Maternal vessels

When And How The Placenta Forms

Fetal capillaries Intervillous space

Chorionic epithelium

Figure 82-5

Fetal capillaries Intervillous space

Chorionic epithelium

Figure 82-5

Above, Organization of the mature placenta. Below, Relation of the fetal blood in the villus capillaries to the mother's blood in the intervillous spaces. (Modified from Gray H, Goss CM: Anatomy of the Human Body, 25th ed. Philadelphia: Lea & Febiger, 1948; and from Arey LB: Developmental Anatomy: A Textbook and Laboratory Manual of Embryology, 7th ed. Philadelphia: WB Saunders, 1974.)

which fetal capillaries grow. Thus, the villi, carrying fetal blood, are surrounded by sinuses that contain maternal blood.

The final structure of the placenta is shown in Figure 82-5. Note that the fetus's blood flows through two umbilical arteries, then into the capillaries of the villi, and finally back through a single umbilical vein into the fetus. At the same time, the mother's blood flows from her uterine arteries into large maternal sinuses that surround the villi and then back into the uterine veins of the mother. The lower part of Figure 82-5 shows the relation between the fetal blood of each fetal placental villus and the blood of the mother surrounding the out-sides of the villus in the fully developed placenta.

The total surface area of all the villi of the mature placenta is only a few square meters—many times less than the area of the pulmonary membrane in the lungs. Nevertheless, nutrients and other substances pass through this placental membrane mainly by diffusion in much the same manner that diffusion occurs through the alveolar membranes of the lungs and the capillary membranes elsewhere in the body.

Placental Permeability and Membrane Diffusion Conductance

The major function of the placenta is to provide for diffusion of foodstuffs and oxygen from the mother's blood into the fetus's blood and diffusion of excretory products from the fetus back into the mother.

In the early months of pregnancy, the placental membrane is still thick because it is not fully developed. Therefore, its permeability is low. Further, the surface area is small because the placenta has not grown significantly. Therefore, the total diffusion conductance is minuscule at first. Conversely, in later pregnancy, the permeability increases because of thinning of the membrane diffusion layers and because the surface area expands many times over, thus giving the tremendous increase in placental diffusion shown in Figure 82-4.

Rarely, "breaks" occur in the placental membrane, which allows fetal blood cells to pass into the mother or, even less commonly, the mother's cells to pass into the fetus. Fortunately, it is rare for the fetus to bleed severely into the mother's circulation because of a ruptured placental membrane.

Diffusion of Oxygen Through the Placental Membrane.

Almost the same principles for diffusion of oxygen through the pulmonary membrane (discussed in detail in Chapter 39) are applicable for diffusion of oxygen through the placental membrane. The dissolved oxygen in the blood of the large maternal sinuses passes into the fetal blood by simple diffusion, driven by an oxygen pressure gradient from the mother's blood to the fetus's blood. Near the end of pregnancy, the mean Po2 of the mother's blood in the placental sinuses is about 50 mm Hg, and the mean Po2 in the fetal blood after it becomes oxygenated in the placenta is about 30 mm Hg. Therefore, the mean pressure gradient for diffusion of oxygen through the placental membrane is about 20 mm Hg.

one might wonder how it is possible for a fetus to obtain sufficient oxygen when the fetal blood leaving the placenta has a Po2 of only 30 mm Hg. There are three reasons why even this low Po2 is capable of allowing the fetal blood to transport almost as much oxygen to the fetal tissues as is transported by the mother's blood to her tissues.

First, the hemoglobin of the fetus is mainly fetal hemoglobin, a type of hemoglobin synthesized in the fetus before birth. Figure 82-6 shows the comparative oxygen dissociation curves for maternal hemoglobin and fetal hemoglobin, demonstrating that the curve for fetal hemoglobin is shifted to the left of that for maternal hemoglobin. This means that at the low Po2 levels in fetal blood, the fetal hemoglobin can carry 20 to 50 per cent more oxygen than maternal hemoglobin can.

Second, the hemoglobin concentration of fetal blood is about 50 per cent greater than that of the mother; this is an even more important factor in enhancing the amount of oxygen transported to the fetal tissues.

Third, the Bohr effect, which is explained in relation to the exchange of carbon dioxide and oxygen in the lung in Chapter 40, provides another mechanism to

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