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What makes blood red?

 
Question:  What makes blood turn red when it is exposed to air?
bzeitler

Answer:  The protein hemoglobin, which carries oxygen in the blood, changes shape when 
it binds oxygen.  When it changes shape, it absorbs different wavelengths of light, 
making it change color.  When blood is exposed to air, much more of the hemoglobin 
absorbs oxygen than had in the vein the blood came from (in the veins, the hemoglobin 
has already given up most of its oxygen to the body).  Therefore, the blood turns red.  
-ProfBill

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