Trace carbon dioxide excretion through the lungs
Connect cellular waste production with its exit route.
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
What process produces CO₂ in the specified tissue example?
What you will learn
- A metabolic product reaches an exchange surface
- Distinguish production from excretion
Trace carbon dioxide excretion through the lungs
| Feature | Meaning |
|---|---|
| Production | Aerobic respiration in body cells |
| Transport | Blood carries carbon dioxide |
| Exit organ | Lungs |
| Removal | Carbon dioxide leaves in expired air |
A metabolic product reaches an exchange surface
Carbon dioxide is a product of aerobic respiration in body cells. It is a metabolic waste that must be removed rather than allowed to accumulate without regulation. Blood carries carbon dioxide from tissues towards the lungs, where it crosses the gas-exchange surface into the air spaces and can leave in expired air.
The required excretory organ for this carbon dioxide is the lungs. A complete short explanation can follow cell production, blood transport, exchange into lung air and removal by expiration. The lungs are an exit route; they are not the only tissue where carbon dioxide is produced, and expiration is not the chemical reaction that makes it.
Distinguish production from excretion
Excretion removes waste products of metabolism and substances in excess of requirements. Carbon dioxide fits because it is produced by cellular metabolism. It is different from undigested material remaining in the digestive tract, which is removed by egestion. Not every substance that leaves the body should be assigned to the same process without considering its origin.
A supplied observation of expired carbon dioxide supports the lung removal route, but does not directly measure production in every tissue or prove that all expired gas is carbon dioxide. Inspired air already contains a small amount, and expired air remains a mixture. Connect the named waste with the correct route without adding unsupported gas-composition claims.
Pause and explain
Which organ is the required carbon-dioxide excretion route?
Worked example
Which organ excretes the carbon dioxide produced by tissue respiration?
Show the worked solution
- Blood transports the waste to the gas-exchange surface.
- Carbon dioxide can leave in expired lung air.
Answer The lungs excrete carbon dioxide.
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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.
State the organ through which carbon dioxide is excreted.
Give me a hint
Choose the gas-exchange exit route.
Compare my reasoning
- Carbon dioxide is carried towards the lungs.
- It leaves via lung air in expiration.
Carbon dioxide is excreted through the lungs.
Look for these in your work
- Name carbon dioxide
- Name lungs
Correct: lungs must make all body CO₂ because it leaves through them.
Give me a hint
A production site need not be the exit site.
Compare my reasoning
- Body-cell respiration produces CO₂.
- Blood carries it to the lungs for removal.
Body cells produce carbon dioxide during respiration; blood transports it to the lungs, which provide its excretion route.
Look for these in your work
- Separate production and exit
- Include transport
Does a positive CO₂ test on expired air prove that expired air is pure CO₂?
Give me a hint
A gas identification test does not describe every component.
Compare my reasoning
- The test indicates carbon dioxide is present.
- Other gases can remain in the mixture.
No. The test indicates carbon dioxide, but expired air is a mixture and is not established as pure CO₂.
Look for these in your work
- State the supported result
- Reject purity inference
Common mistakes
- Calling the lungs the only production site.
- Calling expired air pure carbon dioxide.
What can you explain now?
Which organ is the required carbon-dioxide excretion route?
Compare with the explanation
Lungs
Carbon dioxide enters lung air and leaves in expiration.
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Make it stick
Name the carbon-dioxide exit organ and distinguish it from production sites.
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.