Jigsaw: Hello, Agent Strahm. If you're hearing this, then you've finally found what you've been looking for. But is the discovery of my body enough? Or will your insatiable hunger to uncover the truth push you deeper into the abyss? Heed my warning: do not proceed. For this room can either be you sanctuary, or it can be your grave. The choice is yours.
Agent Strahm: Fuck you.
Brit: No it's not Jigsaw, he's dead. Haven't you watched TV lately?
Jigsaw: Murder is distasteful.
Jigsaw: Hello Seth, I want to play a game. Right now, you are feeling helpless. This is the same helplessness you bestowed onto others. But now, it's unto you. Some would call this karma, I call it justice. Now you served five years of what should have been a life sentence, for murder. A technicality gave you freedom, but it inhibited you from understanding the impact of taking a life. Today, I offer you true freedom. In thirty seconds, the pendulum will drop far enough to touch your body. Within sixty seconds, it will cut you in half. To avoid the pendulum, all you have to do is destroy the things that have killed... Your hands. You must insert your hands and push the buttons to start the devices before you. Your bones will be crushed to dust. Will you destroy the things that have taken life in order to save one, Seth? Make your choice.
Chosen answer: Assuming the woman in the bathtub was able to be moved safely the amount of blood gained from her, while still being helpful, would probably not be enough to save the final two survivors from self harm. Without the heart pumping blood you would only get the blood that was in her arm at the moment. You would have to lift her or raise her so gravity would get other blood moving to get enough. Which the final two possibly could have done, but stress of the situation clouded their judgement.