The blood in your veins is not blue
Venous blood is dark red. Veins look blue through the skin because of how tissue scatters light, not because the blood changed colour.
Established2 min read
Oxygenated blood is bright red. Deoxygenated blood is a darker red. Neither is blue. If you have ever had blood drawn, you have seen the colour of venous blood directly. It does not come out blue and then turn red in the air. It comes out dark red.
Skin scatters short wavelengths differently from long ones, and veins sit at a depth where that scattering, plus the absorption spectrum of blood, shifts the light that gets back out toward blue. A 1996 optics paper worked the transport through and showed the blue is a property of the path, not of the pigment.
Caveats
- Very poorly oxygenated blood is darker still. It is not blue.
- Diagrams that paint arteries red and veins blue are a useful code. They are not a claim about paint.
Sources
- 1Kienle A. et al.. Why do veins appear blue? A new look at an old question. Applied Optics 35, 1996.