Is human blood blue?

Human blood does not appear blue under any circumstances. In fact, vertebrate blood in general is red. An exception is skink blood (genus Prasinohaema), which contains hemoglobin yet appears green because it contains a large amount of the protein biliverdin. read more

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). read more

Human blood is always red. The only difference is that when it is oxygenated, it is a bright red, and when it is depleted of oxygen, it is a darker red. The myth of blue blood may have several origins, 1) veins, which carry the blood once it is low on oxygen, look blueish green, but that's because of the tissue that makes up the veins and is not due to the blood itself. read more

Blue light does not penetrate human tissue as deeply as red light does. As a result, veins that are close to the surface of the skin will be more likely to reflect blue light back to the eye. The oxygen-depleted blood that veins carry also tends to absorb red light more than blue light. read more

Despite these things, nobody's blood has ever been blue. You can double check this the next time you are at a doctor's office and need to have a blood sample taken. If you are brave enough to look at the blood being drawn, it is notable dark red, it is the blood from your veins that is low in oxygen. read more