Your blood is always red — bright red when it's full of oxygen, dark brownish-red when it isn't, but never blue. Veins only look blue because of your skin: it soaks up the red light and scatters the bluer light back to your eye. The blood underneath didn't change colour; the skin is acting like a colour filter. Slide the skin thinner in the simulator and the 'blue' fades back to red.
Most people think deoxygenated blood is blue and turns red on contact with air. In fact blood is always red (bright when oxygenated, dark when not) and veins look blue only because skin scatters bluer light back to your eye.
What's actually happening
Roll up a sleeve and the veins in your wrist look distinctly blue, sometimes greenish. The intuitive conclusion, taught in some schoolbooks, is that deoxygenated blood is blue, and only turns red when it spills out and meets the air. It's tidy, it matches the colour-coded diagrams of red arteries and blue veins, and it is wrong. There is no point, ever, at which the blood inside you is blue.
Blood gets its colour from haemoglobin, the iron-bearing molecule that carries oxygen, and haemoglobin is red in both of its states. Loaded with oxygen (in your arteries) it's a bright cherry red; after delivering that oxygen (in your veins) it's a darker, brownish red — but still unmistakably red. Anyone who has given blood or had a blood test has seen venous blood in the tube: dark red, never blue. The "turns red on contact with air" story has it backwards — the blood doesn't need air to be red; it already is.
So why do the veins look blue? The answer is in your skin, not your blood, and it's the same physics that colours the sky. Light has to travel down through skin, bounce off the blood vessel, and come back up to your eye. Skin strongly absorbs the longer, red wavelengths on that round trip, while shorter, bluer wavelengths are scattered back out more efficiently — so the light that finally reaches your eye from a vein is biased toward blue. Your brain then exaggerates the effect: against the warm pink of surrounding skin, the cooler vein reads as distinctly blue (colour constancy at work). The simulator makes it literal — keep the vein deep under skin and it looks blue; slide the skin thinner and the true red shows through. The blood was red the whole time.
The 'blue' lives in your skin, not your blood: drawn venous blood is dark red, and only copper-blooded creatures like octopuses are truly blue.
- 1Press a fingertip firmly until it pales, then look at the veins on the back of that hand in good light — note how blue they read against the pink skin.
- 2Now look at a vein where the skin is thinnest and most translucent (the inner wrist of a child, say): it reads far less blue, sometimes reddish.
- 3Same blood, different skin depth, different apparent colour — exactly what the simulator shows. The "blue" lives in the skin, not the vein.
Common questions
Light has to pass through skin, bounce off the vessel and return. Skin absorbs red wavelengths on that round trip and scatters bluer ones back more efficiently, so the light reaching your eye is biased toward blue — the same physics that colours the sky.
No, that has it backwards. Venous blood drawn into a test tube is dark red, never blue, even though no air reached it. It does not need air to be red — it already is.
Yes, but not us. Horseshoe crabs, octopuses and snails use copper-based haemocyanin instead of iron, and their blood genuinely is blue-green.