Fetal nutrition: supply, combustion, interconversion, and deposition

The study of placental transport is confounded by species differences, gestational differences, and a permanent non-steady state. The histology of the interhemal membrane, the degree of steric hindrance to diffusional exchange, the discrimination of electric charge, the flow pattern, and the transfer of intact gamma globulins are all species specific. We suggest the creation of order based on the ultrastructure of the placental barrier. The gas diffusion capacity of the membrane changes during gestation. This change is initially due to an increase in placental mass, but the gas diffusion capacity continues to increase after placental growth has ceased, because of changes in surface area and thickness of the membrane. The non-steady state of fetal weight is accompanied by an uptake of nutrients far in excess of what is necessary to satisfy oxygen consumption. Measurements of venous and arterial cord blood chemistry are sufficient to distinguish the fraction of nutrient uptake that is dedicated to catabolism from the fraction that is dedicated to fetal growth.