Application of the Theory of Heat Exchangers to the Transfer of Inert Materials in Placentas

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Abstract
Calculations were made on a variety of models to determine to what extent placental structure (vascular geometry and permeability) and placental function (rate and evenness of distribution of fetal and maternal blood flows, and presence of nonexchanging shunts) affect the rate of diffusional exchange across the placental barrier. The results of the calculations are given in the form of a graphical relationship between three dimensionless variables defined in the text. It is shown that experimentally measured transfers can also be represented in this form and thus compared to transfers that are theoretically predicted. The results indicate that biologically probable degrees of regional variation in permeability are harmless, but that this is not true of uneven distributions of maternal-fetal blood flow ratios. They further show that the relative magnitudes of blood flows through nonexchanging shunts can be found by extrapolation of experimental data obtained with substances whose transfers are entirely flow limited, but that the use of such substances cannot help to separate the effects on placental exchange due to vascular geometry from those due to uneven distribution of flow ratios. Methods are indicated with which the value of the placental permeability can be assessed experimentally under various conditions.