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I have encountered many people in my life that believe that your blood is blue until it hits the air and only then it turns red. The blue color of our veins is also explained the same way, as the deoxygenated blood in the veins is claimed to be blue.

See for example:

When blood gives up oxygen to the different parts of the body, it becomes blue. The veins take blood back to the heart and lungs. They are near the surface of the skin, so you can see them.
Source(s):
I am a medical doctor.

Is the color of human blood ever blue, and if it is not, why are our veins blue? Does the presence of oxygen change the color of blood?

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LOL @ the source "I am a medical doctor". Well if he/she says so... Blue blood it is! –  Rudie May 2 '11 at 23:20
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@Rudie: Doctors can be like that. The difference between God and a cardiologist is that God doesn't believe he is a cardiologist. ;) –  Macke May 3 '11 at 13:35
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cardiologists, on the other hand, do believe they are cardiologists. –  Eric B Sep 4 '13 at 21:45
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2 Answers 2

up vote 60 down vote accepted

This paper holds the answer:

We investigate why vessels that contain blood, which has a red or a dark red color, may look bluish in human tissue.

[...]

To summarize, the reason for the bluish color of a vein is not greater remission of blue light compared with red light; rather, it is the greater decrease in the red remission above the vessel compared to its surroundings than the corresponding effect in the blue.

Why do veins appear blue? A new look at an old question, by Alwin Kienle, Lothar Lilge, I. Alex Vitkin, Michael S. Patterson, Brian C. Wilson, Raimund Hibst, and Rudolf Steiner

In summary: there are other reasons why veins are blue (due to the color of the surrounding tissue), and blood is always different tints of red but never blue.

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I especially like the summary; blood is not blue. –  Rudie May 2 '11 at 23:21
    
@Monkey - I live in Thailand and I saw some crabs being 'milked' for their blue blood just the other day in fact. Quite a sight! –  user2466 May 13 '11 at 8:31
    
Not that Rudolf Steiner I hope... –  Zano Jul 8 at 20:23
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Since Sklivvz already gave the scientific explanation I'll stick to visual evidence.

Have you ever donated blood?
The blood is usually drawn from a vein:

venous blood Image Source

Venous (deoxygenated) blood is dark red.


From The Franklin Institute:

Because it lacks oxygen, the waste-rich blood that flows through the veins has a deep red color, almost like maroon.

Because the walls of the veins are rather thin, the waste-rich blood is visible through the skin on some parts of the body. Look at your wrist, or hands, or ankles. You can probably see your veins carrying your blood back to your heart. Your skin refracts light, though, so that deep red color actually appears a little blue from outside the skin.



The Horseshoe Crab does have blue blood (instead of hemoglobin it uses hemocyanin):

horseshoe crab

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Just out of curiosity, do you know why they were draining them? –  JD Isaacks May 2 '11 at 18:18
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@John - Click the 'Horseshoe Crab' link and your curiosity shall be satisfied ;) –  Oliver_C May 2 '11 at 18:41
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I thought it was for crab juice. –  Monkey Tuesday May 2 '11 at 20:24
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The linked article about horseshoe crabs is awesome. –  jhocking May 2 '11 at 21:41
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I might be wrong, but I think everything is vacuum sealed, since exposing the blood to air also means exposing it to germs and other impurities in the air. –  Oliver_C May 22 '11 at 11:08
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