The umbilical vessels insert onto the placental surface, branch, run within the chorionic plate, and then, at the periphery, turn abruptly toward the maternal surface, branching repeatedly to finally become villous capillaries. Blood is returned from the villous capillary loops to the umbilical cord by veins that merge into the umbilical vein. In the overwhelming majority of cotyledons, there is a 1:1 relation between artery and vein at the periphery, and each artery “supplies” a single cotyledon (placentone). It is remarkable that at least the larger arteries always cross over the veins on the placental surface. They can thus be readily identified by macroscopic examination, while histologically it is nearly impossible to make this distinction. It is interesting to note that the circumferential architecture of the placental surface vessels is asymmetrical. This is due to hemodynamic thinning where pressure in the vessels buckle and thin the superficial portions of the vessels, whereas the “fixed” portions resist this pressure. This phenomenon of thinning of the superficial aspect of chorionic vessels is also shared with cord vessels.
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