Transfer of N2O, CO and HTO in the artificially perfused guinea-pig placenta

Abstract

The fetal vascular system of the guinea-pig placenta at term was perfused with a dextran solution at a rate of 3.6 and 9 ml/min, while the uteroplacental (maternal) blood flow was adjusted to 4-18 ml/min. N2O, CO, and tritiated water (HTO) were added to the perfusion fluid. The effectiveness of placental transfer of these substances, i.e. the actual transfer related to the maximum transfer occuring when the fetal vein equlibrates with the maternal artery, was calculated. It was found that the effectiveness of the CO transfer was 98% (SD = 2%). The effectiveness of the N2O transfer was about 90% (SD = 6%) when the maternofetal flow ratio was two and about 75% (SD = 13%) when the ratio was one. The effectiveness of the transfer was the same for HTO and N2O. It was concluded: (1) The vascular and diffusional shunts on the fetal side of the placenta are only 2%. (2) The placenta has the properties of a countercurrent system in which the effectiveness of gas transfer is decreased by about 25% at a flow ratio of one and by 10% at a flow ratio of two by inhomogeneity of the maternofetal flow ratio. (3) The diffusional resistance for HTO is unmeasurabbly small; its transfer is solely limited by blood flow.