Complex sentence: The fetal circulation works differently from that of born humans, mainly because the lungs are not in use: the fetus obtains oxygen and nutrients from the mother through the placenta and the umbilical cord.
Split into simpler sentences: The fetal circulation works differently from that of born humans, mainly because the lungs are not in use. Instead, the fetus obtains oxygen and nutrients from the mother through the placenta and the umbilical cord.