The Rabbit Placenta as an Organ of Diffusional Exchange

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Abstract
The umbilical arteries of intact, in-situ placentas of one series of anesthetized rabbits were perfused with deoxygenated blood equilibrated with 10% or 20% acetylene and in another series with oxygenated blood without acetylene. The transfer of acetylene, and probably also of oxygen, were shown not to be limited by diffusion. Above the normal physiological rates of fetal placental flow, oxygen uptake in the fetal placental circulation did not increase appreciably with an increase in the rates of perfusion; below it, it decreased almost proportionally with a decrease in the rate of perfusion. To explore the applicability of these results to the human placenta, the experimental data were reduced to dimensionless form. It was shown that three dimensionless variables suffice to describe the characteristics of diffusional exchange in the placenta. The dimensionless diagrams were very similar for the ideal countercurrent exchanger and for the rabbit placenta but these differed from diagrams made from data published on human, sheep, cow, and monkey placentas. The authors propose the dimensionless diagram as a concise description of the passive transfer characteristics of a placenta, at least until measurements of vascular geometry, regional permeabilities and flow patterns allow the calculation of mean diffusion gradients.

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