system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can a baby have a different blood type than the mother if they share a circulatory system?

I assume that the placenta and umbilical chord delivers blood to and reclaims blood from the fetus.  If this is true, how can a fetus develop a different blood type than the host?
gpt
They don't share a circulatory system. The purpose of the placenta is to separate the fetal and maternal blood while allowing the exchange of nutrients, oxygen, and waste. The placenta allows the blood of the mother and child to become extremely close (enough to transport material across a membrane), but never touch. It's like a loading station where trucks from two different routes exchange cargo.