The kidneys and osmoregulation

What you'll learn

This revision guide covers how the kidneys maintain water and nitrogen balance in the body through osmoregulation and excretion. You'll understand the structure and function of the nephron, how ADH controls water content, and medical treatments for kidney failure. These topics appear regularly in AQA GCSE Biology Papers 1 and 2.

Key terms and definitions

Osmoregulation — the regulation of water content in the body to maintain the correct concentration of dissolved substances in cells and bodily fluids

Excretion — the removal of waste products of metabolism from the body, including urea and excess water and ions

Urea — a nitrogenous waste product formed in the liver from the breakdown of excess amino acids

Nephron — the functional unit of the kidney responsible for filtering blood and producing urine

Ultrafiltration — the process where small molecules are forced out of the blood capillaries in the glomerulus into the Bowman's capsule under high pressure

Selective reabsorption — the process where useful substances (glucose, some ions and water) are reabsorbed from the kidney tubule back into the blood

ADH (antidiuretic hormone) — a hormone released by the pituitary gland that controls the permeability of kidney tubules to water

Dialysis — a medical treatment that filters the blood of patients with kidney failure using a dialysis machine

Core concepts

The role of the kidneys in excretion

The kidneys perform several vital functions in maintaining homeostasis:

• Remove urea from the blood (produced in the liver from excess amino acids that cannot be stored)
• Regulate water content by adjusting the volume of urine produced
• Regulate ion content by controlling the concentration of mineral ions in urine
• Filter toxic substances from the bloodstream

The formation of urea occurs because the body cannot store excess proteins or amino acids. In the liver, excess amino acids undergo deamination, where the nitrogen-containing amino group is removed. This produces ammonia, which is toxic, so the liver immediately converts it to less harmful urea. The urea dissolves in blood plasma and travels to the kidneys for removal.

Each kidney contains approximately one million nephrons. Blood enters the kidney through the renal artery and leaves via the renal vein. Urine collects in the ureter and flows to the bladder for storage before excretion through the urethra.

Structure and function of the nephron

The nephron is the microscopic functional unit where blood filtration and urine formation occur. Each nephron consists of:

The glomerulus and Bowman's capsule:

• A tight knot of capillaries (glomerulus) sits inside a cup-shaped structure (Bowman's capsule)
• Blood arrives under high pressure from the renal artery
• The capillary walls and capsule membrane act as filters

The kidney tubule:

• A long, coiled tube surrounded by capillaries
• Divided into sections with different functions
• Eventually joins a collecting duct

The collecting duct:

• Carries urine from multiple nephrons
• Final site of water reabsorption
• Delivers urine to the ureter

The three-stage process of urine formation

Stage 1: Ultrafiltration

High-pressure filtration occurs in the glomerulus:

• The afferent arteriole bringing blood to the glomerulus is wider than the efferent arteriole leaving it
• This creates high hydrostatic pressure that forces fluid through the capillary walls
• Small molecules pass through: water, glucose, urea, ions (sodium, chloride, potassium)
• Large molecules remain in the blood: proteins, blood cells, large plasma proteins
• The filtered liquid entering the Bowman's capsule is called filtrate

Stage 2: Selective reabsorption

Useful substances are reabsorbed from the tubule back into the blood:

• All glucose is reabsorbed by active transport in the first section of the tubule (this requires energy from respiration)
• Some ions are reabsorbed depending on the body's needs
• Some water is reabsorbed by osmosis (water follows the reabsorbed glucose and ions)
• Urea remains in the tubule as it is a waste product
• The tubule is surrounded by a network of capillaries that collect the reabsorbed substances

The reabsorption of glucose is complete under normal circumstances. Finding glucose in urine indicates diabetes mellitus, where blood glucose concentration exceeds the kidney's ability to reabsorb it.

Stage 3: Formation of urine

The remaining fluid continues through the tubule:

• Further water reabsorption occurs in the collecting duct (controlled by ADH)
• The concentration of urea increases as water is removed
• Final urine contains: water, urea, excess ions
• Urine passes to the ureter, bladder, and exits via the urethra

Osmoregulation and ADH

The body must maintain water balance despite varying water intake and loss. Water is lost through:

• Urine
• Sweat
• Exhaled air
• Faeces

The role of ADH in water regulation:

The hypothalamus in the brain continuously monitors blood water concentration using receptor cells.

When blood water content is too low (dehydration):

1. Receptors in the hypothalamus detect the low water concentration
2. The pituitary gland releases more ADH into the bloodstream
3. ADH travels to the kidneys in the blood
4. ADH makes the walls of the collecting ducts more permeable to water
5. More water is reabsorbed from the tubule back into the blood
6. A smaller volume of more concentrated urine is produced
7. Blood water content increases back to normal

When blood water content is too high:

1. Receptors detect high water concentration
2. The pituitary gland releases less ADH
3. The collecting duct walls become less permeable to water
4. Less water is reabsorbed from the tubule
5. A larger volume of dilute urine is produced
6. Blood water content decreases back to normal

This is an example of negative feedback — the response reverses the original change to maintain a stable internal environment.

Factors affecting water balance:

• Hot weather or exercise: Increased sweating causes water loss. More ADH is released, producing concentrated urine. This is why urine appears darker yellow after exercise.
• Cold weather: Less sweating occurs. Less ADH is released, producing dilute urine.
• High fluid intake: Excess water must be removed. Less ADH is released, increasing urine volume.
• Salty foods: Increase the concentration of ions in blood. More ADH is released to conserve water and dilute the blood.
• Alcohol: Suppresses ADH release, causing excessive dilute urine production and contributing to dehydration (hangover symptoms).

Kidney failure and treatment options

When kidneys fail, toxic waste products accumulate in the blood, and water and ion balance cannot be maintained. Without treatment, kidney failure is fatal.

Treatment 1: Kidney dialysis

Dialysis machines perform the kidney's filtering function artificially.

How dialysis works:

• Blood is pumped from an artery through a dialysis machine and returned to a vein
• Inside the machine, blood flows past partially permeable membranes
• These membranes separate the blood from dialysis fluid
• The dialysis fluid contains the same concentration of glucose and essential ions as healthy blood
• Urea and excess ions diffuse from the blood into the dialysis fluid down concentration gradients
• Glucose does not diffuse out because concentrations are equal on both sides
• Essential ions are maintained at correct levels
• The dialysis fluid is constantly refreshed to maintain concentration gradients

Advantages of dialysis:

• No need for immunosuppressant drugs
• No surgery risks (after initial fitting of access point)
• Available treatment while waiting for transplant

Disadvantages of dialysis:

• Time-consuming: typically 3-4 sessions per week, each lasting 3-4 hours
• Restricts lifestyle and travel
• Requires strict dietary restrictions (limited protein, salt, and fluid intake)
• Risk of blood clots and infection
• Expensive for health services over time
• Does not cure kidney disease

Treatment 2: Kidney transplant

A healthy kidney from a donor is surgically implanted into the patient.

Transplant procedure:

• Donor kidney may come from a living relative or deceased donor
• Tissue matching reduces rejection risk
• The donor kidney is placed in the lower abdomen
• Failed kidneys are usually left in place
• The donor ureter is connected to the bladder

Advantages of transplant:

• More normal lifestyle — no time spent on dialysis
• No dietary restrictions
• Better long-term health outcomes
• More cost-effective over time

Disadvantages of transplant:

• Shortage of donor organs — long waiting lists
• Major surgery risks (infection, bleeding, anaesthetic complications)
• Risk of rejection — the immune system attacks the "foreign" kidney
• Lifelong immunosuppressant drugs required to prevent rejection
• Immunosuppressants increase infection risk and risk of some cancers
• Donor kidney may eventually fail
• Not suitable for all patients (e.g., those with certain other medical conditions)

Preventing rejection:

The immune system recognises donor organs as foreign because surface proteins (antigens) differ between individuals. To reduce rejection:

• Tissue type matching: donor and recipient tissues should be as similar as possible
• Immunosuppressant drugs: suppress the immune response but increase infection vulnerability
• Close monitoring: regular tests detect early signs of rejection

In the UK and Caribbean, organ donation systems rely on voluntary registration. Patients with kidney failure may wait years for a suitable donor kidney, making dialysis a necessary bridge treatment.

Worked examples

Example 1: Explain why a person produces a large volume of pale urine after drinking 2 litres of water. (4 marks)

Model answer:

• Drinking 2 litres increases water content of the blood (1)
• This is detected by receptors in the brain/hypothalamus (1)
• The pituitary gland releases less ADH (1)
• The collecting duct/kidney tubule walls become less permeable to water, so less water is reabsorbed (1)
• More water remains in the tubule, producing large volume, dilute (pale) urine (1)

Award any 4 marks

Example 2: A student tested urine samples from three people (A, B, and C) and recorded the results in a table:

Person	Glucose present?	Protein present?	Urea present?
A	No	No	Yes
B	Yes	No	Yes
C	No	Yes	Yes

(a) Which person has healthy kidneys? Explain your answer. (2 marks)

(b) Suggest what medical condition person B might have. (1 mark)

(c) Explain why protein in the urine indicates kidney damage. (2 marks)

Model answers:

(a) Person A has healthy kidneys (1). Their urine contains urea (waste product) but no glucose (all reabsorbed) or protein (too large to filter) (1)

(b) Diabetes / diabetes mellitus (1)

(c) Protein molecules are too large to pass through the filter in a healthy glomerulus/Bowman's capsule (1). If protein is present, the filtering membrane is damaged/not working properly (1)

Example 3: Compare dialysis and kidney transplant as treatments for kidney failure. (6 marks)

Model answer:

• Dialysis does not require surgery after initial setup, whereas transplant involves major surgery (1)
• Dialysis patients need no immunosuppressant drugs, whereas transplant patients require lifelong immunosuppressants (1)
• Dialysis requires several sessions per week (typically 3-4 hours each), whereas transplant allows normal lifestyle (1)
• Dialysis has strict dietary restrictions, whereas transplant patients can eat normally (1)
• Dialysis has no waiting list concerns, whereas transplants have long waiting lists due to donor shortage (1)
• Dialysis does not cure kidney failure, whereas a successful transplant restores normal kidney function (1)
• Transplant carries rejection risk, whereas dialysis has no rejection issues (1)

Award any 6 marks for valid comparisons

Common mistakes and how to avoid them

• Confusing excretion with egestion: Excretion removes metabolic waste products (urea). Egestion removes undigested food (faeces). Faeces are not an excretory product.

• Thinking all glucose stays in the blood during filtration: Glucose is small enough to pass through the filter in ultrafiltration. It is then reabsorbed by active transport in selective reabsorption. State clearly which process you're describing.

• Forgetting that ADH affects permeability, not active pumping: ADH changes how permeable the collecting duct walls are to water. Water then moves by osmosis, not active transport. Don't say "ADH pumps water out."

• Reversing the ADH mechanism: When dehydrated, MORE ADH is released (not less), making tubules MORE permeable, causing MORE reabsorption, producing LESS urine. Create a clear chain of cause and effect.

• Stating that dialysis fluid contains no urea: Dialysis fluid contains no urea (or very little) to maintain a concentration gradient. Some students incorrectly think it must contain some urea. Zero concentration creates the maximum gradient.

• Confusing tissue matching with blood group matching: While blood group compatibility matters for transfusions, kidney transplants require tissue type (antigen) matching to reduce rejection. These are different systems.

Exam technique for "The kidneys and osmoregulation"

• "Explain" questions require cause and effect: Don't just describe what happens. Link each statement to the next. For ADH questions, work through the full chain: stimulus → detection → hormone release → effect on kidney → change in urine → return to normal.

• Use precise anatomical terms: Specify glomerulus, Bowman's capsule, tubule, collecting duct rather than vague terms like "kidney filter." This demonstrates accurate knowledge and gains marks.

• Extended response questions (6 marks): Organise your comparison or evaluation clearly. For treatment comparisons, use a point-by-point structure covering at least three different aspects (lifestyle, risks, effectiveness). Make explicit comparisons using "whereas" or "however."

• Look for mark allocations: A 4-mark "explain" question needs at least four developed points. One sentence will not suffice. Each physiological step in a process typically earns one mark.

Quick revision summary

The kidneys filter blood to remove urea (from amino acid breakdown) and regulate water and ion content. Each nephron performs ultrafiltration (small molecules forced out under pressure), selective reabsorption (glucose and useful substances actively reabsorbed), and urine formation. ADH from the pituitary gland controls collecting duct permeability, regulating water reabsorption through negative feedback. Kidney failure requires dialysis (regular filtering by machine) or transplant (donor kidney surgically implanted with immunosuppressants to prevent rejection). Both treatments have distinct advantages and limitations.