Why do octopus's have three hearts?

Updated: 4/28/2022
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People rarely measure an octopus' blood pressure. I quizzed many experts to find this: The systolic and diastolic pressures of octopuses at rest are 27/15 millimeters of Mercury, says Martin Wells, a reader at the zoology department of Cambridge University and author of Civilization and the Limpet. It is high for many marine animals but not for mammals. Humans' systolic pressure (measured when the heart contracts to squeeze the blood out) is about 120 millimeters of mercury. The lowest (diastolic) pressure (measured when the heart relaxes) is about 80 millimeters. The blood pressure of an octopus is about a fifth that of humans. However, the octopus systolic pressure is twice that of a lobster. Mammals developed more efficient circulatory systems than non-mammals and have correspondingly higher blood pressures. Circulation systems pump stuff through the body via blood. Blood vessels branch repeatedly and get tiny where exchanges take place, for example in the gills or lungs where the blood picks up oxygen and dumps carbon dioxide. The blood pressure drops as the blood spreads out into a jillion small streams (the capillaries). Animals face a problem: If the pump delivers the blood to the gills or lungs with a heady pressure, little force remains to distribute the blood to the rest of the body. Most fishes never solved the problem and that's why their blood pressure is low. The octopus managed a fairly good solution by evolving three hearts. It's got two hearts to force blood through the two gills and then a main heart to force the blood everywhere else. They have another problem, though, that they share with lobsters and insects (but not fishes). The red blood cells are not equipped with hemoglobin (like ours and fish are) but rather with a poor oxygen carrier, called hemocyanin. Consequently, oxygen deprived, octopuses drift through life along a lazy path. Even sex is not "an energetic procedure in octopuses," says Wells. Their group (the cephalopods, which also include squids, cuttlefish, and the chambered nautilus) evolved when most life dwelled in the ocean-465 million years ago- before fishes swam the seas, before land plants developed spores, before vertebrates came ashore. Maybe the octopuses' circulatory system is inefficient, their blood blue, and their blood pressure high. But they manage to kill sharks. By the way, an octopus can squeeze through an opening no bigger than its eyeball.

Octopi have three hearts - one that pumps blue blood throughout their extensive vascular system similar to other molluscs, and two branchial hearts which pump blood to the gills for oxygenation.

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Q: Why do octopus's have three hearts?
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