Octopuses have a closed circulatory system, in which the blood remains inside blood vessels. Octopuses have three hearts; a systemic heart that circulates blood around the body and two branchial hearts that pump it through each of the two gills. The systemic heart is inactive when the animal is swimming and thus it tires quickly and prefers to crawl. Octopus blood contains the copper-rich protein haemocyanin to transport oxygen. This is also why Octopus blood appears to be bluish.