The construction and usefulness of physical models of the placenta

Abstract

Physical models have been constructed of placentas which have exchanging flows in several orientations. The effectiveness of heat transfer was measured in these models and was found to generally conform to that which was predicted from theoretical considerations. The results support the conclusion that the effectiveness of exchange in the primate placenta should be greater than that of a placenta which resembles a concurrent exchanger and less effective than a countercurrent configuration. Using the concurrent model we have demonstrated that the effectiveness of oxygen and carbon dioxide exchange were similar when the permeability of the placenta was high. When the permeability of the placenta was decreased we were able to demonstrate that the transfer of carbon dioxide was decreased, whereas that for oxygen was not changed. This is the same type of result which has been obtained by other investigators in living animals, where the analogous comparison was between inert gas and oxygen transfer and the different behavior of the gases was induced by the use of drugs which bind to cytochrome P450. The earlier results were held as evidence for the existence of a carrier mediated oxygen transport mechanism. The experiments reported here demonstrate that this result can be explained by non-specific decreases in placental permeability. Physical models of the placenta appear to be a useful bridge between theoretical and experimental physiology.