Distribution of maternal and fetal blood flow within cotyledons of the sheep placenta

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Abstract
These studies explore the distribution of blood flow among small samples of placental tissue. Labeled microspheres (14 .+-. 1 .mu.m, SD) were injected into the left ventricle of 6 unanesthetized ewes and into the jugular vein of the fetuses in utero. A total of 3576 samples weighing 31 .+-. 11 (SD) mg were taken from 17 cotyledons and counted for flow labels. Maternal and fetal blood flow within the cotyledons were both normally distributed with SD of 44%. The maternal-fetal blood flow ratio was < 0.5 in 9% of the cotyledon, between 0.5-1.5 in 70%, and > 1.5 in 21%. Errors caused by the method were estimated to contribute < 10% of the flow variances. Maternal and fetal blood flows were significantly correlated (r = 0.57, P < 0.001) and both exhibited spatial organization. Calculations based on mathematical models suggested that the observed distribution of the ratio of maternal to fetal blood flow may explain .apprx. 50% of the uterine-umbilical venous O2 tension difference. The samples studied were small in comparison with the dimensions of placental O2 exchange but may have been similar in size to the dimensions of blood flow control.

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