Blood is blue until exposed to air.
Don’t Believe That!
Most of us first heard this one in grade school – and it makes a lot of sense to an eight-year-old. The “evidence” was, after all, right there before your eyes: just look at blue color of the veins in your wrist.
A surprising number of people continue to believe this myth into adulthood. There’s even a scientific-sounding explanation for our grownup minds: blood flowing through veins toward the heart is blue because it has no oxygen; oxygenated blood flowing away from the heart, through arteries, is red. It doesn’t help that most medical textbooks and anatomical diagrams use blue and red to illustrate the difference between veins and arteries.
When you look at your wrist you are not looking at “blood,” you are looking at the veins that carry the blood, covered by a layer of skin. It’s the skin itself (in Caucasian folks, anyway) that generally refracts red light while reflecting blue light. Deoxygenated blood is actually an even darker red than oxygenated blood, but blood is always some shade of red due to the presence of hemoglobin.
Bonus Fact: Due to a complete lack of skin pigmentation, veins in the wrists of albinos actually appear red, not blue.