Menaal exercises in placental transfer

M A N Y attempts have been made to determine the mechanisms of transfer of substances between mother and fetus across the placental membranes. The technique most frequently employed is that of inducing changes in the composition of the blood entering the maternal or fetal sides of the placenta and measuring the effects of these changes on the blood leaving the placenta. From the data obtained through this approach, placental exchange mechanisms have been postulated which have varied from concurrent to countercurrent or even multicurrent systems. These conflicting results seem to suggest the existence of a basic defect in the technique of determining the nature of intraplacental transfer mechanisms from extraplacental stresses and measurements. In our view, this approach does not take into consideration the many variables which enter into play simultaneously or independently when a substance is transferred across the placental membrane. The problem is further complicated by the fact that the interrelationship of the anatomic structures