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Basic Information

Blood is a liquid substance manufactured in the marrow of your bones. It travels through veins, arteries, and capillaries to transport oxygen and nutrients to the cells of your body. It also transports toxins and waste out of the cells to organs that can eliminate them. Elements of your immune system, such as white blood cells, are also present in blood.

Lots of animals have blood, though the chemical properties vary a bit from species to species. Nearly all vertebrate species have red blood, so colored because of the Hemoglobin compound in our blood. This is a protein that enables the transportation of oxygen, which is so biologically successful that basically all large air-breathing creatures on the Earth with spinal chords and lungs use it. Other forms of life on our planet use other compounds in this role, and thus have blood of other colors.

List of Animals Blood Color Oxygen-carrying protein
All Mammals, and the vast majority of vertebrates in general Red (specifically bright red when oxygenated, and a darker red when not carrying or exposed to oxygen Hemoglobin
Most Mollusks, some Arthropods (such as horseshoe crabs) Blue Hemocyanin
Most Annelid worms such as earthworms and leeches Light Red in concentrated in large quantities, but Green if dilluted Chlorocruorin
Brachiopods and some sea-worms Violet or Pink Hemerythrin
sea squirts Mustard Yellow Hemovanadin
Crocodile Icefish clearly transparent None

Mammalian blood also has a strong, distinctive iron like smell to it (due to the iron in the haemoglobin) - presumably the other main classes of blood have their own distinct odour.

Supernatural interpretations tend to regard blood as tantamount to liquid life - indeed sources as authoritative as The Bible endorse this position … and it is not hard to see why. Our ancestors were not capable of transfusing blood until very recently, but the fact that it is possible would surely only reinforce those beliefs. From such grounds come all manner of blood drinking creatures, blood magic powered by its spilling, and other rituals designed to transfer and redirect such potent stuff. Blood is also seen as tantamount to heredity - again, not an unsurprising position in pre-modern times - hence such things as blood lines, blood relations and corrupt blood.

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Game and Story Use

  • Blood loss and uncontrolled bleeding are pretty dangerous, and likely to be common results in a game with a detailed critical hit chart.
    • Arguably a damage system could do to track blood loss separately - it's not unknown for someone to bleed out from a tiny wound left untended whilst it's possible for a human body to survive some fairly impressive mutilations if you can keep the blood topped up.
  • Blood can carry contamination or disease. A prominent bloodborne pathogen can make a battlefield or disaster site very dangerous. This gets especially bad in some zombie movies, for example.
  • The chart above with all the blood colors could be useful when designing a mystery / crimescene where a monster of the week or mysterious creature was involved in a fight. Or just be a fun bit of descriptive flair when the PCs are duking it out with a giant animal. I've included the protein names in case you want to sound scientific or do your own research.
    • Would certainly give forensics a double-take if what would otherwise seem to be blood splatter is the wrong colour… PCs get a while to wonder about what the hell is going on before the MIBs turn up.
    • One possibility for using this in visual media: show a murder scene, with the blood changed to some unusual color. At first, the audience might think this is just regular censorship, since some codes forbid blood to be shown as red. Then surprise the audience by having a character casually mention the color or an appropriate protein, revealing that non-human humanoids both exist and are not considered unusual.
    • Come to think of it "The Yellow Bastard" from Sin City also had yellowish, foul smelling blood.
    • And the original Reek from A Song of Ice and Fire also had stinky blood.
  • The symbolic associations of blood might also be important.
    • Transfusions might carry a legal or supernatural status of bastardry.
    • If blood is more literally tied to heredity, traits that are normally genetic might jump from a donor's family to a patient's.
    • Since blood is life, transfusing enough of it into a corpse as part of a ritual might literally reanimate it.
  • Makes sense for the fae - with their traditional iron-bane, not to have haemoglobin based blood: have your elves and what have you bleed blue (or whatever).
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