Compared to many other animals, octopuses have rather a complicated circulatory system to ensure oxygenated blood reaches the extremities of their multi-limbed body.
Why do octopuses have three hearts?
Octopuses pump their blood around their circulatory system using three hearts instead of one. While a ‘systemic’ heart supplies the animal’s body, two ‘branchial’ hearts supply each of the two gills where the blood is oxygenated.
One of the bloodstream's main jobs is to distribute oxygen and nutrients around the body as efficiently as possible and return deoxygenated blood back to the oxygen source, like lungs or gills.
In humans oxygen is carried by the iron-based protein haemoglobin, but octopuses have the oxygen-transporting protein hemocyanin instead, as it works better in the cold, low-oxygen environments of the ocean.
However it is only one-fourth as efficient as hemoglobin in transporting oxygen, so extra pumping is needed and the octopus's three hearts come in very handy - and it's also why vertebrates just need a single four-chambered heart.