Diabetics take insulin derived from pigs. Some vaccines are derived from animal diseases. Pigskin has been used for grafts on human skin. When they view the body as a physical machine divorced from any soul or spirit, it is easy. A body is a body.
Parts are parts.
Surgeons take one tube and replace it with another. A piece of intestine may be used to replace an esophagus. A vein in a leg is used to repair a heart. Not all doctors believe that human beings are just a collection of body parts. Some years ago I interviewed Dr. Bernie Siegel for Thresholds Quarterly.