Three Hearts at Work

The three hearts split the job. Two smaller hearts push blood through the gills to load it with oxygen. The third, bigger heart then pumps that fresh blood out to the rest of the body.

There is a strange catch. The big body heart stops beating while the octopus swims. Swimming leaves it short of oxygen and worn out, which is why an octopus would almost always rather crawl along the seafloor than jet through open water.