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The donor is treated and prepared for surgery in a way similar to any other living patient going to the operating room. After the removal of healthy vital organs, what is left is an empty corpse. Such removal is ethically unacceptable. It is the removal of the organs that changes the living person to a dead one.
Anyone familiar with the moment of death knows that once death has occurred, there is no more breathing, moving, grimacing, or squirming and that there is no longer a heartbeat or blood pressure. The argument of some physicians--that such movements in an organ donor are caused by "leftover energy" in the body--has no scientific validity. It is, therefore, unethical for transplantation surgeons to continue performing such procedures that mutilate a living human body. These procedures treat the donors as if they were artificially sustained biologic entities, rather than human persons worthy of dignity and respect...
(Source: "Are Organ Transplants Ever Morally Licit?" by Bishop Fabian Wendelin Bruskewitz, Bishop Robert F. Vasa, Walt F. Weaver, Paul A. Byrne, Richard G. Nilges, and Josef Seifert)