Blood Bath/Headscratchers


 * What exactly is it that keeps the blood in the bathtub from coagulating? Except for the rare examples where magic might play a role in preparing the bath itself, it's never like the blood is being kept warm in a hot tub or jacuzzi or something; it's just blood being drained and dumped into a vat.
 * Assuming the blood was just taken out of cold storage (or a person), you've got at least a few minutes before it starts noticeably coagulating. And you can push that deadline back quite a while if you mix in an anti-coagulant or dilute it with water.
 * Wouldn't diluting blood have some kind of averse affect of reducing the rejuvenating qualities of blood since there would now be less volume of blood in a tub...and would anyone really want to bathe in anti-coagulant?
 * I doubt whatever fictional setting we're talking about that gives blood magical rejuvenating properties would be too terribly bothered by some mild dilution or an anti-coagulant additive. And, of course, in real life, it wouldn't be an issue at all because blood doesn't have magical rejuvenating powers and anyone who thinks it does is mentally unbalanced by definition.
 * It should probably be noted that the large majority of anticoagulants were developed in the 20th Century with the advent of blood donations and blood banks--well after many of the earliest legends of Bathory and other stories have established this trope. Even stories from the 20th Century or later about Bathory wouldn't reasonably be able to fall back on such a justification if a story is set in the 1600's.
 * Don't blood banks filter out the platelets and white blood cells before they store it?
 * Does it solidify completely when it coagulates, or just develop a crust? Nobody said it's comfortable.
 * Let's just say that once blood coagulates, it's not in any state for one to be bathing in it. If you were in a tub of coagulated blood, you wouldn't be bathing in blood at all; you'd only be having dried-up blood sticking to your skin.
 * You can give 83 liters of blood a year without dying. If you make a very human-shaped tub, there's no limit to how little volume it would require. If you're fast enough, you could probably use the same blood a few times. Why bother killing someone over this?
 * Also, bathing doesn't have to be by immersion. You assume they're filling up a whole tub and having a nice soak, but they could just as easily be sponge bathing, so to speak. Unless they're actually shown full on soak-bathing.
 * The opening sentence of this trope's description, the main image on the page, and nearly all the trope examples and wicks explicitly involve immersion "in a bathtub". Nothing here "assumes" anything about people in whole tubs filled with blood; that's exactly what this trope is supposed to be.