The circulatory system
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of the structure and function of cells, tissues and organs.
What is the function of the heart?
To pump blood around the body.
What is a tissue?
A group of similar cells working together for a particular function.
What is an organ?
A collection of different tissues that work together to carry out a particular function.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the circulatory system, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
The Circulatory System
What is the Circulatory System?
The circulatory system transports substances such as oxygen, carbon dioxide, nutrients, and hormones around the body. It is made up of the heart, blood vessels, and blood.
Double Circulatory System
Humans (and other mammals) have a double circulatory system, meaning blood passes through the heart twice on each complete journey around the body:
1. Pulmonary Circulation:
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Deoxygenated blood is pumped from the right side of the heart to the lungs.
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In the lungs, gas exchange occurs - carbon dioxide is removed from the blood and oxygen passes into the blood. This happens by diffusion..
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Oxygenated blood then returns to the left side heart from the lungs.
2. Systemic Circulation:
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The left side of the heart pumps oxygenated blood to the rest of the body.
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Cells use oxygen for respiration and the blood becomes deoxygenated.
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It returns to the right side of the heart to be pumped to the lungs again.
Why is double circulation important?
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Maintains high pressure in the systemic circulation so oxygen-rich blood reaches body tissues quickly.
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Separates oxygenated and deoxygenated blood - improving efficiency.
Key Terms:
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Double circulation – blood passes through the heart twice in one full cycle. The right side of the heart pumps blood to the lungs and the left side of the heart pumps blood around the body.
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Pulmonary circulation – circuit of blood from the heart to lungs and back.
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Systemic circulation – circuit of blood from the heart to body and back.
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Exam Tip
If asked to describe or explain double circulation, make sure you clearly state:
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the two separate circuits, and
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the direction of blood flow (e.g. right side → lungs, left side → body).
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Practice Question
Explain how the structure of the circulatory system in mammals is adapted to supply oxygen efficiently to the body. (4 marks)
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Mammals have a double circulatory system.
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One circuit carries blood from the heart to the lungs and back (pulmonary).
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The other circuit carries blood from the heart to the body and back (systemic).
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This allows oxygenated blood to be pumped at high pressure to the body, ensuring efficient delivery of oxygen to tissues.
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!