Silly question based off of a spell in an anime. This “purify” spell what clean up spillages, turning blood to pure water for a couple of minutes, before reverting to blood .Including all the blood in an enemy's body.
How would such a blood to water spell affect the body in a ME paradigm? What happens with humours and whatnot, since oxygen exchange is unknown?
Someone’s been watching Goblin Slayer I see. 
Yes I think turning blood into water is perfectly possible. After all, you already have a spell that turns the whole human into a puddle of water. As for how we’d do it, that’s a trickier question.
You could just take the Transform to Water spell, and increase the parameters to Touch and Part target in order to affect only the blood of your target. You could possibly get away with lowering the duration to Concentration or Diameter. As for what effect this will have on a target, I am unsure. Muto can be tricky in these instances, cas’ it temporarily changes you to be able to accomodate said changes.
For instance, a spell that gives an Animal a “supernatural” ability to walk on Lava (Steed of Vesuvius) will also alter them to be able to do so without burning to death. Same presumably applies to the example of said guideline to make an animal capable of breathing fire. On the other hand, Base 5 Muto Corpus guideline from Magi of Hermes implies you can impose defective changes onto a target to some degree?
Since your version of Transform to Water would be either a level 45 or 50 spell depending on the parameters, I’d be fine with assuming you can make it a defective and thus a lethal transformation.
As for changing the already spilled blood that isn’t part of a living person, I’d just convert Muto Aquam guidelines one for one to Muto Corpus when trying to manipulate spilled blood. Since Muto Animal can already freely and easily manipulate animal products with greater ease than the living body of an animal, the same should apply to human products.
In the real world, turning a person’s blood into water would have terrible consequences - the body’s cells starved of oxygen is the most obvious consequence.
But if it was only a Sun duration effect, and reverted back to oxygenated blood, then the victim would probably survive as two minutes without oxygen, while debilitating is unlikely to be immediately lethal.
However, would this be what happens in Mythic Europe?
Or would the victim suffer an immediate Humour-imbalance disease? Or something?
And when the blood reverted back from water to blood after Sun duration?
I think most people know blood is really important, and know that it turning to water would be bad.
I would think if it is a Muto spell, it’s just a weird thing that if cut, the person bleeds water. The magic sustains them, just like the spell that turns someone completely to water. To be a kill spell, it should be Perdo.
Yes, most people would consider keeping blood inside a person as rather important. It keeps you warm if I recall Elemental Humour theory.
There is a spell that turns your blood into lava, without hurting you (“Dragon’s Blood”, Hermetic Projects, p15)
With that in mind, I think turning someones blood into water would be quite harmless.
I will have look that up. Thanks