Objective: The study sought stereological evidence for the sluice flow phenomenon in the human term placenta.
Design: Two series of experiments were conducted on caesarean delivered placentae. In both, two fetal arteries were cannulated and perfused. In the first series, five placentae were perfusion-fixed under arterial pressures of 40 mmHg and 60 mmHg, and an external pressure of 10 mmHg in the intervillous space. In the second series, five placentae were perfusion-fixed at arterial pressures of 70 mmHg and 90 mmHg with an external pressure of 30 mmHg. Stereological estimates relating to the size of the capillaries and the thickness of the villous membrane were then made.
Results: In each series, as the arterial pressure rose, there was an increase in the volume fraction and surface area of the fetal capillaries, and a concomitant decrease in the mean thickness of the villous membrane. These changes were negated as the external pressure rose, indicating the importance of the pressure differential between the capillary lumen and the intervillous space in determining the dimensions of the fetal capillaries.
Conclusions: The study demonstrated that placental capillaries are elastic and hence deformable. If a sufficiently high pressure is generated in the intervillous space, they may therefore be compressed. This may explain the increase in umbilical vascular resistance demonstrated by doppler studies in pregnant women scanned in the supine position. In this situation the venous drainage from the intervillous space is believed to be impeded by the gravid uterus compressing the inferior vena cava.