Maternal oxygen capacity

Figure 11. Fractional reduction in maternal oxygen capacity vs. fractional reduction in oxygen transferred...

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... 

Figure 9. Axial partial pressure profiles in the maternal channel for various maternal blood oxygen capacities...

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. 

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. 

The CO diffusing capacity, DPco, of sheep was measured at various 02 tensions in a hyperbaric chamber using a method similar to that outlined above (12). DPco varied as a function of the oxygen tension. In Figure 2 the reciprocal of DPco is plotted as a function of the reciprocal of the diffusing capacity of the maternal and fetal red blood cells. From the plot the slope is the reciprocal of the value of V, the maternal and fetal capillary blood volume while the intercept is the reciprocal of Dmco- From these studies we calculated that the resistance of maternal and fetal red blood cells was approximately one-third of the total resistance while the resistance of the placental membrane per se was about two-thirds of the total resistance to 02 diffusion (12). This is in contrast to previous studies which assumed that the placental membrane per se constituted the total resistance to diffusion. 

Hg) at time t on the maternal and fetal sides, Vm and Vf are the maternal and fetal capillary blood volumes (ml) and Dp is the placental diffusing capacity for 02 [ml/(min X mm Hg)]. While these equations ignore placental tissue oxygen consumption, this will be considered later. 

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