Circulatory systems — Cambridge IGCSE Biology
The circulatory system is a pump, vessels and valves giving one-way flow; compare the single circulation of a fish with the double circulation of a mammal.
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The circulatory system is a system of blood vessels with a pump, the heart, and valves to ensure one-way flow of blood. In a fish's single circulation, blood passes through the heart once per circuit. In a mammal's double circulation it passes through twice, so blood is pumped again after the lungs and reaches the body at high pressure.
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Quick check questions
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.
Supplement How many times does blood pass through the heart in one circuit of a fish?
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Once (single circulation).
Supplement Name the two circuits of a mammal's double circulation.
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Pulmonary (heart to lungs and back) and systemic (heart to body and back).
Supplement Give one advantage of a double circulation.
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Blood is returned to the heart and pumped to the body at high pressure, so substances are delivered faster.
Frequently asked questions
What is the difference between single and double circulation?
Supplement In a single circulation, as in a fish, blood passes through the heart once in each complete circuit: from the heart to the gills, then on to the body. In a double circulation, as in a mammal, blood passes through the heart twice, through a pulmonary circuit to the lungs and a systemic circuit to the body.
Why is a double circulation more efficient than a single circulation?
Supplement After blood passes through the lung capillaries its pressure is low. In a double circulation it returns to the heart to be pumped again, so it reaches the body at high pressure and delivers oxygen and glucose quickly. Mammals have a high rate of respiration, so they need this fast supply. Lung pressure stays low, protecting the capillaries.
Why does the circulatory system need valves?
The circulatory system needs valves to ensure a one-way flow of blood. The circulatory system is a system of blood vessels with a pump, the heart, and valves. Valves in the heart and in veins shut when blood starts to flow backwards, so blood keeps moving in one direction around the body.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026