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"Well, I see your problem: You're bleeding tang!"
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On the outside, they may be Human Aliens, completely indistinguishable from us Puny Earthlings. Or they can be shapeshifters. Or Rubber-Forehead Aliens. Whatever...

What is the best way to quickly establish that a character is not human, sometimes to shock other characters around? Dissection? Squick. They may have Bizarre Alien Biology, but that's usually just an informed characteristic which is rarely appealed to, if ever. Unless it's something very obvious, like having two hearts. Solution? Make them bleed. If their blood (assuming it is actually blood and doesn't serve an entirely different purpose in the body) is not red, or nonexistent at all, they might as well have the words "NOT HUMAN" written on their forehead.

The trope, of course, is not limited to Human Aliens (Plant Aliens often have green blood, for no apparent reason), but for other types, it doesn't allow for The Reveal in such an effective way.

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With Human Aliens, this is a very mild case of Artistic License – Biology: Since the skin is translucent, a creature with different-colored blood wouldn't have the same skin color as a human...if the skin is white. Darker-skinned aliens, or ones with scales, feathers, hair, or something else hiding their skins avert this problem, albeit probably unintentionally.

Of course, here on Earth, all vertebrates (with the exception of certain lizards and genetic conditions), the largest and most complex members of the animal kingdom, have red blood—the reason? Because all animals breathe oxygen. Red blood cells are red because they contain a protein chemical called hemoglobin, which is bright red in color. This is because hemoglobin contains iron, which is vital for transporting life-sustaining oxygen where it needs to go. There are other pigments like hemocyanin or chlorocurorin that come in such vibrant shades as pink, orange, blue, and green, but these are much less efficient at transporting oxygen than hemoglobin and thus only appear in invertebrates, which have smaller bodies and a better surface-area-to-volume ratio, and thus less need of efficient oxygen transport. Usually hand-waved by having different chemicals in their blood.

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In Video Games, aliens and monsters often have different colors of blood than red, frequently appearing as optional Adjustable Censorship, to appease the Moral Guardians. Because it's okay to shoot non-human beings if they don't bleed the same way.

Assuming it's not red like human blood, aliens and monsters most commonly have blood that's either black or some bright color usually associated with something unhealthy, like orange, green, or yellow (blue and violet are possible, but rare).

Not to be confused with Blue Blood or Black Blood, even if it is, in fact, blue or black. (Though some blue-blooded aliens can, in fact, be Blue Bloods.) Also not to be confused with a book of same name by Joan D. Vinge.

See also Machine Blood, which is for bleeding machines/robots.


Example subpages:

Other examples:

    open/close all folders 

    Asian Animation 
  • In Happy Heroes, Shapeshifter's blood is green, while Kalo's blood is blue.

    Card Games 
  • Magic: The Gathering: Old Stickfingers, a boogeyman that haunts the forests of Innistrad, has white blood that's compared to liquid moonlight and to mercury in appearance.

    Films — Animation 
  • Hercules: The Hydra's blood is green slime.
  • In Interstella 5555, Shep's blood is yellow.
  • Stitch's blood from Lilo & Stitch is pink, as seen when Gantu extracts some to put into some gun turrets so they will lock onto his every move if he tries to escape.
  • Though not seen directly, the song "Kidnap the Sandy Claws" in The Nightmare Before Christmas mentions being "beaten black and green", suggesting this is the case.
  • Rahu and Ketu (who are definitely not human)'s blood in The Return of Hanuman is green.
  • The mutants in the Taarna segment of Heavy Metal have green blood, as a side effect of being tainted by the Loc Nar's power.

    Myths & Religion 
  • Christianity: Wine is said to actually be the blood of Jesus Christ. Regardless of one's religious beliefs, such unique biology would be understandable for a demigod. This is not viewed literally though, and even then it's said wine becomes his blood via transubstatiation (invisibly).
  • Classical Mythology: Gods and other divine beings don't bleed red blood but a golden glowing substance called Ichor.

    Tabletop Games 
  • Dungeons & Dragons:
    • Fire trolls have mauve blood that corrodes metal.
    • Oriental Adventures: One way to identify octopus hengeyokai (shapeshifters) is that, even in human form, they have blue blood.
  • Malifaux: The Nephilim have black blood that burns any non-Nephilim that come in contact with it.
  • Vampire: The Requiem downplays this with the "vitae" of vampires: it looks like the mortal blood it's derived from, sometimes growing thicker and darker as the vampire's blood potency grows, but flows according to the vampire's will — it doesn't spill out of injuries, gravity or no, unless the vampire wants it to.
  • Warhammer 40,000 has a number of different takes on this:
    • In an aversion of the "Human Aliens have coloured blood and pink skin" error, the Tau do not look like humans partly because they have deep blue-purple blood (their equivalent of haemoglobin uses cobalt instead of iron), which has the expected effect on their skin tone. In one of the Ciaphas Cain novels, Amberley Vail notes that it smells horrible.
    • The Orks, who are in fact genetically-engineered carnivorous space-fungi, have green skin but red blood (and mouths). It wasn't always this way and earlier sourcebooks for the game described Ork blood as green, but eventually red gore was decided on. This was explained in White Dwarf magazine with an in-universe biologist's report: while Orks' dense green skin is a chlorophyll- and spore-producing layer, the rest of their body is relatively normal meat, and they have a normal (if supercharged) food-based metabolism to support it. On the same page, the game developers gave the real reason: green injuries on green-skinned aliens were hard to notice, and made the models look like "they've had an accident eating a gooseberry squishy". Some sources have indicated that the Orks' blood does double duty as their equivalent of a lymphatic system, and that the colour of the blood will as a result vary depending on their diet, ranging from bright crimson to almost black.
    • The Eldar have red blood like humans, but it crystallizes instead of forming scabs.
    • On the transhuman spectrum, Space Marines are sometimes mentioned in-universe to have extremely bright red blood, due to its efficiency in transporting oxygen. Some also have corrosive or otherwise dangerous blood as a further defense against attackers.
  • Warhammer Fantasy: Older editions allowed Chaos Champions and mutants to gain this — as Lovecraftian Superpowers. Some of the types of blood they could have included molten metal, electrically-charged slime, and a symbiotic Blob Monster that reacts to anyone poking holes in its host's skin by sticking Combat Tentacles out through the wounds to try and kill them. The most detailed list can be found in the Tome of Corruption for Warhammer Fantasy Roleplay, which details twenty different kinds of blood replacement.
    • Acid, electricity, fire, molten metal and poison, obviously, are all lethal to your assailant.
    • There are several variants where "blood" equates to "living animals" — centipedes, ants, beetles, leeches, maggots, worms, even mice and small birds. In all cases, these attack the person who wounded you, usually causing minor damage and fighting penalties (birds just cause penalties and mice cause just damage).
    • Excrement (you do not want details) makes you so disgusting that your enemies find it harder to fight you.
    • Eyeballs give you a tactical advantage because you can still see through them.
    • Glue can temporarily stick an enemy against you.
    • Mucous, mud and wax are disgusting, but harmless.
    • Protoplasm means that if you die, a Blob Monster will emerge and attack your killers.
    • Tar will spurt out and make your enemy all sticky, impairing their mobility.
    • Vines will rip into enemy flesh, harming them and impairing movement.
    • Vomit is not only disgusting, but induces mutations.
    • Wind is deafening and eerie.

    Theme Parks 

    Visual Novels 
  • In Dra+Koi dragons bleed gold. When in human form, this also includes normal human blood.
  • Zigzagged in Marco and the Galaxy Dragon. While many aliens have purple blood and a few have green blood, some—like El Skeleton and Gargouille—have red blood.
  • In Policenauts, the "Frozeners" have white artificial blood, called "first-generation" blood, which is said to be simpler than the "third-generation" blood used on Home (Earth). It's also said to transport oxygen more efficiently than regular blood, making blood loss less of a threat.

    Real Life 
  • Arthropods:
    • While certainly not aliens, most insects bleed light yellow (as mentioned above), some green; crustaceans bleed blue.
    • The most famous of these is the humble horseshoe crab, with blue copper-based blood. Scientists routinely extract blood from the crabs for all kinds of medical procedures & tests, then release the animals after they've regenerated their blood.
  • Mollusks tend to have blue blood.
  • Ditto spiders, whose hemocyanin gives them greenish blue blood.
  • There is a species of pen shell called Pinna squamosa. What makes it unique is that it is the only mollusc to use manganese to store oxygen, in a protein called pinnaglobin. As a result, it has brown blood.
  • Inverted with deep-sea tube worms, which have red blood like ours despite being from a taxonomic line in which iron-based blood pigment isn't expected.
  • The Prasinohaema genus of skinks, in contrast to every other healthy vertebrate, have green blood (as well as green muscles, bone, and everything else). This is caused by levels of biliverdin (bile pigment) in the blood that are so high they would kill any other vertebrate. The name, of course, means "green blood."
  • Due to having little to no hemoglobin, Antarctic icefish have translucent white blood.
  • Underwater, red blood appears green below a certain depth (red blood cells separate from the plasma, turning it yellow, which then combines with the blue tint of the water).
  • Eel blood, although it doesn't look "alien", is toxic to humans and other mammals. Beware eel sushi that looks too pink!
  • In humans...
    • Blood coughed up by accident victims with pierced lungs can be foamy and pink.
    • Sulphur can turn a person's blood green.
    • Carbon monoxide poisoning turns red blood an even brighter, unnatural-looking shade of red.
    • A convention of anatomical illustration is that deoxygenated blood vessels are depicted in blue on drawings and models. This sometimes leads people to the false conclusion that deoxygenated blood is blue; in fact, it's a reference to how the outsides of systemic veins, which carry this type of blood, look blue when seen through the paler shades of human skin.
    • While their actual blood doesn't look different, the bone marrow of a patient with advanced leukemia looks lighter in color than that of a healthy person, due to being overloaded with immature, non-functional white blood cells.
    • Methemoglobinemia is a genetic disorder that causes heightened levels of methemoglobin, which has a lower affinity for oxygen than hemoglobin, making the blood blue or brown. The most famous cases of methemoglobinemia are the "Blue Fugates", a Kentucky family known for their blue complexion.
    • Everyone has a small quantity of "blue blood cells" in their body thanks to the element gallium. Gallium can and will take the place of iron in the red blood cell, turning the hemoglobin blue.
  • Artificial blood:
    • The trope of artificial humans having white blood possibly comes from the early research into oxygen-carrying temporary blood replacements for emergency surgery. Some of these work using very different chemical processes to naturally occurring blood (of any colour) and look a lot like milk.
    • Other scientists are getting in on the fun too, it seems. They have created a oxygen-binding molecule called coboglobin, which is based on cobalt. Like human blood, it changes colors based on if it's oxygenated or not. If it is, then it's clear; if not, it turns amber yellow.
    • Perftoran/Perfukol/Fluozol-DA is blue. In the 1980s Soviet medics called it "blue blood". It's a water emulsion of fluororganic compound that can carry twice more oxygen than erythrocytes.
  • Invertebrates in the phylum Priapulida (also called "Penis Worms") actually have purple blood thanks to the presence of an organic molecule called haemerythrin.
  • Chickens of the Ayam Cemani breed, from Indonesia, have a type of extreme hypermelanism that makes every organ in their body - skin, feathers, legs, combs, meat, guts, bones, etc - pitch black. Their blood is the only non-black thing about them, and it's a very dark red compared to other birds'.

 
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Bug at Oblivion

While most of the aliens have red blood, a bug at Oblivion has green blood.

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