AQA GCSE Biology Foundation (8461), Foundation tier · Homeostasis and response › Hormonal coordination in humans
Practise Maintaining water and nitrogen balance. 4 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the body loses water, how the kidneys make urine, and how kidney failure is treated. Expect data questions comparing blood, filtrate and urine, and questions on dialysis and transplants. This subtopic is in GCSE Biology only.
Key facts
Water lost: lungs (breathing out), skin (sweat), kidneys (urine); only kidney losses are controlled
Kidney: filtration, then selective reabsorption
Reabsorbed: all the glucose, some ions, some water
Urine: urea, excess ions, excess water (no glucose or protein when healthy)
Dialysis fluid: normal glucose and ion concentrations, no urea
Transplant: shortage of donors; drugs needed to prevent rejection
Notes
Water and ion balance
Water leaves the body from the lungs when you breathe out. Water, ions and urea are lost from the skin in sweat.
There is no control over these losses. Excess water, ions and urea are removed by the kidneys in urine.
If body cells lose or gain too much water by osmosis, they do not function efficiently.
How the kidneys work
diagram
Filtration: small molecules (water, ions, glucose, urea) are filtered out of the blood into the kidney tubules. Proteins and blood cells are too large and stay in the blood.
Selective reabsorption: useful substances (all the glucose, some ions and some water) are reabsorbed into the blood.
How the kidney makes urine: filtration, then selective reabsorption.
Urine contains urea, excess ions and excess water. A healthy person's urine contains no glucose and no protein.
Treating kidney failure
diagram
Dialysis: the patient's blood flows through a machine, separated from dialysis fluid by a partially permeable membrane.
The fluid contains the same concentration of glucose and useful ions as normal blood, so these do not diffuse out. It contains no urea, so urea diffuses from the blood into the fluid. Excess ions and water are also removed.
Dialysis: urea diffuses from the blood into the fluid; glucose and useful ions stay in the blood.
Transplant: a healthy kidney from a donor. There is a shortage of donors, and the patient must take drugs for life to reduce the risk of rejection.
How to answer each type of question
Interpret data on filtrate and urine
3 to 5 marksGrade 5
Compare the values in the filtrate and the urine.
Missing from the filtrate: too big to be filtered. Missing from the urine: reabsorbed.
More concentrated in the urine: water has been reabsorbed.
Example. The table shows the concentrations of some substances in the fluid filtered into a kidney tubule and in the urine of a healthy person.
Substance
Filtered fluid (g/dm3)
Urine (g/dm3)
Protein
0
0
Glucose
1.0
0
Urea
0.3
20
(a) Explain why there is no protein in the filtered fluid. (b) Explain why there is no glucose in the urine. (c) Suggest why urea is more concentrated in the urine than in the filtered fluid.Show the model answerHide the model answer
(a) Protein molecules are too large to be filtered out of the blood (1) (b) All the glucose is reabsorbed into the blood (1) because it is useful / needed for respiration (1) (c) Water is reabsorbed into the blood (1), so the urea left in the tubule becomes more concentrated (1)
Don’t lose marks
Saying glucose is not filtered. It is filtered, but all of it is then reabsorbed.
Saying dialysis fluid has no glucose so that glucose is not lost. It has glucose at the normal level so that none diffuses out.
Using 'osmosis' for urea. Osmosis is only for water; urea moves by diffusion.
More tips
Memory tricks
The kidney in two words: filter, then take back (reabsorb) what is useful.
Dialysis fluid copies clean blood: normal glucose and ions, zero urea.
Exam technique
In data questions, follow each substance from blood to filtrate to urine and explain each change as filtration or reabsorption.
In 'evaluate' questions, cover both treatments and end with a justified conclusion.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
List ways water leaves the bodyFrom the lungs when you breathe out, from the skin in sweat, and from the kidneys in urine.
Grade 4
Describe how the kidneys produce urineFiltration of the blood, then selective reabsorption of useful substances such as glucose, some ions and water.
Grade 5
Explain why cell water content must be controlledIf cells gain or lose too much water by osmosis, they do not function efficiently.
Grade 5
Describe the basic principles of dialysisBlood flows past a partially permeable membrane; urea and excess ions diffuse into the dialysis fluid.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the organs that remove excess water, ions and urea from the body in urine.
Kidneys
Name the process by which urea moves from the blood into the dialysis fluid.
Diffusion
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy6 marks
Water, ions and urea are lost from the body in several ways.
(a) Which two organs lose water from the body with no control over the amount of water lost? Tick (✓) two boxes.[2]
Kidneys
Liver
Lungs
Pancreas
Skin
(b) Name the organs that remove excess water, ions and urea from the body in urine.[1]
(c) Which substances are lost from the skin in sweat? Tick (✓) one box.[1]
Glucose, ions and water
Ions, urea and water
Protein, urea and water
Water only
(d) Explain why it is important that body cells do not lose or gain too much water.[2]
Show the answer and mark scheme
(a)Answer: Lungs; Skin
(b)Answer: Kidneys
kidneys
(c)Answer: Ions, urea and water
(d)Answer: Water moves into or out of cells by osmosis; if cells gain or lose too much water they do not function efficiently (they may swell and burst or shrink)
water moves into / out of cells by osmosis
cells do not function efficiently / cells may burst or shrink
Question 2Medium9 marks
A student modelled a dialysis membrane using a length of Visking tubing, a partially permeable membrane. The student filled the tubing with a mixture of starch solution and glucose solution, sealed both ends, and placed it in a beaker of water for 30 minutes.
(a) Iodine solution can be used to test for starch, turning blue-black if starch is present. Describe how the student could use iodine solution to find out whether starch had left the tubing.[2]
(b) Explain why a sample of the water from the beaker would not be expected to turn blue-black when iodine solution is added.[2]
(c) Describe how the student could test a sample of the water for glucose.[3]
(d) The test for glucose on the water is positive. Suggest why this investigation is a useful model of kidney dialysis.[2]
Show the answer and mark scheme
(a)Answer: Take a sample of the water from the beaker and add a few drops of iodine solution; if it turns blue-black, starch is present
take a sample of the water from the beaker (after 30 minutes)
add a few drops of iodine solution: a colour change to blue-black shows starch is present
(b)Answer: Starch molecules are too large to pass through the partially permeable Visking tubing, so no starch gets into the water
starch molecules are too large (to pass through the partially permeable membrane)
so starch cannot pass through the Visking tubing into the water
(c)Answer: Add Benedict’s solution to a sample of the water and heat it in a water bath; a colour change from blue towards green, yellow, orange or brick-red shows that glucose (a reducing sugar) is present
add Benedict’s solution to a sample of the water
heat the mixture (in a water bath)
a colour change from blue to green / yellow / orange / brick-red shows glucose is present
(d)Answer: Like a dialysis membrane, the tubing lets small molecules through but not large ones, so small molecules such as urea can pass from the blood into the dialysis fluid while large molecules such as proteins stay in the blood
the Visking tubing lets small molecules through but not large ones, like the membrane in a dialysis machine
so small molecules such as urea (and glucose / ions) can pass from the blood into the dialysis fluid, while large molecules such as proteins (and blood cells) stay in the blood