Abstrakt
All present thinking about fluid acquisition by the growing fetus assumes that water is driven to the fetus by an external driving force (filtration pressure). As long as the placenta is viewed as a simple membrane the existence of an external driving force must be postulated to explain water transfer from mother to fetus.
However, the placenta is multilayered. If a multilayered membrane contains active transport elements no external driving force may be necessary to explain fluid movement across the membrane.
This has been shown in a theoretical study of solute and fluid transfer across a two membrane system by Patlak, Goldstein and Hoffman (1963).