Placenta, the organ is responsible for feto-maternal exchanges. The placenta is constituted of a part originating from the embryo, the chorion, and a maternal face, the decidua, mucosa issue from the modification of the uterius endometrium. It is highly vascularized (arterioles and venules), so as to bring the fetus oxygen and nutrients that are necessary, and evacuate the wastes. Those vessels converge at the level of the umbilical cord, and regroup to form the umbilical vein (in red), transporting the desoxygenated fetal blood towards the placenta, and two umbilical arteries (in blue) bringing the oxygenated blood to the fetus. Projections of the chorion insinuate and develop inside the decidua of the maternal endometrium; these are chorial villosities (in yellow). Those villosities contain fetal capillaries. The maternal blood circulation is constituted of blood sacs, the intervillous spaces. They are alimented in blood by the spiraled arteries (in red) and drained by the uterine veins (in blue). Intervillous spaces constitutes the interface between the fetal and maternal circulations.