Ok, so in a trauma - the first thing that gets done is to put a couple of big IVs in someone who's lost a lot of blood - then they get a bunch of normal saline (IV fluids, not blood products yet). This will keep the amount of blood circulating enough to keep blood pressure up and keep the vital organs perfused. Eventually you will get a dilutional effect where the amount of oxygen the blood can carry won't be enough to sustain life because there aren't enough red blood cells. So, after 2-3 boluses of IV fluids, then we start using blood products (in humans, O negative if we don't know the person's blood type). I have no idea how O negative blood would work in Kryptonians... You can get type specific or direct donor blood products, but for the direct donor, it's not like you pull it out of one person, turn around and inject it into another person - the blood gets cleaned and concentrated usually. I guess in a pinch it could be done, but it's not a scenario that happens in the majority of the medical world.
As far as how much blood can a person lose or donate at a time... Once you get up to about 10-15% of the circulating volume, you're reaching trouble. Normal human = about 5L of blood, so 1/2 a liter would probably be enough to start causing trouble; once you get to a liter, you're probably reaching scary areas. Really acutely life-threatening blood loss is 30-40% of circulating blood volume - there you're kind of reaching the point of no return. But again, in the short term, you can make up some of that with just fluids - temporize until you can do blood products. First blood products in this situation would be packed red blood cells, then possibly platelets if you're having ongoing bleeding. Third would be FFP (fresh frozen plasma - the liquid part of the blood), this would be after lots and lots of ongoing bleeding - more likely if you have an actual bleeding/clotting problem.
And as far as donating blood, most blood banks will only allow about a pint of blood at a time. Then you do have to wait for a while. Platelets you can donate more frequently.
DocJill
