![]() ![]() The plugs then loosen and permit continuous maternal blood flow into the intervillous space. These trophoblastic plugs obstruct maternal blood flow into the intervillous space and prevent flow until the end of first trimester of pregnancy (10–12 weeks). ![]() The second theory proposes that trophoblasts invade decidual spiral arteries and form trophoblastic plugs. This theory is supported by the presence of endovascular trophoblasts in the decidual spiral arteries of the first trimester placenta. The first theory is that in the first trimester, endovascular trophoblasts migrate along the decidual spiral arteries, invade the vessel walls, and create a path for maternal blood to perfuse the placenta intervillous space. Although the exact mechanism of how the uteroplacental circulation is established is not completely understood, two theories have been proposed. Uteroplacental circulation is not fully established until the end of the first trimester. Figure 2.1 illustrates (1) the relationship of the uterus, placenta, and the fetus, and (2) the directions of blood flow from mother to the placenta as well as fetal blood flow from the placenta to the fetus.Ģ.1. No intermingling of maternal and fetal blood occurs in the placenta. The functional unit of maternal-fetal exchange of oxygen and nutrients occur in the terminal villi. It is estimated that the surface area of syncytiotrophoblasts is approximately 12m 2 and the length of fetal capillaries of a fully developed placenta is approximately 320 kilometers at term. At term, maternal blood flow to the placenta is approximately 600–700 ml/minute. After the exchange of oxygen and nutrients, the umbilical vein carries fresh oxygenated and nutrient-rich blood circulating back to the fetal systemic circulation. The fetal-placental circulation allows the umbilical arteries to carry deoxygenated and nutrient-depleted fetal blood from the fetus to the villous core fetal vessels. ![]() The in-flowing maternal arterial blood pushes deoxygenated blood into the endometrial and then uterine veins back to the maternal circulation. Exchange of oxygen and nutrients take place as the maternal blood flows around terminal villi in the intervillous space. The uteroplacental circulation starts with the maternal blood flow into the intervillous space through decidual spiral arteries. The placenta is a unique vascular organ that receives blood supplies from both the maternal and the fetal systems and thus has two separate circulatory systems for blood: (1) the maternal-placental (uteroplacental) blood circulation, and (2) the fetal-placental (fetoplacental) blood circulation. ![]()
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