Octopuses have three hearts because their blood is exceptionally bad at carrying oxygen. Unlike the iron-based hemoglobin in human blood, octopus blood uses a copper-based protein that is far less efficient, especially in cold, low-oxygen water. A single heart simply can’t generate enough pressure to force this sluggish blood through the gills and then onward to the rest of the body. The solution: split the job across three separate pumps.
What Each Heart Does
Two of the three hearts are called branchial hearts, and their only job is pushing blood through the gills. Think of them as booster pumps. They sit at the base of each gill and force deoxygenated blood through the thin gill membranes, where it picks up oxygen from the surrounding water. Without these dedicated pumps, blood pressure would drop too low by the time it reached the gills for gas exchange to work properly.
The third heart is the systemic heart, the main pump. It receives freshly oxygenated blood from both gills and pushes it out through arteries to the brain, muscles, and organs. In a human, one heart handles both jobs (pumping blood to the lungs and then to the body) by using four chambers. The octopus achieves the same thing with three separate organs instead.
How Blood Flows Through the System
The circuit works like a relay. Deoxygenated blood returning from the body flows to the two branchial hearts. Each branchial heart pumps that blood into its corresponding gill, where carbon dioxide is released and oxygen is absorbed. The now oxygen-rich blood travels from the gills to the systemic heart, which sends it out through a major artery to supply the rest of the body. Then the cycle repeats.
This two-stage pumping system compensates for the inherent limitations of copper-based blood. That blood, which is blue when oxygenated and nearly colorless when depleted, carries significantly less oxygen per unit of volume than mammalian blood. By giving the gills their own dedicated pumps, octopuses ensure blood moves through the gill tissue slowly and under enough pressure for adequate oxygen absorption.
Why Swimming Exhausts an Octopus
Here’s one of the strangest consequences of this three-heart setup: the systemic heart stops beating every time an octopus swims by jet propulsion. When the muscular mantle contracts to blast water through the siphon, that powerful squeeze compresses the veins feeding into the systemic heart. It’s like pinching a straw shut. Blood flow to the main heart pauses completely until the mantle relaxes again.
This means that during sustained swimming, the octopus’s organs and muscles are temporarily cut off from fresh oxygenated blood. The branchial hearts keep working at the gills, but the delivery system to the rest of the body stalls with every jet pulse. The result is rapid exhaustion. This is why octopuses strongly prefer crawling along the seafloor with their arms rather than swimming. Crawling doesn’t require mantle contractions, so all three hearts keep beating normally.
How Three Hearts Stay Coordinated
Keeping three separate hearts beating in the right rhythm requires its own control system. Each branchial heart has a small cluster of nerve cells called a cardiac ganglion that acts as a built-in pacemaker. These ganglia set the baseline rhythm for the branchial hearts and, through nerve connections, influence the timing of the systemic heart as well.
Research on octopus cardiac nerves shows that if these ganglia are disconnected, the branchial hearts still contract but become weak and poorly coordinated. The systemic heart, cut off from its nerve supply, drops in pumping frequency and blood pressure falls. In an intact octopus, though, the system is remarkably steady. Heart rate stays consistent across a wide range of conditions, suggesting the pacemaker ganglia maintain tight control even as the animal shifts between rest, crawling, and brief bursts of swimming.
Other Animals With Three Hearts
Octopuses aren’t the only ones with this arrangement. Squid, cuttlefish, and nautiluses, all members of the cephalopod family, share the same three-heart anatomy. The setup appears to be an ancient adaptation to life with copper-based blood in cold ocean environments, where oxygen dissolves readily in water but is harder to transport once inside the body. All cephalopods face the same fundamental challenge: low-efficiency blood that needs extra pumping power to keep tissues supplied with oxygen.
Some other invertebrates have multiple hearts too, but for different reasons. Earthworms, for instance, have five pairs of simple pumping structures called aortic arches. The cephalopod system is unique in its clear division of labor, with dedicated gill pumps supporting a central distribution heart, essentially solving the same engineering problem vertebrates solved by evolving a four-chambered heart, just from a completely different starting point.