Passage of inert substances and oxygen in the human placenta

Abstract

1. The vascular arrangement and the perfusion pattern of the human placenta is discussed from the point of view of the transfer of substances from the maternal blood through the placental membrane into the fetal blood. The functional unit of the human placenta is a stream of maternal blood in the intervillous space opposed to a large series of fetal capillaries (the multivillous stream bed system).

2. The transfer of each inert substance in the human placenta is dependent upon the concentration difference of the substance in the maternal and fetal blood entering the placenta, the rates of blood flow and the permeability coefficient of the placental membrane. These relationships are graphically presented.

3. The placental transfer of oxygen during maternal hypoxia in the human depends upon the difference in O₂ partial pressure in the entering blood streams, the rates of blood flow, the oxygen affinity and the capacity of maternal and fetal blood and the diffusion capacity of the placental membrane. These relationships are graphically presented.

4. The transfer of oxygen under normal conditions in the human placenta depends upon the rates of blood flow and the diffusion capacity. These relationship are graphically presented for conditions of special composition of maternal and fetal blood entering the placenta.

5. The transfer characteristics of the human placenta are compared with those in placentas with other vascular arrangements and perfusion patterns. The system of the human placenta is more efficient than a concurrent system and a pool system, but less efficient than a countercurrent system if the diffusion capacity of the placental membrane is high in relation to the fetal transport capacity. If the diffusion capacity is low in relation to the fetal transport capacity, all systems have about the same efficiency.