Fishes  have  a two-chambered
   heart and a single circuit of blood flow.
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   Blood  is  pumped  from the ventricle to the gills via arteries (blood
   vessels that carry blood away from the heart).
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   After picking up O[2] and expelling CO[2], blood returns to the atrium
   via veins (blood vessels that carry blood to the heart).
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   There  is  no  segregation of oxygen-rich and oxygen-poor blood in the
   heart.