Unidirectionality of the water exchange between mother and 19-day fetus in the rat.

Tritiated water was injected into 19-day pregnant rats and the rate of incorporation into their fetuses was estimated. The transfer of tritiated water from mother to fetus and then to the surrounding uterus was estimated by measuring the appearance of radioactivity in an albumin-saline solution that bathed externally one conceptus freed of its surrounding uterus except for the placental attachment. There was a very significant transfer of tritium water, even before isotopic equilibration of the fetus, estimated at more than 1.5 ml/h. The reverse, the transfer of tritium from the saline solution towards the mother, resulted in only a small fraction of that figure. The results suggest the existence of unidirectional water circulation in the materno-fetal unit: maternal arterial blood----placenta----fetal circulation----amniotic fluid----amniotic membranes----endometrium----maternal venous blood.