It's kind of like that. What it'll be is I'll insert a needle into a vein in your arm. There is negative pressure in the bag attached to the needle that will pull blood out of you into the bag until it's full, at which point I'll remove the needle from your arm and bandage the site. Then the tube connecting to the bag gets clamped off to keep it sealed, the bag gets labeled with your name, and put in a specialized refrigerator that will be locked to medical staff only. And if, god forbid, you get hurt badly enough to need blood replaced because you've lost more than your body can replenish on its own, there will still be one sterile needle on the bag that can be inserted back into a vein on you and the bag can be hung on a pole so that gravity assists the blood in leaving the bag and going back into you. Does that help it make more sense?