Why is octopus blood blue?

Not all blood is red like ours, the octopus's blood is blue. This blue color comes from hemocyanin, the copper-containing protein that binds oxygen in the octopus. To cope with the low oxygen levels, the octopus maintains a constant high blood pressure and has three hearts. Two of the hearts pump oxygen-rich blood through the gills, while the third circulates it through the rest of the body.