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Why do they call it blue blood?

The term “blue blood” refers to those of noble birth or high social status. It originates from the Spanish phrase “sangre azul,” meaning blue blood. This strange phrase has an interesting backstory related to the physical appearance of blood in different animal species. In this article, we will explore the origins of the blue blood concept, examine the science behind the different blood colors in vertebrate species, and explain why the nobility came to be associated with blue blood.

The Origins of “Blue Blood”

The notion of blue blood originating with noble birth dates back to medieval Spain. At the time, the Spanish nobility claimed never to have interbred with the Moors, who had invaded and occupied parts of Spain starting in the 8th century.

The Spanish nobility used the idea of blue blood to state their blood was never contaminated by Moorish or Jewish blood lines. This concept of pure blood gave them a sense of prestige and aristocracy.

Over time, the phrase “sangre azul” or blue blood became associated with aristocracy in Spain. It was a way for the Spanish nobility to distinguish their ancestry and assumed purity of blood from those of lower social classes.

The idea spread across Europe, with blue blood becoming synonymous with royalty and the hereditary ruling classes in many countries. While the concept has clear racist and discriminatory origins, the notion of blue blood representing nobility persists even today.

The Different Blood Colors in Vertebrates

So where does the association between blood and the color blue originate from? Interestingly, there is some valid physiological reasoning behind the metaphor.

Red Blood in Mammals

In nearly all mammals, blood appears red. This is because mammalian red blood cells contain hemoglobin – an iron-containing protein that transports oxygen through the bloodstream. Hemoglobin is red in color and gives mammalian blood its red hue.

Human blood appears red because the hemoglobin in our red blood cells is oxygenated. The iron in hemoglobin turns red when combined with oxygen. Deoxygenated blood may take on a darker maroon or purple-red shade.

Thus, the red color we associate with blood is primarily driven by the hemoglobin and iron content. All mammals, from humans to dogs to kangaroos, have iron-based hemoglobin and therefore red blood.

Blue Blood in Horseshoe Crabs

While mammalian blood is red, some invertebrate species have blue blood. The most well-known example is the horseshoe crab.

Rather than hemoglobin, horseshoe crab blood contains hemocyanin – a copper-based protein that transports oxygen. Copper turns blood blue when oxygenated, hence giving the horseshoe crab its characteristic blue blood.

Green Blood in Some Lizards

Beyond red and blue blood, some lizard species have green blood! Certain lizards have bile pigments in their blood plasma that give it a greenish color.

For example, some skinks found in New Guinea have bright green blood. The green color comes from high levels of the bile pigment biliverdin.

So while mammalian blood is red, blood can take on a bluish or greenish hue in certain non-mammal species. The metaphor of “blue blood” for nobility stems from this biological difference between mammalian and non-mammalian blood.

The Association Between Nobility and Blue Blood

During the medieval era in Europe, nobility were the only segment of society educated enough to know about the biological differences between mammalian and non-mammalian blood. The blue color of blood in invertebrates like horseshoe crabs was seen as rare and special.

The Spanish nobility latched onto the distinctiveness of blue blood as a symbol of their supposed genetic purity and nobility. As knowledge of the metaphor spread across medieval Europe, blue blood became associated with royalty across many countries.

Of course, human blood is very much red in color regardless of one’s ancestry. But the concept of blue blood as an indicator of nobility persisted through medieval tradition and folklore. It remains an enduring idiom in many cultures today.

While the origins of blue blood are based in questionable racial biases, it continues to evoke a sense of prestige and aristocratic heritage. The metaphor endures as a cultural construct detached from its biological beginnings.

Key Takeaways

– The notion of “blue blood” originated in medieval Spain as a way for the nobility to claim genetic purity from Moorish and Jewish bloodlines.

– Biologically, blue blood refers to the copper-based hemocyanin protein in invertebrates like horseshoe crabs, which turns their blood blue.

– In contrast, mammalian blood contains iron-based hemoglobin, giving it a red color.

– Medieval European nobility knew blue blood was rare in nature and co-opted the concept as a symbol of assumed genetic superiority.

– While human blood is always red, the cultural idiom of blue blood persists today to represent nobility and royalty.


While the idea of blue blood purporting genetic superiority has reprehensible origins, it continues to evoke an aura of aristocracy and prestige. The biological underpinnings that made blue blood seem rare and special in medieval times persist today.

However, we know all humans share the same red blood, regardless of ancestry. Beyond its problematic beginnings, blue blood endures as an interesting metaphor connecting biological physiology and cultural notions of nobility. Its origins remind us to be wary of cultural constructs rooted in racial bias, even if they seem innocuous today.