An octopus has one main heart called the systemic heart and two smaller ones that are located near the its gills. These two smaller hearts function much like the right side of the human heart. They pump oxygen-depleted blood to the gills, where it exchanges carbon dioxide for oxygen, and then pump this refreshed blood to the systemic heart. The systemic heart then propels this new oxygenated blood throughout the octopus’s body, just like the left side of the human heart! The major difference between a human and an octopus’s circulatory systems (other than the number of hearts) is that unlike the red blood of men and women, octopuses have blood that is blue! That is because their blood contains a copper-rich protein called hemocyanin instead of the reddish hemoglobin found in humans!