It is fact that blood is always red so why do the veins transporting it appear blue?
Within the body the varying tissue layers absorb light of different wavelengths. Only blue wavelengths reach venous blood and are reflected back to the viewer’s eyes thus causing the observation of a blue colour.
The diameter and wall thickness of veins influences how much light is absorbed and reflected, meaning that veins can appear different shades of blue. Your brains perception of the colour is also enhanced as the blue colour is contrasted against the warmer skin tone.
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