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

These studies explore the distribution of blood flow among small samples of placental tissue. Labeled microspheres (14 +/- 1 micrometers, SD) were injected into the left ventricle of six unanesthetized ewes and into the jugular vein of the fetuses in utero. A total of 3,576 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 standard deviations of 44%. The maternal-fetal blood flow ratio was less than 0.5 in 9% of the cotyledon, between 0.5 and 1.5 in 70%, and greater than 1.5 in 21%. Errors caused by the method were estimated to contribute less than 10% of the flow variances. Maternal and fetal blood flows were significantly correlated (r = 0.57, P less than 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 about 50% of the uterine-umbilical venous oxygen tension difference. It was concluded that the samples studied were small in comparison with the dimensions of placental oxygen exchange but may have been similar in size to the dimensions of blood flow control.