There is grisly research where a young mouse is sown together with an old mouse and their blood vessels are connected. This seems to benefit the old mouse. And no, I haven’t watched “human centipede” and I have no desire to, although I did find the South Park parody of it moderately funny.

Beyond the stupidity of human centipede, you can imagine a mad dictator or Bond villain somewhere forcing people to be blood donors against their will. A more benign version would have biotech firms trying to synthesize whatever it is that makes young blood good, or possibly using stem cells to make a young version of a person’s blood. What if a version of me could be created that was just a bag of meat with no nervous system, and pumped out blood that kept me alive? That seems creepy but not necessarily unethical. Add a brain and keep that person sedated or otherwise detained against their will, and that would clearly be unethical (not to mention basically the plot of The Island.)