Gas Exchange between Mother and Foetus and Placental Design

Abstract

Our knowledge of placental gas exchange and of foetal blood gas homeo­ stasis is based largely upon animal models and abstract calculations. There has been a revolution in our understanding of these problems in the last few years, as a result of the introduction of new techniques and the greater number of investigators. So this is an appropriate time at which to survey the scene. Placental gas exchange depends on many factors. Some of these have been extensively studied and are very well known. They include the special characteristics of maternal and foetal blood (the size of the O 2 carrying capacity and the shape and position of the Hb-02 and CO2 dissociation curves). We recognize that a proper understanding of these are a necessary prerequisite to physiological studies on placental gas exchange. Yet so much has been written about them that one might be forgiven for sup­ posing that they constituted the single most important variable. From the practical point of view, however, they are factors which are not readily accessible to manipulation in vivo. They can be altered only by intraute­ rine transfusion or, perhaps, by ohanges in the erythropoietin concentra­ tion of the blood. Replacement of foetal with maternal blood in lambs at 0.67-0.86 of term (with a large shift to the right in the Hb-02 dissociation curve) caused a fall in umbilical vein O 2 saturation and a rise in P02 but no systematic changes of blood flows or O 2 consumption rates (1). Human foetuses also survive both thorough transfusion in utero and (in erythro­ blastosis foetalis) a very low O 2 capacity of the circulating blood. Another factor about which much has been written, but of which we know little for certain, is the geometry of the flow pathways in the area of gas exchange. There are four possible arrangements, countercurrent flow, concurrent, crosscurrent (or multivillous) and 'pool' flow systems. It is probable that in the placenta of some species more than one such flow pathway may coexist. For instance in the only situation which has been