MadSci Network: General Biology

Re: What is the color of blood?

Date: Wed Apr 18 04:03:56 2001
Posted By: Bev Fang, Medical student, Medicine, Albany Medical College
Area of science: General Biology
ID: 986001666.Gb

I'm sorry it's taken me more than a long time to respond...

You're right -- blood is always red - bright red when it is oxygenated and a darker red when it's lacking oxygen. Deoxygenated blood just **looks** blue because you're seeing it through your skin.

*One fun website with a pulsating heart presents a discussion of a question similar to yours: http://www.globalclassro

*A simple experiment on how light conduction through skin makes the veins look blue:

"Longer, redder wavelengths, though, can penetrate more deeply into the skin than shorter, bluer wavelengths before reflecting out. A vein looks blue because red light travels far enough into the skin to be absorbed by the blood in the vein. If the blood vessel is far enough below the skin, however, blue light--which would normally also be absorbed by the vein--reflects out of the skin before reaching the vein. So the light reflecting from tissue over the vein contains less red light than blue, giving the vein a bluish cast."

*A number of questions similar to the one you asked are listed in the madsci archive. I've just pasted snippets of the responses and the original message ID numbers.

--Message ID: 840084221.An

HOWEVER, oxygen poor blood is most definitely NOT blue. [I]t is a sort of purplish/maroon color at best. I work in the clinical labs, and have seen countless tubes of blood drawn from oxygen poor veins into vacuum tubes (i.e. never exposed to air), and it has never looked like the color of your veins!

Realize that to see the vein at all, light has to go THROUGH the skin and hit the blood in the vein. The blood absorbs certain colors of light, and reflects others back through the skin. ... For some reason, the combination of these effects (absorption by oxygen poor blood and absorption/deflection in the skin) gives a blue color.

[responder = Dr. Tom Wilson]

--Message ID: 839903302.An

The color of blood comes from the red blood cells (RBC's, also called erythrocytes), which make up about 40% of blood, by volume. Each red blood cell is filled with hemoglobin, the protein which carries oxygen to tissues and carbon dioxide away from tissues. Hemoglobin carries oxygen by using heme, a large ring-like molecule which has at its center a single atom of iron, which is what actually binds to the oxygen.

The nature of heme that gives it these abilities is in the many double covalent bonds that form the ring. These double bonds can be shifted into many different configurations...

When oxygen binds to the iron atom in heme, the iron changes its shape slightly, which alters the resonance of the heme molecule. This new resonance gives off a different frequency of light, so the perceived color of the heme goes from dark red to bright red. When the oxygen is released into the tissues, the iron goes back to its original shape, and the heme returns to its normal resonance, so the color goes back from scarlet to maroon.

[responder = Michael Onken]

Hope this is helpful...


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