Transfer of urea, sodium, and chloride across the perfused human placenta

Clearance indices (clearance of experimental material: clearance of antipyrine, CI) across the perfused human placenta were determined for urea, sodium, and chloride. The selected materials are of relatively small molecular weight and are water soluble. Clearance of urea was determined under conditions of net transfer and of exchange and no difference was detected. Sodium and chloride clearances were measured as exchange rates. The CI were: urea, 0.32 and 0.38 toward fetus and mother, respectively); sodium, 0.28; and chloride, 0.41 (measured toward the fetus). Recalculation of data as the diffusion limitation (LD) facilitated comparison with data in the literature on the monkey, rabbit, and sheep. LD for urea was comparable among the four species whereas the sheep placenta differed, sharply limiting the diffusion of sodium and chloride. The discrepancy in the sheep between urea and sodium indicates that the low transfer rate of the latter cannot be explained by the "thickness" of the syndesmochorial placenta, as is often stated. Inferences concerning nitrogen metabolism in the fetus have been derived by others from data on transplacental urea gradients and urea clearance. Available data in the human are incomplete but are consistent with amino acids providing approximately 10% of fetal energy needs, with glucose serving as the predominant substrate.