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Weeks Since Last Menstrual Period

FIGURE 1 Fetal growth during gestation. The shaded area indicates the range of weight of 95% of fetuses.

Weeks Since Last Menstrual Period

FIGURE 1 Fetal growth during gestation. The shaded area indicates the range of weight of 95% of fetuses.

maternal decidual cells (see Chapter 47) and extravillous syncytiotrophoblast. The mature placenta contains 60 to 70 villous trees, each of which, through repeated branching of the secondary and tertiary villi, gives rise to more than 100,000 intermediate and terminal villi. The villi, whose combined length is estimated to be 90 km, contain increasingly finer branches of arterioles and venules that terminate as clusters of grape-like outgrowths comprised largely of sinusoidal dilated capillaries. The entire villous tree is ensheathed in a continuous layer of syncytiotro-phoblast, which overlays a discontinuous layer of the cytotrophoblastic cells. The chorionic plate is fused at its edges with the basal plate to form a hollow cavity, the intervillous space, which is perfused with maternal blood that enters through multiple spiral arteries that branch off the radial arteries in the myometrium and exits by way of the uteroplacental veins (Fig. 2).

Maternal and fetal blood do not mix or come in direct contact. Exchange of nutrients and gases takes place principally in the terminal villi across a diffusion barrier comprised ofa layer ofsyncytiotrophoblast, basal lamina, and endothelial cells. By late pregnancy, the diffusion barrier thins to about 5 mm and has a surface area of about 12 m2. The multifunctional syncytiotrophoblast, which is richly endowed with ion channels, transporters, and exchangers, is the site of both active and passive exchange processes. Bulk movement and mixing of blood in the intervillous space tends to flatten concentration gradients between umbilical venous and maternal blood.

In addition, a relatively high proportion of placental surface is not available for exchange, creating a sizeable dead space. Consequently, high rates of blood flow are needed to compensate for the relative inefficiency of exchange.

The syncytiotrophoblast is also endowed with versatile biosynthetic capacity and is the source of the placen-tal peptide and steroid hormones (see Chapter 47). Large amounts of progesterone, estrogens, human chorionic gonadotropin (hCG), human placental lactogen (hPL), and other secretory products are released by the syncy-tiotrophoblast directly into the intervillous space and hence the maternal circulation. These placental hormones are largely responsible for orchestrating adjustments in maternal physiology as pregnancy progresses.

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