Steady-state methods for the study of placental exchange.

Published and new data on steady-state exchange of tracers and oxygen are characterized by marked species differences. When the placenta is treated as an ideal diffusion cell of unknown vessel geometry and permeability, the exchange characteristics of oxygen can be used to prove that the exchange of tracers such as acetylene, nitrous oxide, tritiated water and antipyrine is entirely flow limited. The recorded patterns of transfer of flow-limited tracers reveal that some placentas are as effective as counter-current exchangers whereas others mimic the behavior of the less effective types of exchangers. These species differences in apparent geometry are unrelated to the histologic nature of the barrier. The patterns of flow-limited transfer are so similar to those of oxygen transfer that the diffusion resistance to oxygen must be small. The exchange of diffusion-limited (hydrophilic) tracers mimics exchange across capillary membranes in some species and exchange across cell membranes in others. These species differences in diffusion-limited transfer are clearly related to the histologic nature of the barrier and are independent of vessel geometry.