1. How Many Hearts Does an Octopus Have, and What Does Each One Do?
The count is right: an octopus really has three hearts. NOAA's National Ocean Service describes two that pump blood to the gills, where it collects oxygen, and a third that sends the oxygen-rich blood around the rest of the body[1].
The two gill hearts are called branchial hearts. They are extra pumps that help the main heart, which biologists call the systemic heart[2]. Think of a relay: one pair of pumps pushes blood through the gills, and a separate pump takes over from there.
The blood also stays inside tubes, the way yours does. A 2015 paper on octopus blood describes a closed circulatory system with three hearts and veins that squeeze on their own to keep the blood moving[3].
2. Why Octopus Blood Is Blue and How It Carries Oxygen
Our blood is red because the protein that carries oxygen, hemoglobin, is built around iron. An octopus uses a different protein, hemocyanin, built around copper. Hemocyanin is colorless when it carries no oxygen and turns blue when it picks some up[4].
Octopus hemocyanin handles oxygen differently from vertebrate hemoglobin, and its performance is strongly affected by temperature and pH[3]. The 2015 paper also names a limit: hemocyanin has a lower oxygen-binding capacity than fish hemoglobin[3]. How much oxygen a given octopus's blood can carry depends on the species and on how much hemocyanin it has. Keep the puzzle in mind: an animal that is mostly muscle, running on blood with this kind of oxygen carrier.
3. What Happens to an Octopus's Blood Pressure When It Moves? A 1987 Experiment
A team led by the British zoologist M. J. Wells recorded pressure and blood flow in the main artery that runs along the back (the dorsal aorta) of common octopuses, Octopus vulgaris, that were free to move, both at rest and while active[5].
When the animal moved about, its average blood pressure, the size of each pressure pulse, and the flow of blood all doubled[5]. Your own heart handles a sprint mostly by beating faster. The octopus did not. Its heart rate changed very little[5].
So where did the extra blood come from? The paper argues that the octopus has little room to squeeze more oxygen out of each bit of blood, so the increased output of the heart must come almost entirely from a bigger push with each beat by the systemic heart[5]. In plain words, it pumped harder, not faster.
4. Jet Propulsion Can Stop the Octopus's Main Heart
Walking along the bottom is one way an octopus moves. The other is the getaway jet, squeezing water out of its body cavity (the mantle) to shoot backward. The same paper reports that jet propulsion brings on cardiac arrest of the systemic heart, the one that serves the body[5].
The authors measured the pressure inside the mantle during jetting. Their reading is that the veins cannot push blood back toward the heart against the high pressure in the squeezed mantle[5].
The final piece is a number. The oxygen debt an octopus can run up, which is oxygen it has to pay back later, is quite small, about 22 milliliters per kilogram of body weight (roughly 0.34 US fluid ounces per pound). From that, the authors conclude that jet-propelled movement is impossible for distances of more than a few meters[5].
Put it together. The jet is a fast burst, but it is also the moment the main heart falters, and the octopus has almost no oxygen credit to borrow. That is one good reason to expect an animal like this to creep, crawl, and pour itself through gaps instead of racing. That last step is a sensible reading of the measurements, not a proven result about wild behavior.
5. How Cold Water Changes the Way Octopus Blood Delivers Oxygen
Cold makes the oxygen problem harder. Hemocyanin tends to hold on to oxygen more tightly at low temperatures, which makes it harder to hand the oxygen over to the tissues[3].
Researchers compared an Antarctic octopus, Pareledone charcoti, with two warmer-water relatives. At 10 °C (50 °F), the Antarctic animal's blood released oxygen more readily than the others', with a lower oxygen affinity, and it carried much more hemocyanin. The authors report that this combination of lower oxygen affinity and higher carrying capacity helps supply oxygen near freezing[3]. Species differ, so this is not one rule for every octopus.
Back to the opening question: the number of hearts is only part of the story. How well the blood hands over oxygen depends on the blood itself and on the water around the animal, and that helps explain why three hearts are not a guarantee of stamina.
Try It Yourself: Three Ways to Explore How Octopuses and Hearts Work
- Take your own pulse twice. Find it on your wrist and count beats for 15 seconds while sitting still. Then march in place for 30 seconds and count again. Stop if you feel dizzy or unwell. Most people find the beat rate jumps. The octopus study found the rate barely moved while blood pressure and flow doubled. (Skip the jumping if a doctor has told you to take it easy.)
- Watch an octopus in a nature video (or at an aquarium, if you ever visit one). Count how many times it walks or crawls and how many times it jets. If you can, watch what it does after a jet. Keep your hands out of the tank and do not tap the glass.
- Read the original abstract. The 1987 paper, "Blood flow and pressure changes in exercising octopuses," has a short abstract that is free to read on the Journal of Experimental Biology website. See how it words the claim about "a few metres," and notice which parts are measurements and which are the authors' reasoning.