Why do octopuses have three hearts?

An octopus has three hearts to overcome the inefficiencies of its copper-based blood, which doesn't carry oxygen as well as our iron-based blood. Two "branchial" hearts pump oxygen-poor blood through the gills, while one "systemic" heart circulates the oxygen-rich blood to the rest of the body.

This unique three-heart system divides the heavy circulatory workload required to keep these active marine animals alive:

Branchial Hearts (2): These two hearts are located near the gills. Their sole job is to take deoxygenated blood from the body and pump it through the gills to pick up oxygen from the water.

Systemic Heart (1): This central heart takes the newly oxygenated blood and pumps it forcefully throughout the rest of the body, including the arms and organs.