How does utero-placental circulation facilitate fetal gas and nutrient exchange?

• A. Maternal blood directly mixes with fetal blood
• B. Maternal blood enters intervillous space, bathes fetal chorionic villi; gas and nutrient exchange occurs through simple diffusion
• C. Exchange occurs through fetal placental membranes exclusively
• D. No exchange occurs across the placenta

Answer: (B)

Gas and nutrient exchange between mother and fetus happens across the placental barrier. Maternal blood flows into the intervillous space and bathes the fetal chorionic villi, and the thin placental membranes separate the maternal blood from fetal capillaries. Substances move by diffusion driven by concentration and partial pressure gradients, so oxygen moves from mom to fetus and carbon dioxide moves in the opposite direction, while nutrients like glucose and amino acids diffuse as well (often with help from transport systems). The placenta provides a large surface area and a very thin barrier to make this diffusion efficient. Direct mixing of maternal and fetal blood does not occur, which is why the other statements aren’t correct; exchange isn’t limited to only fetal membranes, and it certainly happens across the placental barrier.