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Biology

Comparative Anatomy of Vertebrate Circulatory Systems

Quick fact

Fish have a single circulatory circuit where blood travels from heart to gills to body and back, while mammals have a double circuit—one to the lungs and one to the rest of the body—allowing oxygen-rich and oxygen-poor blood to stay completely separate.

Why this is interesting

You know your heart has four chambers, but did you know a fish’s heart has only two, and a frog’s has three? How does a single pump or a partially divided heart manage to keep an animal alive?