The circulatory systemSpec B9.1.1
In short
Fish have a single circulation, passing through the heart once; mammals have a double circulation, with separate pulmonary and systemic circuits.
The circulatory system is a system of blood vessels with a pump and valves to ensure one-way flow of blood. The pump is the heart. The vessels are arteries, veins and capillaries. Valves stop blood flowing backwards.
Quick check
What is the circulatory system?
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A system of blood vessels with a pump and valves to ensure one-way flow of blood.
What is the pump of the circulatory system?
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The heart.
What is the job of the valves in the circulatory system?
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To stop blood flowing backwards, so it flows one way only.
Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026