Do you have to replace blood if you receive a blood transfusion?
If you receive a blood transfusion, you are under no obligation
to "return" (or "donate") any blood to back the blood bank. This is
true both before and after receiving a blood transfusion - i.e you
do not have to have donated blood previously, before you can have a
transfusion. (At least, this is how it works in the UK). Nice to
know, really.