Circulation Of The Placenta

Cotyledons receive their blood through 80 to 100 spiral arteries that pierce the decidual plate and enter the intervillous spaces at more or less regular

Figure 6.13 The placenta in the second half of pregnancy. The cotyledons are partially separated by the decidual (maternal) septa. Most of the intervillous blood returns to the maternal circulation byway of the endometrial veins. A small portion enters neighboring cotyledons. The intervillous spaces are lined by syncytium.

Placenta Maternal Side

Figure 6.14 A full-term placenta. A. Fetal side. The chorionic plate and umbilical cord are covered by amnion. B. Maternal side showing the cotyledons. In one area the decidua has been removed. The maternal side of the placenta is always carefully inspected at birth, and frequently one or more cotyledons with a whitish appearance are present because of excessive fibrinoid formation and infarction of a group of intervillous lakes.

Figure 6.14 A full-term placenta. A. Fetal side. The chorionic plate and umbilical cord are covered by amnion. B. Maternal side showing the cotyledons. In one area the decidua has been removed. The maternal side of the placenta is always carefully inspected at birth, and frequently one or more cotyledons with a whitish appearance are present because of excessive fibrinoid formation and infarction of a group of intervillous lakes.

intervals (Fig. 6.13). The lumen of the spiral artery is narrow, so blood pressure in the intervillous space is high. This pressure forces the blood deep into the intervillous spaces and bathes the numerous small villi of the villous tree in oxygenated blood. As the pressure decreases, blood flows back from the chorionic plate toward the decidua, where it enters the endometrial veins (Fig. 6.13). Hence, blood from the intervillous lakes drains back into the maternal circulation through the endometrial veins.

Collectively, the intervillous spaces of a mature placenta contain approximately 150 ml of blood, which is replenished about 3 or 4 times per minute. This blood moves along the chorionic villi, which have a surface area of 4 to 14m2. However, placental exchange does not take place in all villi, only in those whose fetal vessels are in intimate contact with the covering syncytial membrane. In these villi, the syncytium often has a brush border consisting of numerous microvilli, which greatly increases the surface area and consequently the exchange rate between maternal and fetal circulations (Fig. 6.8D). The placental membrane, which separates maternal and fetal blood, is initially composed of four layers: (a) the endothelial lining of fetal vessels; (b) the connective tissue in the villus core; (c) the cytotrophoblastic layer; and (d) the syncytium (Fig. 6.8C). From the fourth month on, however, the placental membrane thins, since the endothelial lining of the vessels comes in intimate contact with the syncytial membrane, greatly increasing the rate of exchange (Fig. 6.8D). Sometimes called the placental barrier, the placental membrane is not a true barrier, since many substances pass through it freely Because the maternal blood in the intervillous spaces is separated from the fetal blood by a chorionic derivative, the human placenta is considered to be of the hemochorial type.

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Responses

  • lindsay
    Why is the maternal side of the placenta carefully inspected in the human?
    8 years ago

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