P5 Science

Circulatory and Respiratory Systems

This science revision note aims to help your child

– understand which part of the blood is rich or poor in oxygen.


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The diagram below shows the flow of blood in certain parts of the body.

Which of the following shows the amount of oxygen and carbon dioxide in the blood at A and B?



Let’s start from the lungs and follow the arrows to determine the amount of air in the blood at each part of the body.

1) Lungs –> Heart

Blood that is high in oxygen and low in carbon dioxide is transported from the lungs to the heart so that the heart is able to pump this blood to all parts of the body.


2) Heart –> Legs

The heart pumps the blood that is high in oxygen to the rest of the body, including the legs. At the legs, the body cells used the oxygen in the blood for respiration, and release carbon dioxide as a product of respiration.


3) Legs –> Heart

Since oxygen is used in respiration and carbon dioxide is released, the blood coming from the legs to the heart is low in oxygen and high in carbon dioxide.


4) Heart –> Lungs

The blood that is low in oxygen and high in carbon dioxide is then pumped back to the lungs, so that the carbon dioxide can be released from the body.



Summarise the findings of which blood is high or low in oxygen and carbon dioxide. The question is interested in 1 (A) and 3 (B), so we can take note of those two.

FINALLY, you can answer the question


The answer is 3.

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