Blood is always red, never blue.

[nsh87]

"deoxygenated" blood in the body is really only about 75% deoxygenated. when you cut yourself you're still seeing highly oxygenated blood pouring out. i don't know if the oxygen molecules are ever fully released from the hemoglobin as the blood dries....this would be something to find out. if the oxygen stays attached, then at any point you're still really looking at 75% oxygenated blood.

[biohazard]

The horseshoe crab has blue blood! These odd arthropods have copper-containing protein hemocyanin instead of hemoglobin, and it is indeed blue - although when oxygenated. Although this has nothing to do with human blood, it proves one point: someone earlier in this thread claimed that venous blood turns red because it contacts the air, but this is not the case: hemocyanin turns from colourless or whitish to blue when it contacts air, because it carries oxygen extracellularly. Hemoglobin, to the contrary, carries oxygen intracellularly, and thus the colour doesn't change when you expose it to the air (although on a longer time scale the oxygen saturation changes). And yes, venous blood samples taken with the vacuum method pretty clearly demonstrate that the venous blood is red as well, just darker.

And now, I have revived a funny thread from the past with a horseshoe crab! Yey!

[MrMistery]

What does it have to do whether the pigment is in cells or not? Hemoglobin and other pigments need to be locked inside cells because it is too small and it would clog the excretion systems(be they nephridia, nephrons or whatever). Hemocyanin and other pigments are large enough to float around in the plasma/hemolymph with no ill effects on the excretory system.

[biohazard]

It has to do with the fact that hemocyanin changes its colour right after it contacts the air, whilst hemoglobin, being intracellular, doesn't react instantly unlike someone wrote in a reply in this thread, where they claimed that venous blood is blue and that it turns red because it contacts the air