 The blood of the octopus is composed of copper-rich hemocyanin, which is less efficient than the iron-rich hemoglobin of vertebrates, thus does not increase oxygen affinity to the same degree.Oxygenated hemocyanin in the arteries binds to CO2, which is then released when the blood in the veins is deoxygenated.The release of CO2 into the blood causes it to acidify by forming carbonic acid.