placental cotyledon

Placental septa divide the maternal side of the placenta into rounded lobes called cotyledons, each a partition of the decidua basalis built around a single stem chorionic villus and its branches and subbranches. Fetal blood reaches a cotyledon through chorionic vessels, which give off cotyledon vessels that ramify within the lobe and terminate in capillary networks inside the villous tree. Maternal blood surrounds the cotyledon on the outside of that villous surface, so oxygen and nutrients pass from the maternal circulation into fetal capillaries without the two blood streams mixing. The cotyledon is therefore the functional exchange unit of the placenta: its villous architecture supplies the surface area across which transfer occurs, and its position within the septa determines how maternal blood is distributed across the placental disc.