“The Octopus Has Three Hearts and Blue Blood” packs a lot of chemistry into one stanza: the octopus’s blood is blue, it says, because the molecule that carries its oxygen is built “round copper, not the iron ours chooses, / and copper lends a bluish shade”. That’s true, and it’s the start of a much bigger story about how animals solved the problem of moving oxygen around, and why the octopus’s solution comes with a cost.
Every animal bigger than a speck has the same problem. Its cells need oxygen to burn food, but oxygen doesn’t dissolve well in water, and blood is mostly water. Plain water could carry only a tiny fraction of what a busy body needs. So animals evolved special proteins that grab oxygen where it’s plentiful, at the lungs or gills, and let it go where it’s scarce, in the working tissues. There are two main designs on Earth, and the poem names them both.
Ours is haemoglobin. It’s a protein made of four chains, each holding a flat ring-shaped structure called haem, with a single iron atom at its centre. An oxygen molecule sits on that iron atom, so each haemoglobin molecule can carry up to four. And it’s packed inside red blood cells, which are little more than bags of haemoglobin: each one holds about 270 million molecules of it. It’s the haem that makes blood red: bright scarlet when loaded with oxygen, darker, more maroon, when it has given much of it away. It is never blue.
The octopus uses haemocyanin. Instead of iron in a haem ring, it uses copper, and it uses two copper atoms, side by side, to grip one oxygen molecule between them. When oxygen is bound, the copper atoms change their electrical state, and that changes which colours of light the molecule absorbs. It soaks up orange-red light and reflects blue, so oxygen-rich octopus blood is blue. Without oxygen, it’s almost colourless. Haemocyanin molecules are enormous, built from many units each with their own copper pair, and rather than being kept in cells, they float freely in the blood fluid itself.
Here’s the cost. Haemocyanin carries less oxygen than our system does. There’s less of it in a given amount of blood than there is haemoglobin in ours, and each copper pair holds only a single oxygen molecule. So a millilitre of octopus blood delivers noticeably less oxygen than a millilitre of ours. The octopus makes up for it with a high-pressure circulation, those three hearts the poem describes, two of them dedicated to pushing blood through the gills. But it remains an animal with a limited supply of oxygen. Add the poem’s other detail, that its main heart stops beating while it swims, and you can see why octopuses jet away only in short bursts and spend most of their time crawling and hiding.
So why use copper at all? Partly history: evolution works with what it has inherited. The ancestors of molluscs and arthropods built their oxygen carriers from copper-containing proteins they already had, while our ancestors built theirs from iron-containing ones. And haemocyanin has advantages in the conditions many of these animals live in. Some kinds seem to hold on to oxygen well in cold, low-oxygen water, and to keep working across a range of temperatures. Horseshoe crabs, which have also had blue blood for hundreds of millions of years, are proof that the design lasts.
Which brings us to your veins. Look at the inside of your wrist and the veins there look blue. They aren’t. The blood in them is a deep, dark red. What’s happening is a trick of light in the skin. Red light passes deeper into skin than blue light does, and much of it is scattered back out. Over a vein, the dark blood soaks up the red light that reaches it, so less red comes back from that spot than from the skin around it. Your eyes and brain compare the two areas and report the vein as bluish. It’s the same kind of comparison the human-eye review described, the brain judging colour by contrast. Nobody, human or otherwise, has red blood that turns blue, and only some animals, the octopus among them, have blood that is really blue.
So the poem’s line holds. Copper, holding oxygen, lends a bluish shade. And iron, holding it in a ring inside a cell, makes ours the colour it has always been.