Maternal partial pressure

Pregnancy is also associated with a partially compensated respiratory alkalosis that may affect the protein binding of some drugs. Respiratory changes in pregnancy include a decrease in arterial partial pressure of carbon dioxide to 30.9 mm Hg, most likely due to the effect of progesterone (23,24). In compensation, serum bicarbonate decreases, and maternal serum pH increases slightly to 7.44 (23). 

In the placenta a volume of oxygen sufficient for fetal needs must diffuse across the membranes from maternal to fetal blood during the short time the two circulations are in close contact. This oxygen transfer is a function of several factors which include uterine and umbilical arterial 02 partial pressures, maternal and fetal placental blood flow rates, the 02 capacity and 02 affinity of maternal and fetal hemoglobin, the diffusing capacity of the placenta, the amount of C02 exchanged, and the vascular arrangement of maternal to fetal vessels. 

Figure I. Schematic representation of the maternal to fetal partial pressure differences ana resistances to diffusion.

A further consideration is the relative resistance to diffusion offered by maternal and fetal blood and the placental membranes. The total partial pressure difference between maternal and fetal blood is the sum of the pressure difference from the interior of the maternal erythrocyte to the maternal plasma, plus the pressure difference from the maternal plasma across the placental membrane to the fetal plasma, and the partial pressure difference from the fetal plasma to the interior of the fetal erythrocyte. These relations are diagrammed in Figure 1. The reciprocal of the diffusing capacity, the total resistance to diffusion, is the sum of the resistance of the maternal blood, the resistance of the placental membrane, and the resistance of the fetal blood (12). These resistances are depicted in Figure 1 and may be expressed as ... 

The oxygen contents and partial pressures are related by the hemoglobin dissociation curves for maternal and fetal bloods. We used a modified Hill equation (17, 18) ... 

Figure 7. The change in 02 partial pressure in maternal and fetal erythrocytes, during the course of a single capillary transit for various values of placental diffusing capacity, Dp. The mathematical model was used for the calculations.

After it was determined that changing the linear velocity of the maternal or fetal blood stream did not affect the equilibration pressure, it was possible to determine the maternal to fetal volumetric flow rate ratio that would result in an equilibration pressure which matched experimentally determined values of umbilical vein and artery oxygen partial pressures (37). As the maternal to fetal volumetric flow rate ratio is increased, the partial pressure of oxygen at which equilibration... 

Figure 5. Axial partial pressure profiles in the fetal capillary for various maternal to fetal volumetric flow rate ratios. Maternal volumetric flow rate...

Figure 6 shows the effects of reduced fetal blood volumetric flow rate on the maternal blood axial oxygen partial pressure profile. As fetal blood flow rate decreases and fetal blood oxygen partial pressures increase,... 

Figure 8 shows the effects on the fetal blood axial oxygen partial pressure profile when the maternal blood flow rate is decreased. As the maternal blood flow rate decreases, the fetal profile is shifted downward, resulting in fetal blood oxygen partial pressure values in the fetal capillary that are lower than normal. 

Figure 9 shows the effects of decreasing the oxygen capacity of maternal blood on the maternal axial oxygen partial pressure profile. In the normal case (N2 — 16 ml oxygen/100 ml blood) maternal arterial blood enters the exchange unit at an oxygen partial pressure of 90.5 mm... 

The effects of decreasing the maternal blood oxygen capacity on the fetal blood axial partial pressure profiles are shown in Figure 10. As the oxygen capacity of maternal blood decreases, the fetal axial oxygen partial pressure profile is shifted downward. 

The fetal blood oxygen partial pressure values in the first half of the fetal capillary increase more rapidly than those in the second half. This results from the large pressure gradient initially present between maternal and fetal streams rather than the slope of the dissociation curve. As the last half of the exchange system is reached, maternal and fetal values approach each other, the gradient is decreased, and the rate of oxygen transfer decreases. 

Figure 13. Axial partial pressure profiles in the maternal channel for various fetal blood oxygen capacities. Maternal blood oxygen capacity held constant.

Oxygen Diffusivity. A study was also made to determine the effects of simultaneously decreasing the oxygen diffusivity of maternal and fetal blood. Figures 15 and 16 show the effect on the maternal and fetal axial oxygen partial pressure profiles, respectively. For comparison purposes, normal conditions are represented by a blood diffusivity of D = 0.112 X 10"4 cm2/sec. 

Figure 17. Axial oxygen partial pressure profiles in the maternal capillary for various maternal entrance oxygen partial pressures...

Figures 17 and 18 shows the effects produced on maternal and fetal blood when the arterial oxygen partial pressure of maternal blood is decreased.

Partial pressure of oxygen calculated from the space average fractional saturation of blood, torr Radius of fetal capillary, cm Radius of maternal channel, cm Radius of tissue cylinder, cm Dimensionless radial distance Variable radius, cm Time, sec... 

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