• Question from:Tyler Deem

I recently had a discussion in my biology class on what color blood is.Most of my class, including my biology teacher, were saying that blood is blue until the blood comes in contact with oxygen. I'm very sure that he is incorrect and that blood is always a red. Could I have some feedback on what color blood really is and why it appears blue. I know that there are two types of blood and that venous blood is a very dark red, but NOT blue. Also that veins appear blue from that outside because of the color of skin.

I would like some reputable information on the matter, and would like to show him proof that blood is not blue.

One thing that makes no since if blood is blue is that when you have blood drawn directly from your veins into a vaccuum, the blood is still red, and it never touches oxygen. Also that oxygen is carried by blood cells, and so they would always be red anyway. I do realize that when blood cells have less oxygen it is darker than when it has lots. I researched the web on good websites for information on the topic, and all said that blood is RED. Only then when I couldn't find a good article on the topic did I decide to send this.

Please send some information on this topic so I can, as funny as it sounds, prove my 10th grade BIOLOGY TEACHER wrong.

Thank You,
                 Sincerely,     Tyler Deem

HI Tyler,

I wonder if your teacher was actually referring to the text book scheme used to show the circulatory system?  In text books arteries are most often coloured red, and veins blue. Human blood is red because of the iron in the haemoglobin. This gives red blood cells their red colour.  Veinous blood is much darker than arterial blood, and your teacher is right that oxygenated (arterial) blood is 'redder' than deoxygenated (veinous) blood.  However, human blood is not blue. 

Some animals do have blue blood though.  A good example is the horseshoe crab- Limulus polyphemus (actually this animal is more closely related to spiders and scorpions than crabs).  Its blood is blue, because rather than iron rich haemoglobin, the oxygen binding molecule is 'copper' based, giving the blood a bluish tinge. 

I hope that this answers your question.  I am sorry that I cannot give a reference to a paper to give more details.

Last edited by Neil Gostling (31st Oct 2008 02:14:38)

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  • University of Bristol
  • Posts: 2,467

Agreed - arterial (oxygenated) blood is bright red and and venous is much darker but still unequivocally red! Even in a vacuum it would still be red because of the red haem pigment, as Neil says.

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  • QMUL, London
  • Posts: 1,022

This I understand, but what then causes the optical illusion that makes it *appear* to be blue through the skin. I have veins on my hands that look blue (ish), so why is that?

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  • University of Bristol
  • Posts: 2,467

I have previously answered that question David - see

http://www.askabiologist.org.uk/punbb/v … hp?id=1289

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  • QMUL, London
  • Posts: 1,022

Ah, falling prey to not checking our own index! :) Thanks.