Supplementary Observations on the Oxygen Pressure Gradient Between the Maternal and Fetal Bloods of Sheep *

The observations to be presented here on the average oxygen pressure gradient that exists between the maternal and fetal bloods of the sheep, separated by the five tissue layers that form the syndesmochorial placenta, are in the nature of a supplement to data published earlier.' In that paper data were presented on the form of the oxygen dissociation curves of fetal and maternal bloods obtained from individuals sacrificed at selected stages in gestation, together with the oxygen capacities, contents, percentage saturations, and estimated oxygen pressures of samples of blood drawn from each of the four placental vessels-the uterine artery, uterine vein or venule, umbilical artery, and umbilical vein-after their exposure by caesarian section. From these data, the differences in oxygen pressure of the bloods of the uterine artery and umbilical vein, and the bloods of the uterine vein or venule and the umbilical artery-that is the difference at the two ends of the parallel cords of the placental capillaries'-were estimated. By virtue of the fact that the maternal and fetal bloods do move through more or less parallel cords of approximated capillaries in the placenta of the sheep, some of the published data can be used for the estimation of the changes that occur in the oxygen pressure in the fetal and maternal bloods as they move through the placenta, if some assumptions are made with regard to the flow of the blood and the permeability of the capillaries. These assumptions are: (i) that each stream moves through vessels of constant caliber and at uniform velocity, (ii) that the permeability to oxygen of the placental capillaries and the intervening tissues is uniform throughout their length, and finally, (iii) that the volume of oxygen diffusing per unit time from the blood in one region of the maternal capillaries to the blood in a region of the fetal capillaries is proportional to the difference in oxygen pressure between these two regions.