Sluice flow in placenta: maternal vascular pressure effects on fetal circulation.
In 4 anesthetized pregnant rhesus monkeys the decidual parts of the uteroplacental arteries (spiral arteries) were punctured with glass capillaries. The intravascular blood pressure was measured. Simultaneously, the pressure in the amniotic fluid and in the aorta was monitored. When the uterus was relaxed, the mean blood pressure inside the uteroplacental arteries was found to be 12 Torr (SD=3). At the same time the pressure in the amniotic fluid was 7 Torr (SD=2), and the mean aortic blood pressure was 83 Torr (SD=13). During uterine contractions the blood pressure in the uteroplacental arteries rose and fell simultaneously with the amniotic pressure. It was concluded that the intervillous space of the hemochorial placenta of the rhesus monkey offers only a minor resistance to maternal placental flow when the uterus is relaxed. Maternal placental flow is controlled by the arteries outside the placenta. The correlation between the pressure in the uteroplacental artery and the amniotic pressure substantiates the Starling resistor model of placental circulation.