Water and nitrogen balance (kidneys and ADH) – GCSE Biology exam questions with mark scheme

1Grade 8–9HigherTriple only6 marks

A runner sweats a lot during a race on a hot day.

(a) Explain how ADH helps the runner's body to reduce water loss. [4]

(b) In a healthy person, glucose is filtered out of the blood in the kidney but no glucose is found in the urine. Explain why. [2]

(a) water content of blood falls / blood becomes too concentrated (detected by the brain)(1)
(a) pituitary gland releases more ADH (into the blood)(1)
(a) ADH makes the kidney tubules more permeable to water
allow ADH acts on kidney tubules(1)
(a) so more water is reabsorbed (into the blood)
allow less water lost in urine / a small volume of concentrated urine is produced(1)
(b) glucose is reabsorbed (from the kidney tubules) back into the blood
allow selective reabsorption(1)
(b) all of the glucose is reabsorbed so none is left in the filtrate / urine
the idea of all is needed for this mark(1)
Answer: (a) Low blood water → pituitary releases more ADH → tubules more permeable → more water reabsorbed, so a small volume of concentrated urine (b) All glucose is selectively reabsorbed into the blood.
Examiner tip: ADH is released by the pituitary, not made by the kidney – a common confusion.

2Grade 1–3FoundationTriple only7 marks

The table shows the volume of water lost by different routes from an adult's body in one day.

Route of water loss	Volume of water lost in cm³
Breathing out (lungs)	350
Sweat (skin)	450
Urine	1500
Faeces	150

(a) Calculate the percentage of the total water lost that was lost in urine.

Give your answer to 3 significant figures. [2]

(b) Which route of water loss can the body control? Tick (✓) one box.

☐ Breathing out ☐ Sweat ☐ Urine [1]

(d) On another day the person had a fever. They drank the same volume of water, so the total water lost was the same. The volume of water lost in sweat increased to 1350 cm³. The volumes lost by breathing out and in faeces did not change.

Calculate the volume of urine the person produced that day. [2]

(a) total = 2450 (cm³) or 1500 ÷ 2450 (× 100)(1)
(a) 61.2 (%)
61.22… (not rounded to 3 s.f.) scores 1 mark only(1)
(b) urine (only)
do not accept more than one box ticked(1)
(c) ions / (mineral) salts
allow named ion e.g. sodium / chloride / sodium chloride(1)
(c) urea
ignore glucose / protein / toxins / waste unqualified(1)
(d) 2450 − 350 − 1350 − 150 or 2450 − 1850(1)
(d) 600 (cm³)
correct answer with no working = 2 marks(1)
Answer: (a) 61.2% (b) urine (c) ions and urea (d) 600 cm³
Examiner tip: In (a) you must add up all four routes first – dividing 1500 by any single route is a common way to lose both marks.

3Grade 4–5Triple only6 marks

A student put red blood cells into three different salt solutions, A, B and C.

The diagram shows what one cell from each solution looked like after 10 minutes. The cell in solution A did not change. Soon after the diagram was drawn, the cell in solution B burst.

(a) Which solution, A, B or C, was more concentrated than the inside of the red blood cell? [1]

(b) Explain why the cell in solution B got bigger and then burst. [3]

Suggest why this liquid must have the same concentration of solutes as normal blood plasma. [2]

(a) C(1)
(b) solution B was more dilute than the cell contents
allow higher water concentration outside the cell / lower solute concentration outside the cell(1)
(b) water moved into the cell by osmosis
allow water diffused into the cell through the partially permeable (cell) membrane(1)
(b) (too much water entered and) the cell has no cell wall (so the membrane could not resist the pressure)
allow the cell swelled until the membrane split
ignore ‘the cell exploded’ unqualified(1)
(c) (otherwise) water would move into or out of (blood / body) cells by osmosis(1)
(c) (so) cells would swell / burst or shrink, and be damaged / stop working properly
allow cells are damaged if they gain or lose too much water(1)
Answer: (a) C (b) Solution more dilute than cytoplasm, water enters by osmosis, no cell wall so the cell bursts (c) Otherwise water moves into or out of cells by osmosis, damaging them
Examiner tip: Say ‘water’ moves by osmosis – answers that say salt or solution moved into the cell get no credit.

4Grade 6–7Triple only7 marks

The table shows the concentrations of some substances in the blood plasma, in the filtrate made by the kidney, and in the urine of a healthy person.

Substance	Blood plasma in g/dm³	Filtrate in g/dm³	Urine in g/dm³
Protein	80	0	0
Glucose	1.0	1.0	0
Urea	0.30	0.30	20.0
Ions	7.2	7.2	12.0

(a) Explain why there is no protein in the filtrate. [1]

(b) How many times greater is the concentration of urea in the urine than in the blood plasma? [2]

(d) After the person ate a very salty meal, the concentration of ions in their urine increased. Explain why. [2]

(a) protein molecules are too large to be filtered (out of the blood)
allow too big to pass out of the blood / capillaries
ignore ‘protein is reabsorbed’(1)
(b) 20.0 ÷ 0.30(1)
(b) 67 (times)
allow 66.7 / 66.67 / 66⅔
do not accept 66(1)
(c) (a lot of / most) water is reabsorbed (from the filtrate into the blood)(1)
(c) (but) urea is not reabsorbed (so the same mass of urea is in a smaller volume)
allow less urea is reabsorbed than water(1)
(d) more ions are in the blood / filtrate than the body needs
allow the ion content of the blood is too high(1)
(d) the kidney reabsorbs only the ions the body needs, so the excess ions are not reabsorbed / are lost in urine
allow fewer ions are reabsorbed(1)
Answer: (a) Protein molecules too large to be filtered (b) 67 times (c) Water reabsorbed but urea not, so urea is concentrated (d) Excess ions in blood are not reabsorbed so more leave in urine
Examiner tip: For (c) you need both halves: water is reabsorbed and urea is not – stating only one gets one mark.

5Grade 6–7HigherTriple only6 marks

The flow chart shows what happens to protein that a person eats.

(a) Name process X, organ Y and substance Z. [3]

(b) Suggest why excess amino acids must be broken down instead of being kept in the body. [1]

Predict how this will affect the amount of urea in the athlete's urine. Explain your answer. [2]

(a) X: deamination(1)
(a) Y: liver(1)
(a) Z: ammonia
do not accept ammonium / amino group / urea(1)
(b) (excess) amino acids / proteins cannot be stored in the body
ignore ‘ammonia is toxic’ (does not explain why amino acids are broken down)(1)
(c) more urea (in the urine)
allow higher concentration of urea(1)
(c) (because) more (excess) amino acids are deaminated (in the liver), so more ammonia / urea is made
allow more amino acids broken down in the liver(1)
Answer: (a) X deamination, Y liver, Z ammonia (b) Excess amino acids cannot be stored (c) More urea, as more excess amino acids are deaminated in the liver
Examiner tip: Ammonia is made first and then converted to urea – students who write ‘amino acids are turned into urea’ in (a) miss the mark for Z.

6Grade 8–9HigherTriple only10 marks

A student drank 1 dm³ of water at 30 minutes. The graph shows the concentration of ADH in the student's blood over the next few hours.

(a) Name the gland that releases ADH. [1]

(b) Calculate the percentage decrease in ADH concentration between 30 minutes and 90 minutes. [2]

(d) Explain how the change in ADH concentration affected the urine the student produced between 60 and 120 minutes. [3]

(e) Explain how the graph between 90 and 240 minutes shows that ADH release is controlled by negative feedback. [2]

(a) pituitary (gland)(1)
(b) (4.0 − 1.0) ÷ 4.0 (× 100)
allow 3 ÷ 4(1)
(b) 75 (%)(1)
(c) water content of the blood increased / blood became more dilute
allow change detected by receptors in the brain(1)
(c) (so) the pituitary released less ADH (into the blood)(1)
(d) kidney tubules became less permeable to water(1)
(d) (so) less water was reabsorbed (into the blood)(1)
(d) (so) a large volume of dilute urine was produced
both ‘large volume’ and ‘dilute’ needed
allow more urine and less concentrated(1)
(e) as the excess water is lost (in urine), the water content of the blood falls back towards normal(1)
(e) (so) more ADH is released, and the ADH concentration returns to its original level (4 units)
allow the response reverses the change to keep conditions constant(1)
Answer: (a) Pituitary (b) 75% (c) Blood water content rose so pituitary released less ADH (d) Tubules less permeable, less water reabsorbed, large volume of dilute urine (e) Water level falls back to normal so ADH release rises back to 4 units
Examiner tip: Say ADH changes the permeability of the kidney tubules – ‘ADH absorbs water’ or ‘ADH makes the kidney filter more’ earns nothing.

7Grade 4–5Triple only6 marks

The diagram shows part of a dialysis machine used to treat a person with kidney failure.

(a) The dialysis fluid contains no urea. Explain how urea is removed from the patient's blood. [2]

(b) The dialysis fluid contains the same concentration of glucose as normal blood plasma.

Explain why. [2]

(d) Fresh dialysis fluid is pumped in all the time and used fluid is removed.

Suggest why. [1]

(a) urea diffuses from the blood into the dialysis fluid (through the membrane)(1)
(a) (because) urea concentration is higher in the blood than in the fluid / down a concentration gradient
do not accept ‘along’ or ‘against’ a gradient as the idea of direction(1)
(b) (so there is) no (net) diffusion of glucose out of the blood
allow no concentration gradient for glucose(1)
(b) (so) the patient does not lose glucose (which is needed for respiration)
allow blood glucose concentration stays at a normal level(1)
(c) they are too large to pass through the membrane
allow the molecules / cells are too big to diffuse through the pores
ignore ‘membrane does not let them through’ unqualified(1)
(d) to keep a (steep) concentration gradient (for urea / waste)
allow so urea does not build up in the fluid / so urea keeps diffusing out of the blood(1)
Answer: (a) Urea diffuses from blood into the fluid down a concentration gradient (b) No net diffusion of glucose so it is not lost from the blood (c) Too large to pass through the membrane (d) Keeps a concentration gradient so urea keeps diffusing out
Examiner tip: Use the term ‘concentration gradient’ and say which way urea moves – ‘urea is filtered out’ is not enough for dialysis.

8Grade 6–7Triple only6 marks

Kidney failure can be treated with dialysis or with a kidney transplant.

The table gives information from one hospital about the two treatments.

	Dialysis	Kidney transplant
Time spent on treatment	3 sessions per week, 4 hours per session	one operation; then regular check-ups
Approximate cost	£30 000 per year	£17 000 for the operation, then £5 000 per year for drugs
Diet	strict limits on salt, protein and fluid	mostly normal diet
Availability	can start straight away	average wait for a donor kidney of about 2 to 3 years

Evaluate the use of a kidney transplant compared with dialysis for treating kidney failure.

Use the information in the table and your own knowledge. [6]

transplant is cheaper over time: e.g. over 5 years dialysis ≈ £150 000, transplant ≈ £17 000 + 5 × £5 000 = £42 000; transplant is cheaper even in the first year (£22 000 vs £30 000)(1)
dialysis takes about 12 hours per week (≈ 624 hours per year), so it disrupts work / school / daily life(1)
dialysis needs a strict diet (limiting protein and salt) because urea and ions build up between sessions(1)
dialysis is available straight away, whereas there is a shortage of donor kidneys / a long waiting list(1)
dialysis does not risk rejection; a transplant can be rejected by the immune system (antigens on the donor kidney differ)(1)
transplant patients must take immunosuppressant drugs for life, which increases the risk of infections(1)
a transplant is a major operation with risks (e.g. infection, bleeding, anaesthetic)(1)
a transplanted kidney works continuously, so blood composition is kept constant all the time rather than rising and falling between dialysis sessions(1)
dialysis carries risks such as infection / blood clots at the access point(1)
conclusion with a reason, e.g. a transplant is better in the long term (cheaper, more normal life) but dialysis is needed while waiting for a donor / if no match is found(1)

Level	Marks	What it looks like
Level 3	5–6	Considers advantages and disadvantages of both treatments, using the data (including a correct cost or time comparison) and own knowledge such as rejection or the shortage of donors, and reaches a reasoned conclusion.
Level 2	3–4	Gives some advantages and/or disadvantages of both treatments, with some explanation or use of the data. A conclusion may be stated but is not fully supported.
Level 1	1–2	Gives simple, mainly one-sided points about one or both treatments, often lifted from the table without explanation. No conclusion.
No relevant content	0	Nothing worthy of credit.

Answer: Balanced comparison: transplant cheaper long term (≈£42 000 vs £150 000 over 5 years), frees ~12 h per week and allows normal diet, but has donor shortage/waiting time, rejection risk and life-long immunosuppressants; dialysis available now with no rejection. Conclusion: transplant usually better long-term, dialysis bridges the wait.
Examiner tip: To reach Level 3 you must give a conclusion and back it up – a list of pros and cons without a judgement is capped at Level 2.

9Grade 4–5Triple only6 marks

Each day, the kidneys of a healthy adult filter about 180 dm³ of filtrate from the blood. The same person produces about 1.5 dm³ of urine per day.

(a) Calculate the percentage of the filtrate volume that is reabsorbed into the blood.

Give your answer to 1 decimal place. [2]

(b) Glucose, some ions and most of the water are taken back into the blood from the kidney tubules. Name this process. [1]

Calculate the volume of urine this person would produce per day. [2]

(d) Suggest one problem this person might have. [1]

(a) (180 − 1.5) ÷ 180 (× 100) or 178.5 ÷ 180 (× 100)
allow 100 − (1.5 ÷ 180 × 100)(1)
(a) 99.2 (%)
99.17 / 99.166… (not given to 1 d.p.) scores 1 mark only(1)
(b) selective reabsorption
allow reabsorption(1)
(c) 180 × 0.02 or 2% of 180(1)
(c) 3.6 (dm³)
correct answer with no working = 2 marks(1)
(d) any one from:
• dehydration / loses too much water
• feels very thirsty / needs to drink a lot more
• needs to urinate very often
• blood becomes too concentrated, so cells lose water by osmosis / are damaged
ignore kidney failure / ill unqualified(1)
Answer: (a) 99.2% (b) Selective reabsorption (c) 3.6 dm³ (d) e.g. dehydration / very thirsty
Examiner tip: In (c) work out the 2% that is not reabsorbed – multiplying 180 by 0.98 gives the volume reabsorbed, not the urine.

10Grade 8–9HigherTriple only7 marks

Some people have a condition in which the pituitary gland releases very little ADH.

(a) Suggest two symptoms a person with this condition would have. Explain each symptom. [4]

(b) The condition is treated with a drug that has the same effect as ADH.

Explain why it would be dangerous for the patient to take too much of this drug. [3]

(a) symptom 1: produces a large volume of (dilute) urine
allow urinates very often(1)
(a) (because) kidney tubules are less permeable to water, so less water is reabsorbed (into the blood)(1)
(a) symptom 2: very thirsty or dehydrated
allow feels tired / dizzy / headaches if linked to dehydration(1)
(a) (because) the water content of the blood falls / blood becomes too concentrated
allow cells lose water by osmosis(1)
(b) (too much drug makes the) tubules more permeable so too much water is reabsorbed (into the blood)
allow very small volume of concentrated urine(1)
(b) (so) the water content of the blood becomes too high / blood becomes too dilute
allow ion concentration of blood becomes too low(1)
(b) (so) water moves into (body / brain) cells by osmosis, and the cells swell / burst / are damaged(1)
Answer: (a) Large volume of dilute urine (less water reabsorbed as tubules less permeable); thirst/dehydration (blood water content falls) (b) Too much water reabsorbed, blood too dilute, water enters cells by osmosis so they swell and are damaged
Examiner tip: Link each symptom to a cause – listing ‘lots of urine, thirsty’ without explaining the tubule permeability scores only half the marks.

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