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From Cecil's Mailbag by the Straight Dope Science Advisory Board
Dear Straight Dope:
What color is blood inside the body? If it is red then why do veins
look blue? --WishNmoon
SDSTAFF Hawk replies:
Blood is a bright red in its oxygenated form (i.e., leaving the
lungs), when hemoglobin is bound to oxygen to form oxyhemoglobin.
It's a dark red in its deoxygenated form (i.e., returning to the
lungs), when hemoglobin is bound to carbon dioxide to form
carboxyhemoglobin.
Veins appear blue because light, penetrating the skin, is absorbed
and reflected back to the eye. Since only the higher energy
wavelengths can do this (lower energy wavelengths just don't have
the *oomph*), only higher energy wavelengths are seen. And higher
energy wavelengths are what we call "blue."
In an experiment, glass tubes were filled with blood and immersed
in milk, milk having a similar ratio of fat, proteins, and water in
emulsion as skin. At a certain depth, the tubes appeared blue.
As a phlebotomist (a person who draws blood) at the local
university hospital, I drew hundreds of samples of venous blood.
Almost all samples were dark red. Those that weren't were usually
because people were on enriched oxygen systems.
At the same time, respiratory therapists drew blood from an artery,
not a vein. These samples were characteristically bright red,
unless the patient was having difficulty breathing (asthma, people
with one lung, pneumonia, emphysema, whatever).
So, your veins may look blue, but even a blueblood will bleed
red.
--SDSTAFF Hawk
Straight Dope Science Advisory Board
Cecil's Mailbag is researched and written by members of the Straight Dope Science Advisory Board, Cecil's online auxiliary. Although the SDSAB does its best, these articles are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.
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