Within the placenta, oxygen and nutrients in maternal blood pass into fetal blood. At the same time, carbon dioxide and waste products in fetal blood pass into the maternal blood and are carried away. The exchange of wastes and nutrients between maternal and fetal blood takes place by diffusion through the walls of tiny blood vessels called capillaries. Maternal and fetal blood cells never mix, but instead remain separated by the placental membrane. If the maternal and fetus blood types are incompatible, mixing blood could be dangerous. The mother's immune system may treat the fetal blood cells as foreign invaders.