SITES OF WATER EXCHANGE BETWEEN THE MATERNAL SYSTEM AND THE AMNIOTIC FLUID OF RABBITS1

Abstract

A procedure was devised to permit evaluation of the relative importance of fetus and membranes as sites for exchange of amniotic fluid water in rabbits at the 24th day of gestation. Rates of exchange of water calculated (1) from the rates of passage of deuterium oxide from the maternal system to amniotic fluid, and (2) from the rates of passage of tritium oxide in the opposite direction are approximately equal at about 0.26 ml/minute with intact fetal systems. Crushing the fetus reduces these rates by approximately 0.5-0.12 ml/minute. Ligation of the vitelline vessels produces no significant reduction of the rates. About half the exchange of water occurs across the placenta and half across the membranes by way of the maternal circulation; the vitelline vessels contribute a negligible fraction to the exchange. It is concluded, in addition, that a steady state does not obtain between the maternal system and the amniotic fluid with respect to water and that a close approach to equilibrium does obtain. It is suggested that diffusion rather than secretion is the mechanism of passage of water across the barriers and that the membrane provides a significant restriction to free diffusion. A derivation is presented of equations for the calculation of rates of exchange. This is based on the assumption that the rate of passage is proportional to the gradient of the chemical potential of the substance considered.