Placental oxygen transfer is blood flow limited. Using estimated uteroplacental blood low, Longo (1991) calculated oxygen delivery to be approximately 8 mL 02/min/kg of fetal weight. Normal values for oxygen and CO2 are presented in Figure 7-13. Because of the continuous passage of oxygen from maternal blood in the intervillous space to the fetus, its oxygen saturation resembles that in maternal capillaries. The average oxygen saturation of intervillous blood is estimated to be 65 to 75 percent, with a partial pressure (P02) of 30 to 35 mm Hg.
;: 50 .N 30 . 2010 n = 58 1 mm Hg approx. = 0.•133 kPa
FIGURE 7-13 Umbilical venous cordocentesis samples obtained in fetuses being evaluated for possible intrauterine infections or hemolysis, but who were found to be healthy. A. Oxygen pressure (PoJ B. Carbon dioxide pressure (Pco2). Shaded areas represent 5th to 95th percentiles. (Modified from Ramsey, MM: Normal Values in Pregnancy. Ramsay MM, James OK, Steer PJ, et al (eds). London, Elsevier, 1996, p 106.)
The oxygen saturation of umbilical vein blood is similar but has a somewhat lower oxygen partial pressure. Fetal hemoglobin has a higher oxygen ainity than adult hemoglobin. This is illustrated by the oxyhemoglobin disassociation curve, which is described in Chapter 47 (p. 920).
he placenta is highly permeable to CO2, which traverses the chorionic villus by diusion more rapidly than oxygen. Near term, the partial pressure of carbon dioxide (PC02) in the umbilical arteries averages 50 mm Hg, which is approximately 5 mm Hg higher than in the maternal intervillous blood. Fetal blood has less ainity for CO2 than does maternal blood, thereby favoring CO2 transfer from fetus to mother. Also, mild maternal hyperventilation results in a fall in Pc02ilevels, favoring a transfer of CO2 from the fetal compartment to maternal blood.
Although simple difusion is an important method of placental transfer, the trophoblast and chorionic villus unit demonstrate enormous selectivity in transfer. his results in diferent metabolite concentrations on the two sides of the villus. Importantly, the levels of many substances that are not synthesized by the fetus are several times higher in fetal than in maternal blood. Ascorbic acid is one example. his relatively low-molecularweight substance might be expected to traverse the placenta by simple difusion. he concentration of ascorbic acid, however, is two to four times higher in fetal plasma than in maternal plasma (Morriss, 1994). Another example is the unidirectional transfer of iron. Typically, maternal plasma iron concentration is much lower than that in her fetus. Even with severe maternal iron-deficiency anemia, the fetal hemoglobin mass is normal.
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