What Is the Color of Human Blood? Is It True That Blood Is Blue?

A healthy person's blood is red in hue and flows through their veins.

Our red blood cells, our red blood cells, are termed "red" for a reason: "heme" molecules. Because of their structure, these molecules are red in color. As a result, a blood molecule carrying no oxygen will appear red.

However, the blood in our veins can seem blue, purple, or even green at times, especially if they are dirty and don't contain oxygen, causing some people to believe that blood is blue rather than red. This is entirely incorrect. Even unclean blood is never blue; it only seems that way due to the skin's reflection.

The oxygen-carrying molecules in hemoglobin are known as "oxyhemoglobin," and they are red in color. Hemoglobin molecules make up 96 percent of red blood cells. However, hemoglobin may also contain 10% carbon dioxide, resulting in the formation of the "carboxyhemoglobin" molecule, which appears blue on the skin.

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Cyanosis is a condition in which the blood appears to be blue owing to various disorders. Hemoglobin, for example, becomes a brown molecule termed "methemoglobin" when it is irreversibly oxidized (changed to Fe+3 instead of Fe+2) and can no longer carry oxygen. It has a blue tint when applied to the skin. This accounts for 2% to 3% of red blood cells. Sulphemoglobinemia is a condition in which hemoglobin partly binds oxygen. The veins appear blue in this extremely unusual condition. Blood is blue in several invertebrates (particularly shrimp and crabs). Blood in vertebrates, on the other hand, is always crimson.