Store your current blood
Add functionality to the empty glass vials to store your current blood type. The idea is that this will allow you to empty your current blood pool into an empty glass vial to store for later. Potentially have this be done on an alchemy bench to prevent players from abusing this mechanic. You can already drain blood from prisoners into the glass vials to store blood for later and players can share their blood pool with each other, so allowing players to drain their own blood pool into vials to store for later seems like it still fits within the spirit of the game.