Chapter 42. Placental Circulation - Review of Medical Embryology Book

Introduction: the intervillous space is limited on the maternal side by the basal plate and on the fetal side by the chorionic plate. It is incompletely limited laterally by the decidual septa. The complex division of the villi provides a great deal of placental surface and is a very important factor affecting the rate of fetal-maternal exchange. At term, the placental surface consists of more than 10 square meters. The anchoring or stem villi are attached to the basal plate and define a general circular area. The villus tree, in its entirety, forms a very complex system consisting of a major anatomic unit, the cotyledon
The fetal circulation can be compared to the pulmonary circulation of the adult in that desaturated blood enters through the fetal arteries and oxygenated blood returns by way of the veins

BLOOD ARRIVES via the 2 umbilical arteries which are branches of the iliac arteries of the fetus. It is dispersed in a highly dense network which penetrates even the smallest villous division
BLOOD IS RETURNED via the umbilical vein and finally reaches the inferior vena cava system of the fetus
FETAL CIRCULATION is carried out in a closed vascular system where the average pressure is about 30 mm Hg, which is much higher than that seen in the intervillous space where it is about 10 mm Hg. The difference in pressure prevents the collapse of the villous vessels

BLOOD ARRIVES at the uterus by the branches of the uterine artery, spreads out in the intervillous spaces, and circulates between the branches of the villous trees. It is returned by branches of the uterine veins. The flow in these two circulations is very high, about 500 ml/min, which favors fetal-maternal exchange
MATERNAL CIRCULATION results from a difference in pressure which is very high in the artery (about 70 mm Hg) and relatively low in the intervillous space (about 10 mm Hg). Blood spurts up to the chorionic plate, then comes toward the basal plate and is taken up by the uterine veins where the pressure is even lower than that found in the intervillous space