Placental diffusing capacity

Placental Diffusing Capacity. The diffusion characteristics of the placental membrane may be described by Fick s first law (6) ... 

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. 

Figure 4. The calculated changes in diffusing capacity of fetal and maternal blood and the placental diffusing capacity (Dp) during a...

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.

Figure 8. The change in maternal and fetal end-capillary p02 and the mean rate of O, exchange as a function of placental diffusing capacity. At values of Dp less than 2.0 ml/(min X mm Hg) there is an increased maternal-fetal ena-capillary difference and a decreased rate of Ot transfer. The vertical interrupted line represents the assumed normal values of Dp.

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. 

Normal values for the various determinants of 02 transfer are necessary for quantitative analysis of the exchange process. Some values— e.g., those for the maternal and fetal arterial 02 tensions, 02 capacities, and 02 affinities—are fairly well defined. Others—e.g., the diffusing capacity and maternal and fetal placental blood flows—are less well determined. 

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 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. 

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