Comparative aspects of placental exchange functions.

The anatomic structure of the placenta varies widely at the macroscopic and microscopic levels, and these differences are often reflected in differences in exchange mechanisms. For example, placental iron transfer proceeds by three distinct mechanisms each associated with a distinctive anatomic feature. A number of factors also affect the exchange capacity per unit of placental tissue, these include boundary properties and the arrangement and rates of fetal and maternal placental blood flows. Generally, the exchange rate of a substrate is limited principally by the membranes or by blood flow but these interact in a complex fashion. When exchange occurs by relatively slow passive diffusion, the barrier thickness and exchange area per gram of placenta are most important. The T-diagram reveals that a counter- or crosscurrent arrangement of blood streams is most efficient. However, relatively few placentas make use of these high efficiency designs. It is suggested that only in animals such as rodents and shrews where a large fetus is produced in a short time by a small placenta does placental exchange capacity need to be pushed to its theoretical limits. We define an index termed the 'turbo factor' (= term fetal weight/[placental weight x days of gestation]) to express the extent to which growth taxes the placenta. A value of > 0.2 seems to require a placenta of the high-efficiency type and may predict that, in a given species, the placenta makes use of the most efficient arrangement of blood flows.