Explain why kidney excretion removes excess amounts
Preserve useful water and ions in the account.
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall cell metabolism, blood transport, amino acids and the distinction between excretion and egestion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Can a useful material be excreted when in excess?
What you will learn
- Useful materials can occur in excess
- Read a stated balance rather than a health rule
Explain why kidney excretion removes excess amounts
The numbers describe a closed invented teaching balance. They are not intake guidance or a real learner’s body-water assessment. Original teaching model.
| Feature | Meaning |
|---|---|
| Supply | 14 invented water units |
| Retained requirement | 9 units under the stated model |
| Excess | 14 − 9 = 5 units |
| Evidence limit | Urine alone does not give complete body balance |
Useful materials can occur in excess
Water is needed as a solvent and for transport and cellular processes. Ions also have useful biological roles. Excretion of excess water and ions therefore does not mean that the body should lose every molecule or every ion. It means amounts beyond the organism’s current requirements can be removed through kidney processing.
Urea has a different role in this account: it is a metabolic waste formed from excess amino-acid processing in the liver. It travels in blood to the kidneys for excretion. Do not replace the waste with unchanged protein or say that any useful amino acid must automatically be discarded rather than incorporated into proteins.
Read a stated balance rather than a health rule
A simplified supplied balance can identify an excess only when the input, other losses, retained requirement and time boundary are stated. If a model supplies ten units and requires six retained after all specified accounting, four units are excess in that model. Those invented units are not a drinking recommendation, a urine target or a threshold for a real student.
An observed urine volume alone does not establish total water intake, all other losses or complete body balance. Avoid giving a clinical explanation from one written record. The aim is to understand the qualifier excess and the different roles of waste, useful material and amounts beyond requirements, while the Supplement sequence explains the stated reabsorption outline.
Pause and explain
Which statement preserves the required kidney account?
Worked example
A closed teaching model supplies ten ion units and requires six retained. Find the stated excess.
Show the worked solution
- Subtract the retained requirement from supply.
- 10 − 6 = 4.
Answer Four invented ion units are excess under the stated closed model.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why excess is important in the kidney excretion list.
Give me a hint
Useful materials are still required.
Compare my reasoning
- Water and ions have body functions.
- Only amounts beyond requirements are described as excess.
Water and ions remain useful and required; kidney excretion removes excess amounts rather than all body water and ions.
Look for these in your work
- Retain useful roles
- Distinguish excess from all
A closed model supplies fourteen water units and requires nine retained. Calculate the excess.
Give me a hint
Use only the stated model accounting.
Compare my reasoning
- Subtract nine from fourteen.
- The difference is five model units.
Five invented water units are excess under the stated closed-model assumptions.
Look for these in your work
- Calculate the difference
- Keep the model qualification
A urine-volume observation is supplied without intake or other-loss information. Can total water balance be established?
Give me a hint
Missing routes prevent complete accounting.
Compare my reasoning
- Urine is only one loss route.
- Intake and other losses are not given.
No. Intake, other losses and retained requirements are needed; urine volume alone does not establish the full balance.
Look for these in your work
- Identify missing information
- Restrict the conclusion
Common mistakes
- Treating invented balances as personal intake advice.
- Calling all ions waste in every amount.
What can you explain now?
Which statement preserves the required kidney account?
Compare with the explanation
Kidneys excrete urea and excess water and ions
It keeps waste and excess distinct.
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Make it stick
Explain excess without treating all water and ions as waste.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.