Final answer:
The maternal blood does not mix with fetal blood in the placenta; this statement is false. They come into close proximity within the chorionic villi, allowing exchange of substances without commingling of bloodstreams. Only during birth can fetal erythrocytes potentially enter the maternal bloodstream in Rh incompatibility cases.
Step-by-step explanation:
The statement that maternal blood mixes with fetal blood in the placenta is false. In the placenta, the maternal and fetal blood systems come into close contact at the chorionic villi, but they do not directly mix. The fetal blood components and maternal blood components conduct through the surface of the chorionic villi. This allows for the exchange of nutrients, gases, and waste without mixing the two bloodstreams. It is through this arrangement that the fetus can receive oxygen and nutrients from the mother while carbon dioxide and other metabolic waste products are removed from the fetal blood without the two bloods actually commingling.
It is worth noting that during birth, in cases where the mother is Rh- and the baby is Rh+, fetal erythrocytes (red blood cells) can leak into the maternal blood after the breakage of the embryonic chorion. However, this is an exception to the typical separation of the bloodstreams throughout the pregnancy.