The pig liver lacks an enzyme which the human liver needs for the immune system. The pig heart is usually at the level of its head, whereas a human heart is below the head. If a pig heart were transplanted into a human, the heart may not be able to produce enough blood flow to sustain the human brain. These anatomical and chemical differences between humans and pigs suggests that a cross-species transplant may not be functionally successful. These questions of long term viability of xenogeneic organs are crucial questions but are hard to answer until we come closer to a successful transplantation.
Animal treatment is a factor in the ethical concerns of critics of xenotransplantation.
These activists think it is not morally correct for animals to die to keep humans alive. Some of these activists views are stiffled by the notion that pigs are already farmed for their meat and these organs should just be taken from these pigs which would be sacrificed anyways. Another ethical issue is raised when scientists try to produce disease-free pigs. These disease-free pigs may also produce concern over whether they are safe to eat. Other activists argue that genetically altering pigs can eventually damage their immune system.