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Although octopuses are often imagined as simple sea creatures, their circulatory system is remarkably complex. An octopus has three hearts. Two of these, known as the branchial hearts, sit beside the gills and push oxygen-poor blood through them so it can collect oxygen. The third, the systemic heart, then sends this freshly oxygenated blood out to the rest of the body. The blood itself is blue rather than red, because it carries oxygen using a copper-based protein instead of the iron-based one found in human blood. This copper protein performs well in the cold, low-oxygen conditions of deep water.