors on cell membrane of the distal convoluted tubule & collecting duct
- This increases the permeability of distal convoluted tubule and collecting duct walls
How does ADH increase the permeability of the distal convoluted tubule and collecting duct?
- When ADH binds to receptors on the cell membrane of the distal convoluted tubule and collecting duct
- This leads to activation of the enzyme (phosphorylase)
- This causes vesicles containing aquaporins (channel proteins that act as water channels) to move and fuse with the cell membrane
- This makes them more permeable to water
- More water leaves the collecting duct/distal convoluted tubule by osmosis down a water potential gradient and returns to the blood
When the water potential of the blood is high
If the water potential of the blood rises (e.g. after drinking a lot of water), the osmoreceptors in the hypothalamus detect it and less ADH is released from the posterior pituitary. The distal convoluted tubule and collecting duct become less permeable to water, so less water is reabsorbed and a larger volume of dilute urine is produced.
Summary diagram
How this topic is tested
This analysis is based on past paper data from 2017 to 2025. It is intended for interest only and is not predictive of what will appear in future papers.
- Tested in 7 of 9 years (2017–2025): 25 question parts worth 55 marks.
- 13th most-examined topic overall by marks, 1st in Unit 6.
Marks by year
Most-tested spec points
- Structure & Role of the Nephron: tested in 11 parts (28 marks)
- Osmoregulation & the Role of ADH: tested in 10 parts (19 marks)
Also links to: Osmosis, Receptors & Autonomic Control of Heart Rate, Types I and II Diabetes.
Maths and practical skills
- Percentages, ratios and fractions (MS 0.3): e.g. 2020 P3 Q4.3
- Identifying variables, including control variables (PS 2.4): e.g. 2020 P3 Q4.2
- Units and standard form (MS 0.1): e.g. 2020 P3 Q4.1
- Standard deviation and range (MS 1.10): e.g. 2018 P2 Q5.2
- Choosing and using a statistical test (MS 1.9): e.g. 2025 P3 Q2.3
- Using equations (MS 2.2): e.g. 2019 P3 Q1.3
Tips from examiner reports
What students commonly get wrong
- Protein in urine means the glomerulus is damagedWatch out: proteins are normally too large to pass through the basement membrane of the glomerulus, so they stay in the blood. If protein is found in urine, the basement membrane has been damaged and proteins have been filtered out. It isn't because protein wasn't reabsorbed. 2025 P3 Q2.2 · 2020 P2 Q8.1
- Ultrafiltration: high hydrostatic pressure, through gapsWatch out: small molecules are forced out of the glomerulus by high hydrostatic (blood) pressure, through gaps in the capillary endothelium and the basement membrane, not through the cells. 2019 P3 Q1.1
- Proximal convoluted tubule: features of its cellsWatch out: if a question asks about the cells of the proximal convoluted tubule, give features of each cell and what each one does: microvilli for a large surface area, many mitochondria to make ATP for active transport, and many carrier proteins for facilitated diffusion, active transport and co-transport. Include "many". 2023 P2 Q6.1
- Water moves out of the loop and collecting duct, not into themWatch out: water moves out of the loop of Henle and collecting duct, not into them. The filtrate is becoming urine, so water is removed from it by osmosis and reabsorbed into the blood. 2019 P3 Q1.4 · 2022 P2 Q9.3
- Made in the hypothalamus, released from the posterior pituitaryWatch out: the hypothalamus makes ADH, but it's released into the blood from the posterior pituitary. "Pituitary" alone isn't enough. 2022 P2 Q9.1
- Longer loop of Henle, more sodium ionsWatch out: a thicker medulla means a longer loop of Henle, so more sodium ions are moved into the medulla. This lowers its water potential, so more water is reabsorbed by osmosis. 2019 P3 Q1.4
Exam Question Practice
In a healthy person, all the glucose that is filtered into the nephron is reabsorbed into the blood from the proximal convoluted tubule.
Describe and explain three features of the cells lining the proximal convoluted tubule that allow glucose to be reabsorbed rapidly.
(3 marks)
Hint
Think about the surface of the cells facing the tubule, the transport proteins in their membranes, and the organelles that supply what active transport needs.
Mark Scheme
Max 3 marks. No more than 2 marks can come from points 2 to 4.
Points 2 to 6 need the word “many”. If it’s missing, 1 mark is lost (only once).
- Microvilli give a large surface area (1 mark)
- Many carrier proteins for co-transport of glucose with sodium ions (1 mark)
- Many carrier proteins for active transport of sodium ions out of the cell (1 mark)
- Many channel or carrier proteins for facilitated diffusion of glucose into the blood (1 mark)
- Many mitochondria make ATP for active transport (1 mark)
- Many ribosomes make the carrier proteins (1 mark)
Explanation (not marked): Sodium ions are actively transported out of the cell into the blood, which keeps their concentration low inside the cell. Sodium ions then diffuse in from the tubule through co-transport proteins, carrying glucose with them. Glucose then moves into the blood by facilitated diffusion.
Also accepted / rejected
Tips
- Give features of the cells, not of the tubule. “One cell thick” and “a good blood supply” don’t count
- Microvilli (not villi) give a large surface area
- Say “many”, e.g. many mitochondria make ATP for active transport
- Give the role of each transport protein, e.g. carrier proteins for co-transport
When the water potential of the blood falls, more ADH is released.
Describe how ADH affects the collecting duct, and how this changes the amount of water reabsorbed.
(3 marks)
Hint
What does ADH cause to appear in the membranes of the collecting duct cells? How does this change the movement of water, and by which process does water move?
Mark Scheme
- Aquaporins (water channel proteins) move into the cell-surface membrane (1 mark)
- The membrane becomes more permeable to water, so more water is reabsorbed (1 mark)
- By osmosis (1 mark)
Explanation (not marked): Water moves out of the collecting duct into the surrounding tissue of the medulla, which has a lower water potential, and then into the blood.
Also accepted / rejected
Also accepted
- “Channel proteins open” (point 1)
- Water “enters the blood” or “leaves the collecting duct” for reabsorbed (point 2)
Tips
- ADH causes channel proteins (aquaporins) to move into the cell-surface membrane. Include “membrane”
- This increases permeability, so more water is reabsorbed out of the collecting duct (not into it)
- Remember to say “osmosis”
ADH is made by neurones in the hypothalamus.
Name the part of the body that releases ADH into the blood.
(1 marks)
Hint
ADH is made in one place but stored and released somewhere else nearby. Which gland, and which part of it?
Mark Scheme
The posterior pituitary gland (1 mark)
Tips
- ADH is released by the posterior pituitary. Include “posterior”. The hypothalamus makes ADH but doesn’t release it into the blood
Also accepted