Explain oxygen transport by red blood cells
Connect haemoglobin, oxygen loading at lungs and delivery to body cells.
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 gas exchange and aerobic respiration.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which blood-cell substance binds oxygen?
What you will learn
- Haemoglobin carries oxygen
- Structure and function are related but distinct
Haemoglobin carries oxygen
Red blood cells contain haemoglobin, the pigment protein that binds oxygen. At lungs, oxygen enters blood and binds to haemoglobin to form oxyhaemoglobin. Blood circulation carries these red cells to body tissues, where oxygen is released for diffusion towards respiring cells. Haemoglobin transports oxygen; it does not manufacture it.
The same circulation repeatedly loads and releases oxygen. The red cell normally remains inside the vessel while oxygen moves across the exchange boundary. Do not describe the whole cell as being used up as fuel by every tissue it passes.
Structure and function are related but distinct
The biconcave shape has a thin centre and a large surface area relative to volume, supporting gas exchange. Mature human red cells lack a nucleus, leaving space for haemoglobin. These features help explain function, but the required central statement is that haemoglobin in red cells transports oxygen.
The original route cards separate lungs, transport and tissues. They show a reversible loading/release pattern rather than a new red cell forming at lungs on each circuit. A supplied haemoglobin-binding comparison is a model of capacity, not a medical diagnosis or a claim that plasma carries no gases.
Pause and explain
What happens at respiring tissues?
Worked example
Explain how a red cell helps an actively respiring muscle obtain oxygen.
Show the worked solution
- Haemoglobin loads oxygen after lung exchange.
- Circulation carries it to muscle, where oxygen is released and diffuses towards cells.
Answer Haemoglobin carries oxygen from lungs and releases it at tissues for aerobic respiration.
Try a topic test
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Test this topic →From guided practice to a new situation
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.
Complete lungs → haemoglobin loading → transport → tissue release with the carried substance.
Give me a hint
The substance supports aerobic respiration.
Compare my reasoning
- Oxygen enters blood at lungs.
- Haemoglobin carries and releases oxygen at tissues.
Oxygen is loaded at lungs, transported by red-cell haemoglobin and released at tissues.
Look for these in your work
- Name oxygen and haemoglobin
- Keep the loading/release sequence
Explain why “red cells supply oxygen by entering tissue cells” is wrong.
Give me a hint
Separate carrier location from molecule movement.
Compare my reasoning
- Red cells normally stay in the vessel.
- Released oxygen diffuses through the exchange route towards tissues.
Red cells remain in blood; oxygen released from haemoglobin diffuses towards tissue cells.
Look for these in your work
- Keep the carrier in blood
- Describe oxygen delivery
A model has the same number of red cells but half as many available haemoglobin binding sites. What prediction is justified if all other conditions are equal?
Give me a hint
Compare transport capacity rather than diagnose a person.
Compare my reasoning
- Fewer available sites can bind less oxygen.
- The model predicts reduced oxygen-carrying capacity, without clinical conclusions.
Lower maximum oxygen-carrying capacity in the model; the supplied data alone do not diagnose a learner or establish actual tissue oxygen supply.
Look for these in your work
- Link binding sites to capacity
- Retain the model limit
Common mistakes
- Saying haemoglobin makes oxygen.
- Moving whole red cells normally out of capillaries into muscle.
What can you explain now?
What happens at respiring tissues?
Compare with the explanation
Oxygen is released from haemoglobin for delivery
Cells use the delivered oxygen.
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Make it stick
Explain a complete oxygen loading, transport and release cycle.
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.