Blood circulation
Different organisms have different circulation pathways for blood to travel around the body. All animals need blood to be transported to the lungs (or gills in fish) in order to be oxygenated.
After the blood is oxygenated, it travels back to the heart in amphibians, reptiles, mammals and birds in order to be transported to the rest of the body. This is called double circulation.
In fish, once the blood leaves the gill capillaries it travels directly to the rest of the body before travelling back to the heart. This is called single circulation.
Image caption, Two-chambered heart
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