Previously, I wrote about various foods that aren’t actually their original natural colors for various reasons. These include cherries, oranges, margarine, wasabi, and Blue Curaçao. Now, I’m going to go for the flip side of that one.
When I ask, “What color is blood?” I’d guess that your immediate answer would be “red.” And if you’re a member of certain species, then that is true, those species being humans and most vertebrates.
But that’s not true of every species at all. It depends entirely upon chemistry.
So, if you’re red-blooded, what does it really mean? It has nothing to do with courage, valor, patriotism, or any of those silly attributes. What? Goldfish have red blood. So do dogs and cats. But why is that the case?
It’s simple. Well, it’s actually ultimately complicated, but all you need to really know is that the hemoglobin in our blood, which is the molecule that binds to oxygen and circulates it through our body, contains an iron molecule at the center of a ring structure.
This is what allows your red blood cells to circulate oxygen, out from your lungs, around your body, and back again as carbon dioxide.
If you’re wondering, “Okay, why red? I can’t see oxygen in the air,” think about this. Have you ever seen rust? What color is it? And what is rust? Oxidized iron.
In the body, in reality, the blood in the lungs starts out bright red and winds up a duller and more rust-like color by the time it comes back. But it’s red because of that iron.
But blood doesn’t necessarily need to use iron.
Swap the iron out for the metal vanadium, and you get yellow blood, which is found, for example, in beetles and sea cucumbers. Surprise, though: vanadium does nothing to circulate oxygen, so its presence is still a mystery.
While you might associate green blood with a certain popular Star Trek character, one human did surprise surgeons by bleeding green during surgery, although that was due to a medication he was taking rather than alien origins.
Otherwise, it’s really not normal for humans. But there are a few species of lizard that are very green on the inside and, ironically, it’s due to the same chemical that our bodies produce as a waste-product of red blood cell death, but which would kill us if it built up to levels that would actually turn our blood green.
That chemical is biliverdin, which is filtered out by human livers as quickly as possible via conversion to bilirubin.
It’s not such a problem for these species of lizards discovered in New Guinea, which have levels of biliverdin more than twenty-times that ever seen in a human.
Figuratively, “blue blood” refers to a member of the noble class. The English expression is actually a direct translation of the Spanish sangre azul, and it came from the noble classes of Spain wanting to distinguish themselves from the darker skinned Moorish invaders.
The nobles of Spain claimed descent from the Visigoths, who were actually Germanic and when one has paler skin, the veins that show through their skin appear blue, hence the term. Although, keep in mind that while veins may appear blue, the blood in them actually isn’t.
If you want to find real blue blood, you’ll have to seek out certain octopodes, crustaceans, snails and spiders, which are all related. Instead of hemoglobin to transport oxygen, they use hemocyanin, and you can see the clue in the name: cyan is a particular shade of blue.
Instead of iron, hemocyanin uses copper as the oxygen-binding element. When copper oxidizes, it doesn’t rust. Rather, it corrodes, so while corroded copper picks up a green patina, when it carries oxygen in blood, it imparts a blue color.
One of the most famous blue animal bloods came from horseshoe crabs, who until recently were harvested in order to collect their blood because it could be used to test for bacteria, contamination, and toxins during the manufacture of any medicine or medical device intended to go inside of a human.
While the blood harvesting isn’t intended to harm the animals, many of them were still dying in the process, so scientists finally switched to an artificial substitute.
Finally, we come to the blood color that Romans would have considered the most noble, but find it mostly in lowly worms. These animals use the molecule hemerythrin to transport oxygen, which has two molecules of iron. Before it’s oxygenated, it’s transparent. Once it’s oxygenated, it turns light purple, almost violet.
So there’s a rainbow tour of blood, proving that we have plenty of “alien” biology already here on Earth, as well as that the simplest of molecular changes can make a huge difference in a surface appearance.
Image via (CC BY-SA 4.0)