The effects of maternal vascular pressure on the dimensions of the placental capillaries

Abstract

Objective The study sought stereological evidence for the sluice flow phenomenon in the human term placenta. Design Two series of experiments were conducted on Caesarian 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 concomittant 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.

Developmental Dynamics of the Definitive Mouse Placenta Assessed by Stereology1

“…Thus, mean capillary diameter d was estimated using the equation [21] 1/2 d ϭ 2(mean area/) (6) Interhemal Membrane Thickness…”

“…A 100ϫ objective lens was used and 12 fields of view within the Lz were selected by meander sampling to determine volume densities, surface densities, length densities, and interhemal membrane thickness. Shrinkage associated with resin embedding was considered insignificant at 1.08% and was assessed by comparing the mean diameter of 100 erythrocytes in the resin-embedded placentas with fresh maternal erythrocytes [21].…”

The placenta: a multifaceted, transient organ

“…It requires the trophoblast to tap into branches of the maternal uterine arteries that carry blood at a higher pressure than the fetus can ever generate. Hence, there is a danger that the fetal capillaries within the terminal villi will be compressed, impeding the umbilical circulation and preventing the formation of vasculosyncytial membranes [33]. Equally, the high velocity of maternal arterial blood flow can potentially cause mechanical damage to the delicate villous trees [34], with high shear rates also causing oxidative stress [35].…”

Regulation of vascular growth and function in the human placenta

“…This may explain the location of the dilated sinusoids at the apices of capillary loops (Burton & Tham 1992). The observation that the capillary sinusoids are capable of expansion/compression dependent upon the pressure differential between the maternal and fetal circulations indicates that the capillary walls have elastic properties (Karimu & Burton 1994). The pressure differential is likely to rise during gestation as the fetal heart matures, and so one might envisage a continually increasing distending force being applied.…”