Electrical potential difference across the mid-term human placenta.

Electrical potential difference across the mid-term human placenta was recorded during hysterotomy. The average value registered was 2.7 mV (S.E. of mean=0.4 mV, n=7), fetus negative. From this value and the concentrations of Na, K, Ca, Cl and inorganic phosphate in maternal and fetal plasma the possible mechanisms of net transport from mother to fetus are inferred. It is concluded that of the above ions only the transport of Na is compatible with simple diffusion.