Circulatory and Respiratory systems
Study the diagram
This science revision note aims to help your child
– interpret the diagram representing the circulatory system (your child must identify oxygen-rich or oxygen-poor blood in different parts of the human circulatory system)
– how to write out the answer using the proper scientific keywords.
But before you read on, you might want to download this entire revision notes in PDF format to print it out for your child, or to read it later.
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The diagram below shows the flow of blood between the lungs, heart and legs.
Which blood is richer in oxygen, the blood at X or Y? Explain why.
STEP 1
Identify which blood is poorer in oxygen.
You should know that blood flowing towards the body is richer in oxygen than blood flowing away from the body. This is because blood transports oxygen to the body, and the body uses up this oxygen for respiration.
Hence, blood at X is richer in oxygen than blood at Y.
STEP 2
To explain why blood at X has more oxygen than blood at Y, you need to know where the blood is coming from and consider what happens to oxygen at that body part.
1) Blood at X is coming from the lungs.
What happens in the lungs? Oxygen is taken into the body from the surroundings and enters the blood.
2) Blood at Y is coming from the legs.
What happens in the legs? Oxygen is used up by body cells for respiration and carbon dioxide is released into the blood.
FINALLY, put your answer together
Question: Which blood is richer in oxygen, the blood at X or Y? Explain why. (2m)
Answer:
Blood at X is richer in oxygen than blood at Y.
Evidence: Blood at X is coming from the lungs.
Reasoning: In the lungs, Oxygen gas is taken into the body from the surroundings and enters the blood.
Evidence: Blood at Y is coming from the legs.
Reasoning: Oxygen is taken in by the muscular cells at the legs for respiration and carbon dioxide enters the blood.
Hence there is more oxygen in the blood at X than at Y.
Common mistakes
Incomplete answer: Blood at X is richer in oxygen than blood at Y because body cells in the leg undergo respiration.
Why is this answer incomplete?
Firstly, this answer does not link respiration and oxygen. Hence, it is unclear how respiration causes oxygen levels to be lower
Secondly, since the question asks for which blood is richer in oxygen, the explanation needs to show a comparison between X and Y. However, this answer only tries to explain why blood at Y is low in oxygen but does not explain why blood at X is high in oxygen.
Hence answer is incomplete.
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Andy Ling
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