Why Is Blood Red, and Why Do Veins Look Blue?

Blood is red because of haemoglobin, the iron-containing protein in red blood cells that absorbs blue and green light and reflects red. It is red whether or not it is carrying oxygen: oxygen-rich blood is bright scarlet and oxygen-poor blood is a darker, more purplish red, but it is never blue. Veins look blue for a different reason entirely. Skin scatters light and absorbs red more than blue over the depth of a vein, so of the light that makes it back out to your eye from that spot, blue wins. The blood underneath is dark red the whole time.

• Blood is red because haemoglobin's iron absorbs blue and green light and reflects red.
• Deoxygenated blood is darker red, never blue.
• Veins look blue because skin scatters blue light back and absorbs red on the way to and from the vein.
• Octopuses have blue blood because their oxygen carrier uses copper instead of iron.

Here is each half of the answer in turn.

The iron at its heart. Each haemoglobin molecule holds four iron atoms, each sitting in a ring structure called a haem group, and it is the arrangement of electrons around that iron-and-ring that determines which wavelengths the molecule absorbs. Haemoglobin absorbs strongly in the blue and green, around 400 to 550 nanometres, and reflects the longer red wavelengths. White light in, blue and green removed, red out. When oxygen binds to the iron, the electron arrangement shifts slightly and the absorption changes with it, which is why oxygenated blood is a brighter, more orange-red and deoxygenated blood is darker. The colour change is what a pulse oximeter on your finger is reading.

All blood is red. This is one of the most persistent myths in biology and it is straightforwardly false. Blood drawn from a vein, which has given up much of its oxygen to the tissues, is dark red, sometimes almost maroon. Anyone who has had a blood test has seen it. There is no point in the circulation at which human blood is blue. The idea probably comes from two places: the convention of drawing veins in blue and arteries in red on anatomical diagrams, and the fact that veins genuinely look blue through skin. The diagram is a convention. The blue vein is an optical effect, explained below.

Because of how skin handles light. Skin is not transparent; it scatters light strongly, and it scatters short wavelengths, blue, more than long ones. Skin and the tissue beneath it also absorb red light, and a vein a millimetre or two down is deep enough that much of the red reaching it and returning is absorbed on the round trip. Meanwhile, the blue light mostly scatters back out from the shallow layers before it ever reaches the vein. So over a vein, the returning light is a little poorer in red than the skin next to it, and by contrast it reads as blue. Over the vein-free skin beside it, the red survives and the skin looks pink. Your brain compares the two patches and calls the darker, less-red one blue. The blood has nothing to do with it. Under bright, direct illumination with the skin pressed thin, the same vein looks dark red or grey.

Because the blood has left the vessel and is being broken down. A fresh bruise is the dark red of pooled deoxygenated blood, seen through skin, so it looks red-purple or blue-black by the same scattering trick as a vein. Over the following days enzymes break haemoglobin down into other pigments: biliverdin, which is green, then bilirubin, which is yellow. The bruise cycles through purple, green and yellow as one molecule is turned into the next, and each stage has its own absorption spectrum. A bruise is a small chemistry lesson running under the skin.

No, and the exceptions are instructive. Octopuses, squid and horseshoe crabs use a copper-based protein called haemocyanin instead of iron-based haemoglobin, and it is blue when it carries oxygen and nearly colourless when it does not. Some marine worms use a protein that makes their blood green. A few Antarctic icefish have no oxygen-carrying pigment at all and their blood is almost clear. In each case the colour comes from which metal sits at the centre of the protein and how it absorbs light. Iron gives red. Copper gives blue. The animal's blood colour is a fact about chemistry, not about oxygen.

Because the light has to contain red for blood to reflect it. Under a warm incandescent bulb or in daylight, blood is a vivid red. Under some cool LED and fluorescent lights that are weak in the red, it looks darker and browner, and under a green or blue stage light it can look almost black. The blood has not changed. The light offering it something to reflect has. This is the same principle that makes a red car look grey under yellow sodium streetlights, and it is the whole idea behind a colour filter.

That red is what remains. Hold the magenta face of a CMY Cube to the light and green is removed; layer the yellow face behind it and blue is removed too; the light that comes through is red, made by taking away everything blood takes away. Now look at your own wrist through the cyan face, which removes red. The veins go from blue to almost black and the skin loses its pink, because you have subtracted the one wavelength that was doing the contrast work. You have just switched off the illusion. The science page explains the subtraction behind all three faces.

Blood is red because haemoglobin's iron absorbs blue and green light and reflects red, and it stays red, brighter or darker, whether or not it carries oxygen. Veins look blue because skin scatters blue light back before it reaches the vein and absorbs red on the way down and back, so the patch over a vein comes out less red than the skin beside it. The blood is dark red the whole time. Look through a cyan filter and the blue vanishes.

Switch the illusion off with one filter, and see light through a different lens.