In Person of Interest, Shaw inserts a line into a guy's arm that goes directly into her arm for a blood transfusion. See clip: https://www.youtube.com/watch?v=eYQRa6IYf0A&t=83
What determines the direction the blood flows? How does she ensure it the blood flows to her rather than away? My guess is that because she's lost a lot of blood due to a gun shot, her blood pressure would be lower. Since his blood pressure would be higher, it pushes the blood to her accordingly. Once she receives a decent amount of blood, her pressure will increase until there's an equilibrium. Is that an accurate assumption or is there something else going on?