What would happen if we had three hearts and one of them stopped? From, Marko, 8, Melbourne, Australia

Dear Marko,

It’s hard to say exactly what would happen if you had three hearts and one of them stopped. Humans, and cats, have just one heart, so we have no experience with this. Octopuses, on the other hand, do have three hearts.

When I called my friend Kirt Onthank, a professor at Walla Walla University who studies how octopus bodies work, he told me all about the three hearts. Before becoming a professor, he also studied biology here at Washington State University.

Onthank says the answer to your question depends on which of an octopus’s three hearts stops working. Octopuses have two types of hearts. Two of them are called branchial hearts and one is called a systemic heart.

altEach branchial heart sits right next to each of the octopus’s gills. The branchial heart pumps blood through the gills and after the blood leaves the gills, the single systemic heart pumps it to the rest of the body.

“The branchial hearts kind of work like the right side of your heart, pumping blood to the lungs, and the systemic heart works like the left side of your heart, pumping blood to the rest of the body,” Onthank says.

If one of the branchial hearts failed, the octopus would probably be okay. Of course, it wouldn’t be able to use the gill next to it anymore. Just as humans can live with one lung, octopuses can live with one gill.

“In fact, an octopus with one gill is likely better off than a human with one lung,” Onthank said,

That’s because, unlike humans, octopuses can also breathe through their skin. They don’t have to depend on just their gills to breathe. This ability to breathe through their skin is why they can move around on land for short periods of time.

But if the octopus’s systemic heart failed, it would be bad news. The octopus would not be able to survive because that is the heart that provides the whole body with blood, which also helps deliver important oxygen around the body.

If you thought three hearts was a lot, you might be even more surprised to learn about the hagfish, which looks kind of like a slimy, sticky eel. It actually has four hearts. One is the main heart, while the other three support it. The supporting hearts are what scientists have named auxiliary hearts.

Of course, humans only need one heart. Put your hand on your chest and you’ll feel it beating. At this very moment, the right side of your heart is receiving blood from your veins and is pumping it into to your lungs, which pick up oxygen and get rid of carbon dioxide. The left side of your heart is receiving blood from your lungs and pumping it through to the rest of your body. It pumps with a strong force—about the same force it takes for your hand to squeeze a tennis ball— and helps keep you going each day.

Dr. Universe