Note: Readers commenting under today’s Friday Happy Hour post brought up an age old question: “Is blood blue when it’s inside the veins?” Matt Soniak kindly put together this response.
Why do veins look blue? One answer you’re likely to hear is that veins look blue because the blood inside actually is blue, because it’s deoxygenated. If you wonder why you’ve never seen blue blood before, someone might tell you that’s because when you bleed, the blood is oxygenated upon contact with air, and immediately turns red.
First things first: Our blood is not blue. It is always red.1 Even when it’s deoxygenated. Even in the absence of oxygen in a vacuum. (Remember, when you get blood drawn at your doctor’s office, they use a vacutainer, which is essentially a vacuum in a tube. The tube is attached to the needle in your arm, exposing the inside of the vein to the vacuum and drawing the blood out.)
How red it is varies. After your blood is pumped to your lungs by your heart, it’s bright red because hemoglobin — the iron-containing, oxygen-transporting protein in our red blood cells — binds to the oxygen the blood just picked up. From the lungs, the blood goes back to the heart (this is called pulmonary circulation), which pumps it out to the rest of the body via the arteries and into tiny blood vessels called capillaries, where it gives its oxygen to the body’s tissues (systemic circulation). On its return trip to the heart through the veins, the oxygen-depleted blood is dark red or maroon, because the hemoglobin is no longer bound to oxygen.
Why So Blue?
Now, I’m no surgeon, but real doctors will tell you that when you poke around inside a human being and see a vein or artery in its naked glory, it isn’t blue. If blood isn’t blue, and veins and arteries aren’t actually blue, why do our veins look blue through our skin?
When you look down at the veins in your arm, light of different wavelengths is hitting the skin, the veins and the blood. Some of that light is being absorbed, and some is getting scattered and reflected back to your eye. Different wavelengths of light have different properties and abilities. As it turns out, blue light, compared to red light 1) doesn’t penetrate the skin as well, 2) is absorbed by the blood more, and 3) is more likely to be scattered and make it back to your eye.
So, if a vein is close to the surface of the skin, most of the blue light will be absorbed, and even though red light doesn’t reflect as much, the red light:blue light ratio is high enough to make the vein appear red. With deeper veins, the blood doesn’t absorb as much blue or red light. But the blue light’s inability to penetrate as deeply as red light makes the veins appear blue.
1 Note the “our” in that statement. Humans and all other animals with backbones have red blood, but some animals, such as lobsters, crabs, crawfish, octopodes, squid, mussels and clams, do have blue blood.
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Comments (13)
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Blue light, compared to red light, IS absorbed by the blood more?
posted by Tdl on 3-19-2010 at 4:55 pm
Thats such a horrible picture!
posted by septerr on 3-19-2010 at 4:59 pm
Matt,
Thanks for the answers. Maybe you could comment on another urban legend, royalty having blue blood. Heard long ago that since royalty didn’t have to spend long days in the field farming or guarding the realm, they didn’t get tanned and thus their veins looked like they were filled with blue blood. Thoughts?
posted by Prism on 3-19-2010 at 8:26 pm
@Tdl,
Right. Shorter-wavelength, higher-frequency light like blue and violet are absorbed much more by objects than light towards the red end of the spectrum. It’s then scattered and reflected more by whatever particles it’s hitting and being absorbed by.
posted by Matt Soniak on 3-19-2010 at 9:34 pm
I think there’s an inaccuracy here. I’m pretty sure blood does not go from the lungs to the rest of the body. I think it goes from the lungs back to the heart. The heart also sends out blood to the rest of the body via the aorta. I hope I’m right here…
posted by brian on 3-19-2010 at 9:39 pm
Just had to comment on my recaptcha –
regulations aorta (no kidding!)
posted by Michael on 3-19-2010 at 9:48 pm
@Brian,
Good catch. Oxygenated blood does indeed go back to the heart before going out to the rest of the body. I appear to have skipped that step in my explanation. That’ll get fixed in a bit.
posted by Matt Soniak on 3-19-2010 at 11:00 pm
Where are my manners? Great article and topic. Forgot to mention that. I only remember the blood pathways from a single semester with the best professor I’ve ever had.
posted by brian on 3-20-2010 at 8:51 am
@Prism
A theory I’ve read is that excessive ingestion of silver can turn the skin blue. Royalty were more likely to eat with silver utensils and drink from silver goblets, so their skin took on a faint bluish tone. The assumption was there was a genetic reason for the bluish tint rather than an environmental one. The hue was mistakenly attributed to their blood being different, hence the term “blue blood.”
Click my name for a link to “the Blue Man” who ingests large amounts of colloidal silver on a daily basis. Papa Smurf, anyone?
posted by Michelle on 3-20-2010 at 4:10 pm
Brilliant explanation, my wondering has ceased. Hemo The Magnificent would be proud!
posted by Joe Maz on 3-20-2010 at 7:12 pm
Hmm… if blue light is reflecting. . . it’s blue. What’s what colour is o.o Colour is subjective.
Still agree that blood is not blue, though.
posted by Bakedpotatoes on 3-20-2010 at 11:35 pm
@Prism
did you know that women used to paint blue veins on themselves to make them look more like royalty.
posted by Pho on 3-21-2010 at 12:31 am
glad this was worked out as the back and forth in the comments on Friday’s Happy Hour was starting to get kinda harsh.
posted by Katie Rose on 3-22-2010 at 10:14 am