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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?

Sklivvz
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JD Isaacks
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    LOL @ the source "I am a medical doctor". Well if he/she says so... Blue blood it is! – Rudie May 02 '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 03 '11 at 13:35
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    cardiologists, on the other hand, do believe they are cardiologists. – Eric B Sep 04 '13 at 21:45

2 Answers2

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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.

Sklivvz
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  • I especially like the summary; blood is not blue. – Rudie May 02 '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! –  May 13 '11 at 08:31
  • Not [that](http://en.wikipedia.org/wiki/Rudolf_Steiner) Rudolf Steiner I hope... – Zano Jul 08 '14 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

Community
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Oliver_C
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    Just out of curiosity, do you know why they were draining them? – JD Isaacks May 02 '11 at 18:18
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    @John - Click the 'Horseshoe Crab' link and your curiosity shall be satisfied ;) – Oliver_C May 02 '11 at 18:41
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    I thought it was for [crab juice](http://www.youtube.com/watch?v=Skurheg3P6I). – Monkey Tuesday May 02 '11 at 20:24
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    The linked article about horseshoe crabs is awesome. – jhocking May 02 '11 at 21:41
  • `...a quart of horseshoe crab blood has a price tag of an estimated $15,000...` Wow, just wow – Move More Comments Link To Top May 03 '11 at 04:44
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    Just being a devil's advocate, "Have you ever donated blood? The blood is usually drawn from a vein"; the equipments used to extract these bloods are not air-tight, aren't they? Isn't it possible that the blood turned from blue to dark red when extracted since they get into contact with the air in the equipment. – Lie Ryan May 21 '11 at 14:05
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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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    Surely the needle is exposed to the air (and thus to all its impurities) and I don't remember ever having a vacuum seal around the point where it broken my skin... – configurator Oct 27 '11 at 00:57
  • [Venipuncture and evacuated tube system](http://en.wikipedia.org/wiki/Venipuncture). There will be air in the needle, but that's a miniscule amount. Last time I donated blood it was half a liter. – Oliver_C May 25 '12 at 08:00
  • Yeah, it's not like your skin, which doesn't like holes in it, wouldn't be snug around the needle. – Rig May 25 '12 at 14:42
  • PS: Having lived in the mid atlantic area there are tons of dead horseshoe crabs littering the beach. Wonder how much this has to do with that... – Rig May 25 '12 at 14:45
  • @Rig Quite a lot. Read the paragraph [**Impact of Amebocyte Harvesting on Horseshoe Crab Behavior and Physiology**](https://www.frontiersin.org/articles/10.3389/fmars.2018.00185/full#h4) in [The Role of Horseshoe Crabs in the Biomedical Industry and Recent Trends Impacting Species Sustainability](https://www.frontiersin.org/articles/10.3389/fmars.2018.00185/full). – cdlvcdlv May 16 '19 at 18:38