Interpret an invented urea production and removal balance
Calculate a retained-amount change and keep the evidence limits.
Section 13.1 · Excretion in humans
Exam scope and assessment notes
Extended only (Supplement) · 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
What is the net-change calculation for this closed model?
What you will learn
- Use production minus removal over the same time
- Distinguish retained amount from concentration
Interpret an invented urea production and removal balance
| Feature | Meaning |
|---|---|
| Formation | 12 invented units/h in liver |
| Removal | 10 units/h by kidney excretion |
| Net change | +2 units/h; +6 units over 3 h |
| Missing information | Starting amount, volume and clinical evidence |
Use production minus removal over the same time
The invented model forms twelve urea amount units per hour in the liver and removes ten units per hour by kidney excretion. It assumes constant rates for three hours, the same substance and accounting boundary, and no other route changing the amount. The net retained-amount change is production minus removal, not the sum of the rates.
Twelve minus ten is two units per hour of net increase. Over three hours, the retained amount rises by six units relative to its unspecified starting amount. Six is a change, not necessarily the final total: a starting value would also be needed. Equal formation and removal rates would give zero net change under the same closed assumptions.
Distinguish retained amount from concentration
The amount units are invented teaching values rather than a concentration, clinical blood test or intake recommendation. A concentration calculation needs a defined volume as well as an amount. These data cannot establish a medical diagnosis, a dangerous cutoff or the kidney performance of any student; they illustrate how continued production with insufficient removal increases retention.
The biological reason for removal is the toxicity of urea and prevention of harmful accumulation. Keep the named organ roles in the calculation explanation: liver formation and kidney excretion. The model does not mean urea is produced in the bladder, that all amino acids are excess or that real production and removal must remain constant for every person and time interval.
Pause and explain
What do the supplied values establish after three hours?
Worked example
Find retained-amount change for twelve formed and ten removed per hour over three hours.
Show the worked solution
- Net rate: 12 − 10 = 2 units per hour.
- Change: 2 × 3 = 6 units.
Answer The retained amount increases by six invented units; its final total is not given.
Try a topic test
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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.
Calculate the retained-amount change using the stated rates and duration.
Give me a hint
Subtract rates before multiplying by time.
Compare my reasoning
- 12 − 10 = 2 units per hour.
- 2 × 3 = 6 units.
Increase of six invented amount units over three hours; final total and concentration are not established.
Look for these in your work
- Use the correct subtraction
- Preserve amount-change limit
Under the same closed model, formation and removal are both twelve units per hour. Find the change in four hours.
Give me a hint
Equal rates balance.
Compare my reasoning
- 12 − 12 = 0 units per hour.
- 0 × 4 = 0 units change.
Zero net amount change over four hours under the stated closed constant-rate assumptions.
Look for these in your work
- Show equal rates
- State the model assumptions
A student treats the six-unit increase as a dangerous concentration. Evaluate.
Give me a hint
Amount change does not include volume or clinical interpretation.
Compare my reasoning
- No volume or starting amount is supplied.
- No clinical threshold is supplied.
The data show a six-unit amount increase only; volume, starting amount and clinical evidence are missing, so a dangerous concentration is not established.
Look for these in your work
- Distinguish change and concentration
- Reject unsupported diagnosis
Common mistakes
- Adding production and removal rates.
- Reporting a change as a final concentration.
What can you explain now?
What do the supplied values establish after three hours?
Compare with the explanation
An increase of six amount units
They establish change, not a final concentration.
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Make it stick
Calculate the net change, give the correct organs and state what the model cannot establish.
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.