Objective: Our purpose was to determine whether the human placenta is able to adapt structurally to hypobaric hypoxia so as to increase its diffusing capacity.
Study design: Material from 10 normal term placentas collected at each of three altitudinal levels in Kirghizstan was analyzed stereologically. Data were compared by one-way analysis of variance.
Results: Within the overall sample group birth weight and placental weight remained constant. There were no differences in villous and capillary surface areas or capillary length, but capillary volume increased from 37.9 cm3 in controls (700 m) to 50.8 cm3 at high altitude (2200 to 2800 m). The harmonic mean thickness fell from 6.9 microns in controls to 4.8 microns at high altitude. Consequently, the morphometric diffusing capacity of the villous membrane for oxygen increased from 4.22 to 7.56 cm3.min-1.mm Hg-1 (F = 6.45, p < 0.05).
Conclusions: The placenta is capable of adapting to hypobaric hypoxia to increase its functional capacity for gaseous exchange. This is principally through a reduction in the thickness of the villous membrane.