Why is blood red?

Blood is red. You have known that since before you can remember, which is exactly why it is worth stopping on, because the simple answer hides a much stranger set of facts. The colour comes down to a single metal atom and the precise way it catches light. But the truly interesting part is what blood is not. It is not blue in your veins, however blue they look. It is not the only colour blood can be. And on other branches of the tree of life, it is not even built from iron.

01 · The moleculeIron, held in a ring that catches light

Inside every red blood cell you carry are hundreds of millions of copies of a protein called haemoglobin. Its working part is the haem group: a flat, ring-shaped molecule with a single atom of iron sitting at its centre. That is where the colour lives. The whole haem unit absorbs light strongly across the blue, green and yellow part of the spectrum, everything below roughly 600 nanometres, and reflects the longer red wavelengths back out. Stack up enough of it and you get the deep, unmistakable red of blood.

A quick correction to a common half-truth: it is not the iron by itself that is red. Loose iron is not blood-coloured, and rust is a completely different reaction. The red is a property of the arrangement, the iron cradled inside its ring, poised to grab oxygen. Chemistry, not just the metal.

02 · The two redsWhy fresh blood is brighter

Blood is not one shade of red but two, and the difference is oxygen. When haemoglobin picks up oxygen in your lungs, the iron atom shifts slightly into the plane of its ring, and the molecule reflects red light more strongly. That is oxyhaemoglobin, and it gives arterial blood its bright, almost cheerful scarlet. Once the blood has delivered its oxygen to your tissues, the haemoglobin becomes deoxyhaemoglobin, which absorbs more red light and takes on a darker, more purplish crimson.

The gap is bigger than it sounds. Around 650 nanometres, deoxygenated blood absorbs something like ten times as much red light as the oxygenated kind. That is why the blood a nurse draws from a vein looks so much darker than the blood from a cut fingertip. Both are red. One is just carrying less oxygen.

03 · The mythYour blood is never blue

Here is the claim to bury, because almost everyone is taught it: that blood is blue inside your body and only turns red when it hits the air. It is completely false. Blood is never blue at any point inside you. The dark, oxygen-poor blood in your veins is exactly that, dark red. It does not perform a colour-change trick on contact with oxygen. It is already red in there, just a deeper shade than the bright stuff in your arteries.

The myth is so sticky partly because of how it is drawn. Anatomy diagrams colour arteries red and veins blue so you can tell the plumbing apart at a glance, and generations of us quietly mistook that colour code for the real thing. The “loses oxygen, goes blue” story then sounds just plausible enough to survive. But it does not survive a paper cut, or a blood test, both of which are stubbornly red.

04 · The illusionWhy veins still look blue

So why do the veins on your wrist look distinctly blue when the blood inside them is dark red? The answer is optics, and it happens in your skin, above the vessel, before the light ever reaches the blood. When light lands on you, the longer red wavelengths penetrate deepest, and down at the vein they are largely absorbed by the blood. The shorter blue and green wavelengths do not get that far: they scatter and reflect off the skin and tissue and bounce back toward your eye.

The upshot is that the light returning to you from over a vein is disproportionately blue, even though the object underneath is red. Your eye compares that patch to the pinker skin around it and calls it blue. It is genuinely the same family of effect that tints the daytime sky: short wavelengths scattering more than long ones. The vein is not blue, the blood is not blue, but the light that reaches you has been filtered into looking that way.

05 · The blue-bloodedThe animals that really do bleed blue

Now for the twist that redeems the whole blue-blood idea, because real blue blood does exist, just not in us. Plenty of animals ditch iron entirely and carry their oxygen on haemocyanin, a protein built around copper instead. Horseshoe crabs, octopuses, squid and cuttlefish, snails, many crabs and spiders: molluscs and arthropods across the board. When haemocyanin binds oxygen, the copper-oxygen complex turns a vivid, genuine blue. And when it lets the oxygen go, it does not darken like ours does, it goes clear. Their blood is blue when full and colourless when empty, a mirror image of our red-to-darker-red.

Why copper rather than iron? It comes down to the life you live. Iron-based haemoglobin packs more oxygen into a given volume and moves it fast, which suits active animals that burn through a lot of it. Haemocyanin carries less per drop but performs better in cold, low-oxygen water, which is exactly where so many of these creatures live. Iron did not simply win. Each chemistry is a good answer to a different question.

06 · The rainbowGreen, violet and the strangest lizards alive

Blue is not even the end of it. Some marine worms run on chlorocruorin, an iron pigment related to haemoglobin that looks green, especially when dilute. A handful of invertebrates like peanut worms and brachiopods use haemerythrin, which is colourless when empty and turns a soft violet-pink when it grabs oxygen, despite, confusingly, containing no haem ring at all.

And then there are the green-blooded skinks of New Guinea. Several lizards in the genus Prasinohaema have blood, muscles and even bones tinted lime green, not from any oxygen pigment but from staggering levels of biliverdin, a bile pigment. Their biliverdin is the highest recorded in any animal, far past the amount that turns a human jaundiced, and well into the range that would poison most creatures. Yet the lizards are perfectly healthy. Green blood seems to have evolved among them several separate times, which usually signals it is good for something. Nobody has pinned down what. A resistance to malaria-like blood parasites is one guess, still unproven.

07 · The payoffThe blue blood we can't do without

There is a final reason to care about all this, and it is on the shelf of every hospital. The blue, copper-based blood of the horseshoe crab contains cells that clot the instant they meet bacterial toxins. An extract of it, called LAL, is the standard test that vaccines, injectable drugs and medical implants are free of contamination that could kill you. Crabs are caught, drained of about a third of their blood, and returned to the sea, but a fraction do not survive, and their numbers are under real strain, a genuine conservation problem that a synthetic replacement, recombinant Factor C, is slowly starting to solve.

So the honest, full answer to a child’s question is longer than the child expects. Blood is red because iron in haemoglobin catches light a particular way. It is never blue, however blue your wrist looks, that is your skin playing tricks. And out across the living world, blood is a whole palette: copper blue, worm green, lizard green, invertebrate violet. Red is just the version that happened to win on our branch of the family tree, and even that came down to nothing grander than which metal was lying around.