Maintaining water and nitrogen balance in the body (biology only) — AQA GCSE Biology

Maintaining water and nitrogen balance involves the kidneys filtering blood to excrete toxic urea produced by the liver, excess ions, and regulated volumes of water. Anti-diuretic hormone (ADH) controls water reabsorption in kidney tubules via a negative feedback loop to preserve blood osmolarity.

Maintaining water and nitrogen balance is an essential homeostatic process that protects cells from osmotic damage and removes toxic metabolic by-products. If water levels in body fluids fluctuate, cells either lose water by osmosis and shrivel, or absorb excess water and burst.

Protein metabolism produces excess amino acids that cannot be stored and must be deaminated by the liver into urea. The kidneys filter this urea from the blood while regulating ion and water loss under the control of anti-diuretic hormone (ADH), ensuring steady internal conditions.

• →Deamination: The liver converts excess amino acids into ammonia, which is immediately converted into less toxic urea for transport to the kidneys.
• →Filtration and Selective Reabsorption: The kidneys filter all small molecules (glucose, water, ions, urea) out of the blood, then selectively reabsorb all glucose, some ions, and regulated amounts of water.
• →ADH Regulation: Pituitary release of ADH controls the permeability of kidney tubules, functioning as a negative feedback loop to maintain osmotic balance.
• →Treatments for Kidney Failure: Dialysis acts as an artificial kidney using partially permeable membranes and diffusion gradients, whereas kidney transplants offer a long-term cure despite the risk of organ rejection.

• Osmosis is the movement of water from a dilute solution to a more concentrated solution across a partially permeable membrane.
• If body fluids are too dilute, water enters animal cells by osmosis.
• Water entering an animal cell causes it to swell and potentially burst, as it lacks a cell wall.
• If body fluids are too concentrated, water leaves the cell by osmosis.
• Water leaving an animal cell causes it to shrink, which disrupts normal cellular activity.
• The kidneys regulate the water content of body fluids to prevent these harmful osmotic changes.
• Water evaporates from the moist surfaces of the lungs into the air in the alveoli.
• The water vapour is removed from the body when air is exhaled.
• The lungs are a route of water loss, not a route of urea loss.
• The rate of water loss through the lungs increases with faster or deeper breathing.
• Exhaled air contains more water vapour than inhaled air.
• This loss contributes to the body's overall water balance and must be replaced by drinking and food.
• Sweat is produced by sweat glands in the skin.
• Sweat contains water, dissolved ions and urea.
• Water is lost from the body when sweat evaporates from the skin surface.
• Ions such as sodium and chloride are lost in sweat.
• Urea is lost in sweat, although the kidneys remove most urea in urine.
• Sweat production increases during exercise or in hot conditions, increasing these losses.
• The loss of water and ions in sweat must be replaced to maintain balance.
• Water is lost from the lungs as water vapour in exhaled air.
• Water, ions, and urea are lost from the skin in sweat.
• The loss of water, ions, and urea from the lungs and skin is uncontrolled and not adjusted to maintain internal balance.
• The kidneys are responsible for the controlled loss of water, ions, and urea by adjusting urine production.
• The kidneys filter the blood, removing water, ions and urea from it.
• Excess water, ions and urea are excreted in urine, which passes down the ureter to the bladder.
• Reabsorption returns useful substances such as glucose and some water and ions to the blood.
• The volume and concentration of urine are adjusted by the kidneys to help maintain balance.
• Urea is a waste product from the breakdown of excess amino acids and is removed in urine.
• Water moves into or out of cells by osmosis through the partially permeable cell membrane.
• If cells lose too much water, they become dehydrated and their reactions do not work efficiently.
• If cells gain too much water, they may swell and burst, so they cannot function efficiently.
• The kidneys help maintain the water balance of body fluids by adjusting water loss in urine.
• Efficient cell function depends on keeping the water content of cells and body fluids within a narrow range.
• Dietary proteins are digested into amino acids, which are absorbed into the blood.
• Excess amino acids cannot be stored by the body and must be broken down.
• Deamination in the liver removes the amino group from excess amino acids.
• The amino group is converted to ammonia and then to urea, which is less toxic.
• Urea is transported in the blood to the kidneys and excreted in urine.
• Deamination occurs in the liver, not in the kidneys or blood plasma.
• Excess amino acids, not all dietary amino acids, are deaminated.
• The amino group is removed from the amino acid during deamination.
• Ammonia (NH₃) is the nitrogen-containing product formed.
• The remaining part of the amino acid can be respired or converted to carbohydrate or fat.
• Deamination helps maintain nitrogen balance by removing surplus nitrogen from the body.
• Ammonia is toxic to cells and must not accumulate.
• The liver converts ammonia to urea.
• Urea is less toxic than ammonia and can be transported safely in the blood.
• Urea is excreted by the kidneys in urine.
• The conversion happens immediately after deamination.
• A high-protein diet increases urea production because more amino acids are deaminated.
• The kidneys regulate the water content of body fluids.
• Water is reabsorbed from the filtrate back into the blood.
• The amount of water reabsorbed determines urine concentration and volume.
• When blood is too concentrated, more water is reabsorbed and a smaller volume of concentrated urine is produced.
• When blood is too dilute, less water is reabsorbed and a larger volume of dilute urine is produced.
• Filtration of blood occurs in the kidneys and forms a filtrate.
• Small molecules such as water, glucose, ions and urea enter the filtrate.
• Blood cells and large proteins are not filtered into the filtrate.
• Selective reabsorption returns useful substances to the blood.
• Glucose is fully reabsorbed, so it is normally absent from urine.
• Some ions and some water are reabsorbed, while urea is not reabsorbed and is excreted in urine.
• Identify that glucose, ions and urea are present in the blood plasma and are filtered into the initial filtrate.
• State that glucose is completely reabsorbed back into the blood, so it is absent from the final urine.
• Compare numerical values from the table or bar chart, quoting specific concentrations for at least two substances.
• Explain that ions and urea are partially or not reabsorbed, so they remain in the urine.
• Recognise that the data show selective filtration based on molecule size, with large molecules such as proteins retained in the blood.
• State that ADH increases the permeability of the kidney tubules to water.
• Explain that increased permeability leads to more water reabsorption by osmosis into the blood.
• Describe that a decrease in ADH reduces the permeability of the kidney tubules to water.
• Link changes in ADH concentration to the volume and concentration of urine produced.
• Use the term 'osmosis' correctly when describing water movement from the tubule into the blood.
• State that ADH is the hormone that controls the water level in the body.
• Identify the pituitary gland as the source of ADH.
• Explain that ADH acts on the kidney tubules to increase water reabsorption.
• Describe the negative feedback mechanism: changes in blood water concentration trigger changes in ADH release.
• Link the action of ADH to the maintenance of a constant internal environment (homeostasis).
• State that ADH is released by the pituitary gland when blood is too concentrated.
• Describe that ADH causes more water to be reabsorbed from the kidney tubules into the blood.
• Link the effect of ADH to the production of a smaller volume of more concentrated urine.
• (Higher Tier) Explain that ADH increases the permeability of the kidney tubules to water.
• (Higher Tier) Explain that increased permeability allows more water to be reabsorbed by osmosis.
• Negative feedback reverses a change away from a normal level, returning the internal environment to its set point.
• A receptor detects the stimulus, a coordination centre processes the information, and an effector produces the response.
• In water balance, changes in blood concentration are detected and the pituitary gland releases ADH.
• ADH increases the permeability of the kidney tubules to water, increasing water reabsorption when blood is too concentrated.
• When blood is too dilute, less ADH is released, so less water is reabsorbed and a larger volume of dilute urine is produced.
• The response counteracts the original change, which is why the mechanism is described as negative feedback.
• Dialysis uses a partially permeable membrane to separate blood from dialysis fluid.
• Dialysis fluid contains glucose and mineral ions at normal blood concentrations but no urea, so useful substances are not lost.
• Urea diffuses from the blood into the dialysis fluid down its concentration gradient.
• Excess water and ions are also removed from the blood during dialysis.
• A kidney transplant can restore normal kidney function but the immune system may reject the donor organ.
• Patients with a transplant usually need immunosuppressant drugs, whereas dialysis patients do not.

• 💡Define osmosis using GCSE terminology: movement of water from a dilute to a concentrated solution through a partially permeable membrane.
• 💡Explain the direction of water movement first, then the effect on the animal cell (swelling/bursting or shrinking).
• 💡Name the process as evaporation from the moist surfaces of the lungs.
• 💡State clearly that the water leaves during exhalation, not inhalation.
• 💡Link the loss to breathing rate or environmental conditions if the question asks for factors.
• 💡Keep this route separate from urea loss, which occurs mainly in urine.
• 💡List all three substances lost in sweat: water, ions and urea.
• 💡Name the sweat glands as the structures that produce sweat.
• 💡Link sweat production to temperature or exercise when explaining why losses change.
• 💡State that evaporation of sweat also has a cooling function, but keep the focus on the substances lost.
• 💡Clearly distinguish between the routes: state that lungs lose water, while skin loses water, ions, and urea.
• 💡Use the term 'uncontrolled' for losses via the lungs and skin, contrasting this with the 'controlled' regulation by the kidneys.
• 💡Use the sequence kidney → ureter → bladder → urethra when describing the route of urine.
• 💡State that the kidneys adjust both the volume and the concentration of urine to control water and ion balance.
• 💡Link urea to the breakdown of excess amino acids so the source of urea is clear.
• 💡Use the terms swell and shrink to describe the effects of too much water gain and loss on cells.
• 💡Link the effect on cells to the role of the kidneys in keeping water balance stable.
• 💡State that osmosis involves water moving across a partially permeable membrane from a dilute to a concentrated solution.
• 💡Name the liver as the organ where deamination of excess amino acids occurs.
• 💡State the sequence excess amino acids → ammonia → urea → kidneys → urine.
• 💡Explain that converting ammonia to urea makes the nitrogen waste safer to transport and excrete.
• 💡Name the organ and the product: liver and ammonia.
• 💡Use the word deamination accurately rather than describing it as digestion.
• 💡Link the process to nitrogen balance so the answer addresses the section context.
• 💡Use the sequence deamination → ammonia → urea → urine.
• 💡Compare toxicity explicitly: ammonia toxic, urea less toxic.
• 💡Mention the liver as the conversion site and the kidneys as the excretion route.
• 💡Use the terms reabsorption, filtrate and urine concentration.
• 💡Give a cause-and-effect chain: blood concentration → water reabsorption → urine concentration.
• 💡Include a specific example such as drinking little water or sweating heavily.
• 💡Use the two-stage structure: filtration then selective reabsorption.
• 💡Name substances reabsorbed and substances excreted to show selectivity.
• 💡Link the process to water and nitrogen balance in the body.
• 💡When describing data, quote specific numbers from the table or chart, including units, to support your comparison.
• 💡Distinguish clearly between blood plasma, initial filtrate, and final urine.
• 💡If asked to translate data, structure your answer by comparing each substance at different stages of kidney function.
• 💡Use comparative language such as 'more permeable' or 'less permeable' rather than just 'permeable'.
• 💡Link each change in ADH to a change in water reabsorption and then to urine volume or concentration.
• 💡Remember that this is higher tier only, so expect questions that require a clear causal chain.
• 💡Use the term 'negative feedback' when describing how ADH maintains water balance.
• 💡Make clear that ADH travels in the blood to reach the kidney tubules.
• 💡When explaining, always state the stimulus, the hormone response, and the effect on water reabsorption.
• 💡Use the phrase 'blood too concentrated' to describe the stimulus for ADH release, rather than just saying 'low water'.
• 💡Higher Tier students should explicitly use the terms 'permeability' and 'osmosis' when explaining how ADH increases water reabsorption.
• 💡Use the sequence stimulus, receptor, coordination centre, effector and response when explaining negative feedback.
• 💡Link each change in ADH concentration to a change in the permeability of the kidney tubules and the resulting volume of urine.
• 💡Practise explaining both directions of the feedback loop, not only the response to dehydration.
• 💡Compare dialysis and transplant by referring to lifestyle, risk and availability of donors.
• 💡Explain diffusion in dialysis using the concentration gradient of urea between blood and dialysis fluid.
• 💡State clearly that the membrane is partially permeable, not fully permeable.
• 💡Always specify that all glucose is reabsorbed in a healthy kidney tubule; finding glucose in urine is an indicator of diabetes, not normal kidney function.
• 💡Use the phrase 'tubule permeability to water increases' when describing the direct effect of ADH, rather than vague statements like 'the kidney works harder'.

• Using A-level terminology like 'water potential'; correct this by using standard GCSE terms such as 'dilute solution' and 'concentrated solution'.
• Claiming that animal cells become turgid; correct this by stating that turgidity applies to plant cells with a cell wall, while animal cells swell and burst.
• Forgetting to mention the partially permeable membrane; correct this by always including it in definitions and explanations of osmosis.
• Stating that water is lost from the lungs during inhalation; the correction is that water vapour leaves the body during exhalation.
• Confusing water loss with urea loss; the correction is that the lungs lose water but urea is removed mainly by the kidneys.
• Saying that the lungs produce water; the correction is that water evaporates from moist lung surfaces and is carried out in exhaled air.
• Ignoring the effect of breathing rate; the correction is that more frequent or deeper breathing increases water loss.
• Stating that sweat contains only water; the correction is that sweat also contains ions and urea.
• Claiming that sweat glands are found only in the palms; the correction is that sweat glands are distributed widely in the skin.
• Saying that urea is lost only in sweat; the correction is that urea is mainly lost in urine, with a smaller amount in sweat.
• Confusing sweat with urine; the correction is that sweat is produced by sweat glands and contains water, ions and urea, while urine is produced by the kidneys.
• Thinking the skin controls water or ion balance by sweating; correction: sweating is for temperature regulation, and the resulting water and ion loss is uncontrolled.
• Believing urea is breathed out from the lungs; correction: urea is lost in sweat from the skin and in urine from the kidneys, while the lungs only lose water and carbon dioxide.
• Assuming all water loss is regulated; correction: only the kidneys actively control water loss, whereas lung and skin losses are unregulated.
• Confusing the ureter with the urethra; the correction is that urine leaves each kidney through a ureter and leaves the body through the urethra.
• Thinking all filtered water and ions are excreted; the correction is that some are reabsorbed back into the blood.
• Believing the kidneys remove only urea; the correction is that they also remove excess water and ions in urine.
• Thinking water movement by osmosis does not affect cell function; the correction is that too much water loss or gain disrupts cell processes.
• Believing cells only burst when they gain water and never shrink when they lose water; the correction is that both too little and too much water cause inefficient function.
• Confusing osmosis with diffusion of solutes; the correction is that osmosis is the movement of water across a partially permeable membrane.
• Thinking excess amino acids are stored like fat; the correction is that they are broken down in the liver by deamination.
• Believing urea is formed directly from proteins in the stomach; the correction is that proteins are digested to amino acids, and urea is made from excess amino acids in the liver.
• Confusing ammonia and urea; the correction is that ammonia is toxic and is converted to the less toxic urea before excretion.
• Saying deamination happens in the kidneys; correct this by stating it occurs in the liver.
• Writing that amino acids are converted directly to urea; correct this by stating ammonia is formed first.
• Stating that all amino acids are deaminated; correct this by specifying excess amino acids only.
• Stating that ammonia itself is excreted in urine; correct this by stating ammonia is converted to urea first.
• Saying urea is more toxic than ammonia; correct this by stating urea is less toxic.
• Naming the kidneys as the site of conversion; correct this by naming the liver.
• Saying the kidneys make water; correct this by stating they reabsorb water from the filtrate.
• Confusing water balance with temperature regulation; correct this by focusing on water content of body fluids.
• Stating that urine concentration stays constant; correct this by linking it to water reabsorption.
• Saying all filtered substances are reabsorbed; correct this by stating reabsorption is selective.
• Stating that glucose is normally present in urine; correct this by stating glucose is fully reabsorbed.
• Confusing filtration with reabsorption; correct this by describing filtration as movement into the tubule and reabsorption as return to the blood.
• Assuming that glucose is not filtered into the kidney tubule; correction: glucose is small enough to be filtered into the initial filtrate, but is then completely reabsorbed.
• Confusing the direction of reabsorption, thinking that substances move from the blood into the filtrate; correction: reabsorption moves useful substances such as glucose from the tubule back into the blood.
• Misreading bar charts by comparing bar heights without checking the axis scale; correction: always read the numerical scale carefully and quote values with units.
• Saying that ADH makes the tubules permeable to glucose or ions rather than specifically to water; correction: ADH primarily affects water permeability.
• Confusing the effect of ADH with the effect of aldosterone or other hormones; correction: focus only on ADH and water reabsorption.
• Stating that ADH is released by the kidney; correction: ADH is released by the pituitary gland.
• Thinking that ADH is produced by the kidneys; correction: ADH is produced by the pituitary gland and acts on the kidneys.
• Believing that ADH directly controls ion balance; correction: ADH primarily controls water reabsorption.
• Confusing the effect of ADH with that of insulin or glucagon; correction: ADH is specific to water balance.
• Saying that ADH causes water to be reabsorbed from the blood into the tubules; correct this by stating water moves from the tubules back into the blood.
• Stating that ADH is released when blood is too dilute; correct this by explaining ADH is released when blood is too concentrated (water levels are low).
• Assuming the detailed mechanism of ADH is required for Foundation tier; correct this by noting Foundation students only need to describe the effect, while Higher Tier students must explain permeability and osmosis.
• Thinking that negative feedback amplifies a change: it reverses the change, returning conditions towards the normal level.
• Using A-level terminology like 'collecting duct' or 'water potential': stick to the GCSE terms 'kidney tubules' and 'blood concentration'.
• Stating that ADH is released when blood is too dilute: ADH release increases when blood is too concentrated and decreases when blood is too dilute.
• Thinking that dialysis fluid contains urea: it contains no urea so that urea continues to diffuse out of the blood.
• Believing that dialysis fluid is pure water: it contains glucose and mineral ions at normal blood concentrations to prevent their loss.
• Assuming a transplant always works immediately and permanently: rejection is a risk and immunosuppressant drugs are usually needed.
• Believing protein is filtered into the kidney tubule: Large protein molecules and blood cells remain in the capillaries because they are too large to pass through the filtration membrane.
• Thinking all substances are reabsorbed by diffusion: Glucose is selectively reabsorbed against its concentration gradient via active transport, ensuring 100% of it is normally reclaimed.
• Assuming ADH is produced by the kidneys: ADH is synthesized in the brain and released by the pituitary gland, acting on the kidneys as its target organ.

1. 1Day 1: Master the biochemical pathway of nitrogen excretion (excess protein -> amino acids -> ammonia in liver -> urea -> kidneys).
2. 2Day 2: Draw and label the functional stages of a nephron: ultrafiltration, selective reabsorption of glucose, variable reabsorption of water.
3. 3Day 3: Practice drawing flowcharts for the ADH negative feedback loop under both dehydration and overhydration conditions.
4. 4Day 4: Create a comparison table evaluating kidney dialysis versus kidney transplant (cost, lifestyle impact, rejection risks, diet restrictions).

• 📋6-mark extended response questions detailing the negative feedback loop controlling blood water potential.
• 📋Data analysis questions interpreting dialysis fluid compositions or changes in urine volume and concentration under varied physical activities.
• 📋Evaluation questions weighing the advantages and disadvantages of kidney dialysis versus organ transplantation.