Una mentions a “couple of litres” of blood. Did she mean pints, and the writers did a find/replace to make it metric and more futurey? Because “a couple of litres” is a lot.

This is months after the previous episode. I think the implication was that Una had been steadily donating blood so they can study it and/or have a stockpile for emergencies. It makes sense as long as they avoid the “hey everyone, magic blood is the cure for death” crap from the second JJ movie.